You've one job and earn less than £100,000? It's likely your tax code won't change, but there's no harm in checking

For the vast majority who are on a version of the 1250L tax code (S1250L in Scotland, C1250L in Wales), a change in circumstances isn't likely to mean your tax code changes. We've spoken to HMRC, which advises that you don't need to contact it if you've been furloughed or otherwise had your income affected. It'll know about changes to your income or employment from your (old) employer, or through self-assessment. It also says if you're now paying too much or too little tax due to a change in circumstances, this will usually be corrected during the tax year (tax information is now submitted in real time, rather than checked at the end of the tax year). But if you've not checked your tax code in years, it's worth checking it now using the calculator above , in case you've been on the wrong code for a long time and just not realised.

More than one source of income or earn £100,000+? It's definitely worth checking in case of changes

There are a few sets of people who should keep an eye on their tax code (and the amount of tax they pay) this tax year, as coronavirus could have some consequences for them. See if you might be affected...

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Helen Saxon

Updated 5 May 2020

It looks like an innocuous set of digits, but your tax code can have a big impact on your finances. Every year, many people are hit by errors – and some are due £1,000s back. Use our Tax Code Calculator to check you're on the right tax code, and read our guide for how to handle underpaid and overpaid tax.

Think the code's wrong?
- I've overpaid – how do I get cash back?
- I've underpaid – what happens now?
Watch your future codes
- Get free tax help

Thanks to Tony Tesciuba (Tesciuba Ltd) and Matthew Brown (Chartered Institute of Taxation) for feedback/suggestions. While every effort's been made to ensure accuracy, this guide isn't authorised, tailored tax advice (get help here). We can't take responsibility nor accept liability for damage or losses; you use the info at your own risk.

The Tax Code Calculator

If you already know your tax code, use our calculator below to get a ROUGH idea of whether it's correct (it's impossible to be exact). If it seems wrong, it could be worth taking action to check whether you're owed (or owe) cash.

Don't know your code? The rest of this guide explains how you can find it, and what to do if you think your tax code is wrong.

Got one employer, no work perks, live in England/NI and earn under £100,000? Your 2020/21 code should be 1250L.

If you've been furloughed or you've been made redundant from a job due to coronavirus, we've added some help to guide who might need to check their tax code because of this.

New. Coronavirus and your tax code

It's an unsettling time for many, and the coronavirus crisis means changes to our work, whether that's being furloughed and potentially adapting to a lower salary, having to stop work as you can't run your business or being made redundant from your job.

Whether these changes are something you need to let HMRC know about, or mean you need to keep an eye on your tax code or the amount of tax you're paying will depend on your circumstances.

• You've one job and earn less than £100,000? It's likely your tax code won't change, but there's no harm in checking

For the vast majority who are on a version of the 1250L tax code (S1250L in Scotland, C1250L in Wales), a change in circumstances isn't likely to mean your tax code changes.

We've spoken to HMRC, which advises that you don't need to contact it if you've been furloughed or otherwise had your income affected. It'll know about changes to your income or employment from your (old) employer, or through self-assessment.

It also says if you're now paying too much or too little tax due to a change in circumstances, this will usually be corrected during the tax year (tax information is now submitted in real time, rather than checked at the end of the tax year).

But if you've not checked your tax code in years, it's worth checking it now using the calculator above, in case you've been on the wrong code for a long time and just not realised.
• More than one source of income or earn £100,000+? It's definitely worth checking in case of changes
There are a few sets of people who should keep an eye on their tax code (and the amount of tax they pay) this tax year, as coronavirus could have some consequences for them. See if you might be affected...
You usually earn more than £100,000 a year

If you usually earn more than £100,000, your personal allowance will be lower than the standard £12,500, and this will be reflected in your tax code, which will look different to the normal 1250L and will tell your employer to take more tax.

If you're furloughed and your furlough pay is the maximum £2,500 a month, then your tax code's likely to mean you're paying too much tax on the furlough pay. However, HMRC says this is the sort of change that would be corrected within the tax year.

Of course, the hope is that when furlough is lifted, you will be able to go back to your normal salary and the amount of tax you pay would be evened out over the tax year. But if the worst were to happen, and you were made redundant after furlough, then you may be due tax back.

You don't need to tell HMRC you're furloughed, but it's worth checking at the end of the tax year that you've paid the right amount of tax (use our income tax calculator to help).

