What Is a P800? HMRC Tax Calculation Letter Explained

A P800 lands with no warning. One morning it is just there on the doormat, bearing HMRC's logo, and most people's first instinct is either to worry or to ignore it. Neither is quite right. Each year HMRC issues millions of P800s, and roughly half of them result in a refund rather than a demand for more money.

Why Does HMRC Send a P800 in the First Place?

The PAYE system is designed to collect the right amount of tax automatically, but it relies on your employer knowing your full financial picture — and it often does not. HMRC sends a P800 when, after the tax year closes on 5 April, it reconciles data from your employer, pension provider, DWP and its own records and finds a discrepancy.

Common triggers include:

• Changing jobs mid-year, especially if an emergency tax code (such as BR or 0T) was applied at the new employer.
• Two or more sources of PAYE income at the same time — for example, a main job and a part-time role, where both applied a full Personal Allowance.
• Starting or stopping a company pension partway through the year.
• Untaxed interest on savings or state benefits (such as the State Pension) that push total income above your coding notice allowances.
• Claiming Marriage Allowance or other reliefs that were processed after payroll had already run for much of the year.

Overpayment vs Underpayment: What Each Means for You

If Your P800 Shows an Overpayment

This is the scenario you want. The letter will confirm the amount HMRC owes you — this is what most people call a tax rebate. If HMRC already has your bank details on file, you may receive a notification that the refund will land within five working days. If not, you can claim online at the government's repayment portal (usually a link is shown on the letter itself) or request a cheque.

Beware: third-party claims management companies routinely advertise alongside P800-related searches and will take a significant cut (sometimes 30-50% plus VAT) of a refund you could claim yourself for free in under five minutes.

If Your P800 Shows an Underpayment

This is the scenario most people dread, but HMRC's standard approach is remarkably painless. For underpayments below £3,000, the amount is collected by adjusting your PAYE tax code for the next tax year — spreading the repayment across twelve months' payroll deductions. You will not receive a separate payment demand.

For underpayments above £3,000, HMRC may contact you separately to arrange repayment.

A Worked Example: The Two-Job Overlap

Suppose Sarah is a nurse who earns £28,000 from her NHS trust. In November 2024 she picks up a second role at a private clinic, earning £6,000 for the remainder of the tax year. Her main employer correctly applies her full Personal Allowance of £12,570 against her NHS salary. Her second employer, lacking a tax code instruction in time, uses a basic-rate BR code and taxes the entire £6,000 at 20%, deducting £1,200.

At year-end, Sarah's combined income is £34,000. After her single Personal Allowance of £12,570, taxable income is £21,430. Tax due: £21,430 x 20% = £4,286.

Tax actually paid: £3,086 (NHS) + £1,200 (clinic) = £4,286. In this case the numbers balance perfectly. But if the clinic had instead applied a 0T code (taxing from the very first pound at 20%), Sarah would have paid the same £1,200 and the result would be identical. The mismatch more typically arises when the Personal Allowance is accidentally applied twice.

Had a second Personal Allowance been applied by the clinic, Sarah would have paid £0 on the first £12,570 from that job, meaning she underpaid by £2,514. A P800 would land in autumn 2025, and HMRC would collect that £2,514 through her 2025/26 tax code.

What to Do When Your P800 Arrives

1. Check the figures against your own records. Compare the income figures on the P800 against your P60 (end-of-year certificate) and any P45s from employers you left during the year. Errors in HMRC's data do happen.
2. Verify your tax code was correct throughout the year. You can check your tax code to see what HMRC had on file and whether any adjustments are needed going forward.
3. Claim your refund promptly if one is due. You generally have four years from the end of the tax year to claim a P800 refund. Leaving it risks missing the window.
4. Do not ignore an underpayment notice. If HMRC's figures look correct, the code adjustment will happen automatically. If you believe the underpayment is not your fault (for example, it arose from an HMRC administrative error), you can apply for Extra-Statutory Concession A19, which can write off the debt in certain circumstances.

Does a P800 Affect Your Self Assessment Position?

Sole traders and the self-employed file their own returns through Self Assessment and will never receive a P800 for their trading income — their reconciliation happens through their annual tax return. However, if a sole trader also has a PAYE job or pension income alongside their self-employment, any under or overpayment on the PAYE side of their affairs could still prompt a P800 for that element alone. In practice, HMRC usually folds PAYE discrepancies into the Self Assessment calculation instead, so a sole trader receiving a P800 is unusual but not impossible.

P800 vs P60: What Is the Difference?

These two documents confuse a lot of people. Your P60 is issued by your employer every April and shows exactly what you earned and what tax was deducted in that employment during the year. A P800 is issued by HMRC and looks across all your income sources to check whether the total tax collected was right. Your P60 feeds into HMRC's calculation; the P800 is HMRC's verdict on the outcome. You need both if you want to verify HMRC's arithmetic yourself.