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The P800 letter: HMRC's tax calculation, explained

HMRC's calculation of your finished tax year, showing a refund or an underpayment.

Last reviewed 2 September 2026 · How Tax Letters checks its facts

The short answer

What it asks
Usually nothing except reading it. A refund P800 tells you how to claim, online or by cheque. An underpayment P800 tells you how HMRC intends to collect, most often automatically through next year's tax code, and only asks you to act if the figures look wrong or a different way of paying suits you better.
Who gets it
People taxed through PAYE whose year did not add up exactly: employees and pensioners whose employer, pension or bank figures, once HMRC reconciled the year, show too much or too little tax was taken.
Is the deadline legally binding?
Where this letter gives a reply date, the letter sets it, not the law. The deadline section below explains what that means and what ignoring it can cost.
Your options at a glance
Claim the refund, if it shows one · Check the calculation against your records · Query a figure with HMRC

Sources: GOV.UK: Tax overpayments and underpayments (checked 2 Sept 2026) · All 2 sources and dates

What the deadline means

Reply window: set by the letter, not by law

A P800 sets no reply deadline: it reports the result of HMRC's check of your PAYE year, and refunds can be claimed as soon as it arrives.

Nothing fines you for reading a P800 slowly: the letter itself sets no deadline, and most underpayments are collected automatically through your tax code without you doing anything. For refunds, the letter says how and when to claim.

The figures only get checked if you check them. A wrong P800 quietly becomes a wrong refund or a wrong code, so comparing it against your P60 and interest statements is the one job the letter really asks of you.

Source: GOV.UK: Tax overpayments and underpayments

Why you got it

After each tax year HMRC compares the tax collected through your codes with the tax the reported figures say was due. Employers, pension providers and banks all report to HMRC, and when the reconciliation finds a gap either way, a P800 sets out the calculation.

Holding the letter now? Paste it into the checker for a plain-English identification.

Your options, honestly

  1. Claim the refund, if it shows one

    The letter explains the claim route, normally online through your personal tax account with the money paid into your bank account. Whether an unclaimed refund is later posted as a cheque has changed over the years, so treat the letter's own words as the rule. Claiming online is faster and free; no company is needed to do it for you.

  2. Check the calculation against your records

    Put the letter beside your P60 or P45, pension statements and bank interest figures and compare line by line. Tax Letters' letter checker identifies the letter and explains it, and the deadlines page explains what each date on it means.

  3. Query a figure with HMRC

    If a line does not match your documents, contact HMRC through the route on GOV.UK with the letter and your document in front of you. HMRC checks its side and either corrects the calculation or explains where its figure came from.

A P800 is not a penalty and does not start one. It is arithmetic about a finished year. Penalties live in different letters entirely, and nothing about a P800 changes by being read a week later.

You may owe nothing

Plenty of P800s are simply correct, and some exist because HMRC owes you money. If the lines match your documents, the letter needed nothing from you beyond the read, and a refund claim if one is shown.

If you do nothing

An unclaimed refund sits with HMRC until you claim it; whether one is eventually posted as a cheque depends on the year's process, and the letter's own words are the rule. An underpayment P800 is normally followed by collection through your tax code, or by a Simple Assessment letter that does carry a statutory query window. Ignoring the arithmetic does not make it wrong or right; it just means nobody checked it.

The letters themselves say it: if your health or personal circumstances make dealing with this difficult, you can tell HMRC and they can give you extra support. That is a standing HMRC service, and using it is normal.

Already past the date in your letter? Read this before anything else.

Questions worth asking an adviser

  • Does the employment and pension income on the P800 match my P60s and P45s exactly, and if not, whose figure is wrong?
  • The untaxed interest line: which accounts is HMRC counting, and does it match my own statements?
  • If I owe tax, is collection through next year's code the cheapest and simplest way for me to pay it?

Quick answers

What is a P800 letter?

A P800 is HMRC's tax calculation for a finished tax year. It compares the tax taken through PAYE with the tax due on the income reported to HMRC, and shows either a refund or an underpayment.

Is there a deadline to respond to a P800?

The letter itself sets no reply deadline. Refunds can be claimed as soon as it arrives, and most underpayments are collected through a later tax code. Any date printed on your own letter is the one that counts.

How do I claim a P800 refund?

Through the route printed on the letter, normally online via your personal tax account with payment to your bank account. The letter states what happens to an unclaimed refund. The claim is free and needs no company acting for you.

Should I just trust the P800 figures?

Check them. The calculation uses figures reported by employers, pension providers and banks, and any of those can be wrong or incomplete. Compare each line against your P60, P45, pension statements and interest figures before treating the result as settled.

Want a date in your calendar?

This letter sets no reply deadline. If you plan to claim or query, pick a date and add it to your own calendar.

The calendar file is made in your browser. Nothing is sent to Tax Letters.

Want a professional to look at it?

Some letters are worth handing to a specialist, and knowing when is half the value of a check. An adviser will treat what you say in confidence and will not judge how it got here. If we cannot point you anywhere useful, we will say so.

Where to get help

You can deal with HMRC directly, free, without any adviser. If money is tight: TaxAid (low incomes, older people included) and Citizens Advice help for free.

If Tax Letters ever introduces you to a firm, the firm may pay for the introduction. You never pay Tax Letters for it, and it never changes what these pages say. How Tax Letters is funded.