You've more than one job and you've been furloughed on one or more of your jobs

If you have more than one job, you will usually have different tax codes for those jobs – for example, your 'main' job (often the one you got first) will likely have a coding that takes your personal allowance into account, whereas your second job will often have a BR code, telling the employer to tax all income at the basic 20% rate. It's easier for HMRC to keep track this way, rather than trying to split your personal allowance between employers.

Yet if you've been furloughed on one or more job, it may be that these codings mean you pay the wrong amount of tax, especially if your furlough pay from your main job takes you below your personal allowance.

This should be sorted out within the year as HMRC will realise you've paid too much tax and will let your employer know to correct it by taking less tax later in the year. You won't need to tell HMRC you've been furloughed and you won't need to change tax codes – furlough is, after all, designed to be a temporary measure, so the hope would be that in a few months' time your employment is back to its normal pattern.

But it's worth keeping an eye on how much tax you pay this tax year (which runs from 6 April 2020 to 5 April 2021) and making sure it's the right amount (use our income tax calculator to help).

You've lost your main job (but still have others)

If you have more than one job, your tax-free allowance is usually held in your tax code for your main job, with other jobs attracting tax codes like BR, D0 or D1 (depending how much you earn at them).

If you get made redundant from your main job, then what was a second or third job will become your main job. In this case, contact both HMRC and your new main employer's HR department to let them know that your main job has changed.

You'll need to give your now main employer your P45 from your old job, but the employer will also need to wait for notification of your new tax code from HMRC before it can change the tax code it pays you on (something that will happen once your old employer has notified HMRC that you've been made redundant).

The new tax code will transfer your tax-free personal allowance to your new main job. As there are a few moving parts here, keep an eye on your tax code on your payslip, and if nothing's changed in a few months' time, call HMRC to find out what's going on.

You had pension and work income and now just have the pension income

If you have income from a state pension, a private pension and employment, then your tax codes can get complex, as usually only one of these will have your tax-free allowance coded into it.

In this example, you could have the 1250L tax code on the state pension, a BR coding on the private pension (to show it should all be taxed at the 20% rate) and potentially even a K coding on your employment income (a K coding implies a negative personal allowance, meaning tax is taken from your employment income that you actually owe on other income).

In this scenario, HMRC will need to adjust your tax codes so that you're paying the right amount of tax on the right sources. Again, it should get all the information it needs from your employer letting it know you've been made redundant, but do keep an eye on the tax code(s) on your pension(s) – if nothing changes over the next few months, you should call HMRC to let it know so it can look into your tax file and correct anything that needs changing.

You retired from the NHS but have gone back to help during the crisis

Tax codes can get complicated if you have the state pension and a private pension, and then you go back to work. As you're one of the medical heroes who has stepped in to help the country during the coronavirus crisis, you will need to make sure you're not penalised for doing so by paying too much tax.

One accountancy firm, Moore, has offered to do a free tax check for retired medical staff who have gone back to the NHS. If you'd like its help, you can fill in your details on this form.

The form asks for your tax details, but if you're uncomfortable filling those in on that form, you only need to fill in your contact details and tick the box to say you've read the terms and conditions, then submit the form. Moore will call you back to ask about your tax details.

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What's a tax code?

Before we start explaining what a tax code is, it's important to understand that not everyone has one.

Who has a tax code?

Full or part-time employees

Those receiving a private pension

Who doesn't?

Fully self-employed or unemployed people

Those ONLY receiving a state pension

On the face of it, a tax code is a dull and harmless series of numbers and letters – 1250L, S1250L, C1250L, BR and K497 are just some examples for the current tax year.

But these hieroglyphics are used by your employer to calculate the amount of tax that should be deducted from your wages or pension before they hit your bank account.

Therefore if you pay tax through the pay-as-you-earn (PAYE) system, the tax code tells your employer or pension provider what it should take – and even small errors can lead to mistakes of £100s.

So the aim here is to find your tax code and decipher what it means, to see if it's correct.

Quick questions

Why are there so many tax code mistakes?

In summer 2010, errors started coming to light and MORE THAN 30 MILLION mistakes for the tax years between 2003/04 and 2015/16 (the latest figures available) were uncovered by HM Revenue & Customs (HMRC).

People pay the wrong amount of tax because they've changed jobs midway through the tax year, or they've retired, or some other change.

