Tax · United Kingdom
Tax calculation (P800) — HMRC says you paid too much or too little tax
A P800 is a letter from HMRC saying it has recalculated your Income Tax for a past tax year and found you paid too much or too little through PAYE. It is not a bill in itself: it either announces a refund or explains how underpaid tax will be collected.
How serious: routineAlso called: P800, Tax calculation letter, P800 refund letter
A P800 tells you HMRC has looked again at your Income Tax for a past tax year. The recalculation found you paid either too much or too little through PAYE, the system that takes tax from wages and pensions. The letter shows the tax year, your income, the tax paid, and a line saying whether you are owed a refund or owe tax.
Common triggers are being on the wrong tax code, finishing one job and starting another and being paid by both in the same month, or starting a workplace pension. HMRC sends these letters between June and March of the following tax year.
The letter is not a demand in itself. Where you overpaid, it announces a refund and explains how to get it. Where you underpaid, it explains how HMRC intends to collect the difference — usually through your tax code rather than as a separate payment.
Who sends it
HM Revenue and Customs (HMRC), the UK's tax authority. HMRC collects Income Tax and other taxes and holds the PAYE records the calculation is based on.
The letter is headed 'Tax calculation' with the form reference P800 and comes from HM Revenue and Customs. It shows a tax year, your income, the tax paid, and a line saying you are owed a refund or owe tax. Refund letters carry a reference number used to claim online.
The deadline
There is no fixed response deadline on a P800. Where the letter says a refund cheque is coming, it arrives automatically within 14 days of the date on the letter. For an underpayment, the letter itself explains when and how the tax will be collected.
If you do nothing
For a refund: if the letter says HMRC will send a cheque, no claim is needed — 'you'll automatically get the cheque by post' within 14 days. For an underpayment: 'HMRC will usually collect the tax you owe by changing your tax code', so more tax is deducted from wages or pension, 'usually in equal instalments for 12 months from the start of the following tax year'. This applies if you have PAYE income, earn enough above the Personal Allowance, and owe less than £3,000. If HMRC cannot collect this way, 'HMRC will write to you about how you can pay'.
How it escalates
- 01Refund case: nothing is lost immediately. Where the letter says so, a cheque is sent automatically within 14 days of the letter date; refunds claimed online arrive within 5 working days.
- 02Underpayment case: HMRC changes your tax code and collects the amount automatically from your wages or pension in instalments over 12 months from the start of the following tax year.
- 03If automatic collection through your tax code is not possible, HMRC writes to you again to explain payment options.
Common questions
Is a P800 a bill?
No. It is a calculation. It either announces a refund or explains how an underpayment will be collected — usually through your tax code, not as a separate payment.
How do I get the refund?
If the letter says a cheque is coming, it arrives by post within 14 days of the letter date. Otherwise the refund can be claimed online with the reference number on the letter and your National Insurance number; online claims arrive within 5 working days.
How does HMRC collect tax I underpaid?
Usually by changing your tax code so that more tax comes out of your wages or pension, in equal instalments over 12 months from the start of the following tax year. If that is not possible, HMRC writes to you about how to pay.
What if the figures look wrong?
The calculation can be queried with HMRC by phone.
Sources
Facts verified against the sources below on 28 August 2026.