Commentators often point the finger at HMRC, though many tax nerds dispute this, saying its reconciliation at the end of each year reveals errors – but didn't create them. Current employers can only use the tax code they are told to use, so the key is having correct info flowing through the system.
How likely am I to be on the wrong tax code?
The errors are indiscriminate, but some groups of people are likely to be more affected than others. You should take action quickly if one of the following situations has applied to you in the recent past.

Changed jobs? The tax system can incorrectly assume you have two jobs if your former employer hasn't let HMRC know you've moved on.

Have more than one income? If you've been earning money from more than one source (eg, you have a second job), you could find you've been taxed incorrectly on a chunk of your earnings.

Receive employee benefits? If part of your salary is made up of company benefits such as a company car, healthcare cash plan or medical insurance, it's possible you're being taxed wrongly.

Just started your first job? Young people embarking on first jobs in the middle of a tax year can easily be shunted on to an emergency tax code. Never assume the amount you receive in the first few pay packets is correct.

Have more than one pension or recently retired? If you receive money from more than one pension source, have retired in the last couple of years or have recently started to receive the state pension, you could have been taxed erroneously.
Important! Have you changed address and not told HMRC?

If you've moved home and not told the taxman, any correspondence from HMRC about your tax code may have gone to the wrong address. Your employer can't update this, only you can, so if you haven't, a possible refund could've gone to the wrong address.

You must contact HMRC directly to update your address. The easiest way to do this is at HMRC's website.

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How do I find my tax code?

Taking on your tax code is not an appealing task for even the most dedicated MoneySavers. But it doesn't have to be that gruelling.

Your tax code is listed on your 'coding notice', payslips or P45s. The most important thing to remember is...

Each income source (job, private pension) will have different tax codes. Check 'em all!

Here are the best places to look for your tax code...

Your PAYE coding notice (or P2)

A copy of this is sent to you and your employer around March, just before the start of the tax year. It tells it how to deduct tax, and explains to you how this code was generated.
Your payslip

Perhaps the easiest place to look is on your payslip, which you'll receive from your employer every time you get paid.
Your P45

If you've dumped your payslips (though it's always best to keep them for your records), hunt down a copy of your P45. This is the form your employer gives you when you stop working for it – and the one you give to your new employer when you change jobs.
Your P60

This form is an annual summary of your salary and the tax that's been deducted. Your employer is required to give you this at the end of each tax year.
HM Revenue & Customs

If you can't lay your hands on any of these, you can check your tax code with HMRC online. You'll need to create an account or sign in with your Government Gateway/Gov.uk Verify ID if you already have one.
Tax code for pensions

If you're receiving a private pension, the easiest place to find your code will be on any pension advice slip or on your P60 sent once a year.

What does my code mean?

Tax codes are made up of two main elements, which determine the amount of tax your employer will take. If you work for multiple employers (or work and also draw a pension), you'll have more than one code.

Here's an example of a common tax code for the new tax year (similar ones for past years include 1185L and 1150L). It will usually be made up of numbers and letters:

What do the numbers show?
It indicates the first three or four digits of your tax-free allowance – the amount you can earn in a year before your employer needs to deduct tax. The size of this layer depends on your income and whether there are any deductions (eg, company car) or additions (eg, pension contributions) to this.

You need to add a zero to get the real number, so 1250 means you can earn £12,500 a year tax-free. This is called your personal allowance. Above that, you pay tax on income, though the amount you pay depends on your total earnings.

For the 2020/21 tax year (which runs from 6 April 2020 to 5 April 2021):

You pay 20% tax on the portion of your income between £1 and £37,500 above your personal allowance. This is called basic-rate tax.

You pay 40% tax on the portion of income between £37,501 above your personal allowance and an overall salary of £150,000. This is called higher-rate tax.

You pay 45% tax on the portion of income above £150,000. This is called additional-rate tax.

If you live in Scotland, tax rates are different. For the 2019/20 tax year:

You pay 19% tax on the portion of your income between £1 and £2,085 above your personal allowance. This is called starter-rate tax.

You pay 20% tax on the portion of income between £2,086 and £12,658 above your personal allowance. This is called basic-rate tax.

You pay 21% tax on the portion of income between £12,659 and £30,930 above your personal allowance. This is called intermediate-rate tax.

You pay 41% tax on the portion of income between £30,931 above your personal allowance and an overall salary of £150,000. This is called higher-rate tax.

You pay 46% tax on the portion of income above £150,000. This is called additional-rate tax.
What does the letter show?
This may relate to a number of different factors, and you should use the Tax Code Calculator to check that the definitions are relevant for you.

It usually refers to your age, at what rate employment is being taxed and whether you have any unusual circumstances. Here are a few examples:

L – most common code – you're eligible for standard personal allowance.

BR – whole income taxed at 20% – usually for second jobs/pensions.

NT – no tax to be deducted from this income – often used if you live overseas.

What should my tax code be?

This is where you need to switch your brain on. The important bit to check is correct is the number...

1. Find your personal allowance

The first thing that HMRC does to establish your tax code is to tot up all of your tax allowances – in other words, how much you can earn before you start to pay tax. In many cases, this will just be your personal allowance.
2. Check for any deductions

Any income you haven't paid tax on at source is known as your deductions. The usual suspects are taxable employment benefits or extra income, eg, renting out a property or state pension.

Common taxable benefits include discounted rent or household bills, vehicle use, medical insurance, healthcare cash plans, some travel costs, payment in vouchers, and goods bought on company credit cards.
3. Use these to make the number in your tax code

These deductions are subtracted from the total amount of tax allowances you get (probably your basic personal allowance), and what's left is the total amount of tax-free income you are permitted in each tax year.

HMRC then removes the last digit of this number (so 1250 in the case of the 2020/21 standard £12,500 personal allowance) – and hey presto, you've established the number part of your tax code.

In the majority of cases, these numbers will be followed by a letter. And this letter will vary according to your particular circumstances.

Possible differences in your code

The HMRC slogan that "tax doesn't have to be taxing" is well intentioned, but rarely true. If your code's not the standard 1250L, click on the following statements for an explanation of how your situation differs – it may explain any discrepancies in the code. There's lots of extra info on Gov.uk too.

I've more than one job
The amount of tax you pay is based on your total income for the tax year – whether this is from one, two or more jobs, interest on savings or rental income from a second property that you own. But you will still only have one personal allowance (the amount you can earn before tax) for all of them.

You'll be issued with a separate tax code for each job – and these are likely to be different. For example, if your main income does not take you above the basic 20% rate of tax, you may be on a 1250L tax code for your main job and a BR code for your second job.

Or if you work four days a week in one job and earn £50,000, and have a second job paying £30,000, your tax code could be 1250L for the first job and D0 for the second job because the £30,000 earnings will be taxed entirely at the higher rate of 40%.

It's important to ensure HMRC knows which is your MAIN job (generally the one that pays you most) as if it's the wrong way round, it can cause problems.

Your PAYE coding notice (also known as a P2) should tell you what each tax code is for each job and how it was worked out. If it's unclear, call HMRC on 0300 200 3300 to get some answers.

How to use the calculator with multiple jobs...

Put each code in the calculator separately, ideally starting with your main job, ie, the one that pays you the most.

Your overall personal allowance is the sum of allowances from all the tax codes you have. For example, if you had tax codes 300L and 250L, the tax-free allowance given to you by your tax codes is £5,500.

Note down all the personal allowances given to you by your tax codes, and add them up. If these aren't equal to the allowance that your age and salary predict for you (the calculator lists this), it's worth taking things further.
I've never worked but will be starting soon

If you've just left education and are going into your first job, you should be put on the tax code that reflects your earnings and position. For example, if you qualify for a basic personal allowance of £12,500, you should be put on the standard tax code of 1250L.

However, a code of BR (with no numbers) or something looking like 1250L X, 1250L M1 or 1250L W1 may also be used if you've started a new job, don't have a P45 and haven't completed a starter checklist before your first payday.

Sometimes it can take a month or so for HMRC to get you into its system and on to the right tax code. But any tax you have overpaid should come back to you automatically in your next wage packet. If not, chase up HMRC.
I've taken several years off work

If you took time out from your job to have a family and are going back into work, you should automatically be put on the tax code that reflects your earnings and position. For example, if you qualify for a basic personal allowance of £12,500, you should be put on the standard tax code of 1250L.

However, a code of BR (with no numbers) or something looking like 1250L X, 1250L M1 or 1250L W1 may also be used if you've started a new job and haven't completed a starter checklist before your first payday. Even if you can lay your hands on it, your last P45 won't be any good to you now as it is only valid for the current tax year.
I've not been living in the UK

If you've been working overseas and paying tax to a different country, you should be put on the tax code that reflects your new earnings and position as soon as you return to the UK and start working again. For example, if you qualify for a basic personal allowance of £12,500, you should be put on the standard tax code of 1250L.

However, a code of BR (with no numbers) or something looking like 1250L X, 1250L M1 or 1250L W1 may also be used if you've started a new job and haven't completed a starter checklist before your first payday.

But if there are still tax issues hanging over from the country you had previously been working in, you may find you are put on a T code, which indicates that your tax position is not settled and will need to be reviewed regularly by HMRC.
I have employee benefits, eg, a company car or healthcare insurance

This is where things can get complicated. The letter after your numbers – which will be your total personal allowance with the last digit removed – could be an L or a T.

Which it is depends on the value of the taxable benefits, such as company cars, dental care, private healthcare, mobile phones, vouchers or laptops.

If you're a higher-rate taxpayer (earning more than £37,500, or more than £30,930 in Scotland, above your personal allowance in 2019/20) or an additional-rate taxpayer (earning over £150,000) and have the perks to match, you are likely to be on the T code, denoting that your tax affairs need regular review.
I'm receiving universal credit, tax credits or child benefit

Claiming any of these WON'T make any difference to your tax code.

Universal credit and working tax credits are classified as 'means-tested' benefits, so are not deducted from your individual personal allowance. See our full Universal Credit and Tax Credits guides.

Child benefit was withdrawn from some higher earners in January 2013, with people earning between £50,000 and £60,000 a year only remaining eligible for a portion of it, and those earning £60,000 or above not eligible at all.

If this is you, you can still claim child benefit, though it must be repaid at the end of the year. You can also choose to pay through your tax code, which will lower your personal allowance. You'll also have to fill in a self-assessment tax return.
I'm retiring or retired

If you're eligible for the full personal allowance (£12,500 in 2020/21) and were born between 6 April 1938 and 5 April 1948, you used to have a boosted allowance, and thus the letter P was tagged on to the end of the numbers in your tax code. However, as the personal allowance for everyone is now £12,500, it's likely your tax code will be 1250L.

If born before 6 April 1938, you also used to have a boosted personal allowance, with the letter Y tagged on to the end of the numbers in your tax code. But again, as the personal allowance for everyone is now £12,500, it's likely your personal allowance will be 1250L.

The last tax year that a P code was used was 2014/15, and the last tax year a Y code was used was 2015/16.

Income from state pensions will also be subtracted from your personal allowance. If your total taxable deductions exceed the personal allowance, you'll have a negative coding – where a K is used.
I earn more than £100k

High earners lose some of their personal allowance – for every £2 you earn above £100,000 a year, your personal allowance decreases by £1. An example may help...

Anna earns £110,000 a year. As she earns £10,000 over the £100k threshold, her personal allowance decreases by £5,000. Her new personal allowance is £7,500.

Because the number in your tax code usually depends on your personal allowance, earning over £100k generally means that this number changes.

Assuming Anna has a standard L letter in her code, and no other changes to her personal allowance, her code would be 750L.

Think your tax code's wrong?

Now you understand what your tax code means, you'll be able to assess whether it's likely to be correct for your earnings, age and situation. If your tax code doesn't look right, it probably isn't.

The onus is on you to get on the right tax code. We now have two possible scenarios, and one is much more fun than the other.

But don't panic...

If you have overpaid tax, the money will come back to you.
And if you have underpaid, not only do debtors' prisons no longer exist, you probably won't have to pay it back all in one go either.

I've overpaid – how do I get my cash back?

If your tax code is wrong...

The first thing to do is to tell HMRC you think your tax code may be wrong, and why.

It's probably simplest to call HMRC on 0300 200 3300 so you can resolve your situation with a human and ask questions along the way. Alternatively, you can contact HMRC online via your personal tax account to let it know your tax code is wrong – you'll need to log in/set up an account using your Government Gateway or Gov.uk Verify ID.

If HMRC agrees you've overpaid tax it will only ever inform you of a refund by post. This is crucial to remember following a spate of email and phone scams, so never discuss this with anyone over the phone, or via email.

So usually it simply means identifying the overpayment and informing HMRC is the key way to get any overpaid cash back.

How and when will I be repaid?

This depends on the tax year your claim refers to. If it's the current tax year and you're paying too much tax as a result of your tax code right now, HMRC will inform your employer, the tax code will be amended and the overdue tax will be refunded to you via your wages.

After the end of the tax year, HMRC will send you a P800 (or in some cases a 'simple assessment' letter). If you're self-employed or if the tax refund refers to previous tax years, you'll be able to claim your refund online (your P800 will tell you if you can) or HMRC will send a cheque in the post.

In some cases, HMRC will pay a paltry rate of interest on any tax you have overpaid – since September 2009, you earn 0.5% on overpaid tax.

How far back can I claim?

You can claim back up to four years of overpaid tax, if the problem's been going on that long. Here are all the time limits for claiming a refund:

Tax year (ends 5 April)	Deadline for claiming
2016/17	5 April 2021
2017/18	5 April 2022
2018/19	5 April 2023
2019/20	5 April 2024
2020/21	5 April 2025

However, even if the deadline has passed for the tax year in question, don't let this put you off getting back what's yours. In certain circumstances – including when HMRC is at fault – your claim will be considered. So fight your corner.

I've underpaid – what happens now?
In most cases, you'll have to pay it back – generally HMRC can only go back four tax years, but this extends to six if you acted carelessly and 20 if you acted deliberately.

How you go about paying what you owe depends on the amounts involved and how HMRC has dealt with your case – though it's always important to check you agree with HMRC about how much tax you've underpaid.

But it wasn't my fault I'm on the wrong code!

The official line from HMRC is that it is each individual's responsibility to check they are on the right tax code.

However, if your code was wrong there are some situations where you might not have to pay the tax bill. This could include:

If your employer made a mistake and put you on the wrong tax code despite being sent the correct one by HMRC. In that case, HMRC should try first to recover the tax from your employer.

If HMRC reviews your tax and finds an underpayment of £50 or less in the last year, it'll write off the tax.

If the underpayment was made in a tax year ending more than a year ago, you may be able to challenge via the not-so-catchy name of an extra-statutory concession – or an A19 (read more about this on Gov.uk and below).

See the Low Incomes Tax Reform Group's guides on underpaid tax for more.

How to reject underpayments more than a year old

Using an A19 – a little-known clause which lurks deep in HMRC's complex book of rules – ISN'T guaranteed, and in fact it's far from likely to work. But there is a possibility, so it may be worth giving it a go.
To even think about going down this route, you must strictly fulfil the following criteria:

The underpayment is more than 12 months old.

HMRC was given the CORRECT information.

Lastly, you will need to demonstrate a 'reasonable belief' that your tax affairs were in order, which can become pretty tricky – if you suddenly started receiving loads more pay, it can be argued you should have spotted it!

This means that at present, if you underpaid in the 2018/19 tax year or earlier you may be able to go the A19 route (from 6 April 2021, you'll be able to claim for the 2019/20 tax year too).

You will also need to have all your paperwork and dates in perfect order.

However, if you feel as though injustice has been done, you can overcome these hurdles and apply for an exemption under an A19 simply by calling or writing to HMRC.
You can download a template letter here to help if you plan to write to HMRC.

With thanks to the Low Incomes Tax Reform Group.

This is definitely worth a go if you qualify, though remember – it's far from a sure thing. But we have received emails telling us of successes – one MSE user had a £1,700 repayment written off after HMRC didn't follow procedures properly.

I am delighted to tell you that my underpayment (£1,700) has been written off under the A19 ruling. The underpayment HMRC had already started deducting from my pension has been refunded.

A19 doesn't apply to me – I have to repay. How do I do that?

If you do owe the tax HMRC claims you do, you will have to repay. But the good thing is you don't have to repay it all at once. For smaller amounts, you can pay through your tax code over the next year. And for larger amounts, you can agree a repayment plan with HMRC. We've answered some common questions about tax underpayment here:

Quick questions
Will I have to pay it all in one go?

Any underpayments for the tax year immediately before won't be requested if they are under £50 (you likely won't even be aware of this, as HMRC won't contact you if you owe less than £50).

If you do need to make a payment, you won't have to pay it all back in one go – you can call HMRC and arrange a payment plan instead. This usually involves it taking instalments from your earnings through your tax code.

And you'd better brace yourself – because this will mean your tax code will have to be altered accordingly, meaning higher bills in future.

How do I repay it?

That will depend on the amount you owe...

If less than £3,000, HMRC may simply adjust your tax code at the start of the new tax year to claw back the sum you owe. For example, if you had underpaid £300 during the last tax year, you would need to be taxed on an additional £1,500 of income (if you paid basic-rate tax of 20%) over the next tax year to repay this sum.

This means £1,500 would need to come off your personal allowance for the next tax year. Assuming you have the standard tax code of 1250L, it would be adjusted to read 1100L (12,500 minus 1,500 with the last digit removed).

If that happens, ensure you do a full budget (see the free budget planner) to incorporate your decrease in disposable income.

If more than £3,000, HMRC will send you a bill regardless of the point in the tax year and give you 30 days in which to cough up. If you cannot make the payment, don't worry – you can call HMRC and arrange a repayment schedule.

In cases where the underpayment has been quite serious – for example, you have a couple of rental properties you have not declared income on – you may be required to enter the self-assessment system and may have to pay interest and fines.

Will I have to pay interest and charges on what's owed?

HMRC has a right to ask for backdated interest on the tax owed to put itself back in the 'commercial position it would be in' had you paid on time. However, if your tax shortfall is a one-off through a tax code error, you will probably be let off.

Can I challenge the deadline HMRC sets me?

For most, there are no 'challenges' allowed against payment deadlines when it comes to the taxman – just consequences of missing them.

For more information, read HMRC's 'Time to Pay' guidance and what to do if you're in difficulties.

What if I just can't afford to pay it back?

In this case, you should inform HMRC of your hardship (and be able to back it up).

In some extreme cases, the underpayment could be wiped. One example might be if your underpayment came from a past salary, and you're now retired, earning only the state pension. It's unlikely for most people, but worth discussing – see the tax help agencies at the end of this guide.

People have used this guide to reclaim up to £11,500

Since launching this guide in late 2010, we've heard of many MoneySavers who reclaimed money after realising they were on the wrong code. Here are two for inspiration...

I want to thank you from the bottom of my heart. You recently stressed the importance of checking your tax code. I had never done this before, but you gave me the confidence to do it.

After I was recently made redundant, I got my P45 and phoned HMRC. I said 'Look, can you just check this for me?' They checked a couple of things, said 'Can you stay on the line?', then said 'We owe you £11,486 – how would you like it paid?' The cheque arrived 10 days later.

I had just trusted the amount of money HMRC had taken off my redundancy pay – then my instinct kicked in after your warnings. This success has allowed us to clear our debts and plan for the future. I am so grateful. I just want one person who reads this to pick up the phone to HMRC and have the belief to do it.

Tina, via email

Following your guidance, I checked my tax code and found that I have been overpaying my personal tax. I've now received confirmation of my tax refund for £2,240. Fantastic result.

Morag, via email

Please report successes/failures getting money back in the Tax Code Calculator successes forum discussion.

Keep an eye on future codes

With any luck, once you have established the right tax code, it'll all be sorted with HMRC from then on, unravelling the web of complexity surrounding tax codes and what you ultimately should be paying. Yet don't bank on it...

Small changes can change your code

Every time your circumstances change – whether it's a promotion at work with a larger salary, new employee benefits, taking on another job, giving up work to have children or leaving the country and the UK tax system altogether – your tax code may change, so it can be worth getting in touch with HMRC to establish your new tax code.

A phone call today can save an awful lot of hassle and expense in future...

Get free tax help

This guide provides general information about tax codes to help you see if you're on the right track. But it's no substitute for personal advice if you need it – and you should always take care to ensure you're definite about any actions you're taking.

The following organisations all give help and advice and some don't charge a fee, so give them a try if you're struggling:

Tax Aid: Lots of info is available here, but TaxAid can only help those on a low income who can't resolve their own issues with HMRC.

Citizens Advice: Visit Citizens Advice's website to get the number of your local Citizens Advice bureau.

Low Incomes Tax Reform Group: This is an initiative from the Chartered Institute of Taxation, aimed at those with low incomes who have tax problems. Visit LITRG.

Tax Help for Older People: If aged over 60, you can try Tax Help for Older People online or by calling 01308 488066.

Find a tax adviser: If you can afford it and have more complex affairs, you can use the Chartered Institute of Taxation's 'Find a Tax Adviser' search.