Estimated P800 refund or underpayment.
Outcome
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Tax actually due
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Your breakdown
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What a P800 is telling you
A P800 is HMRC's end-of-year reconciliation for people taxed through PAYE. After the tax year closes on 5 April, HMRC compares the Income Tax you actually paid against the tax that was genuinely due on your income, and if the two do not match it issues a P800 showing a refund or an underpayment. This tool runs the same comparison in advance. You enter your total taxable income and the Income Tax already deducted, and it works out the tax that should have been paid, then the difference. It is built for employees and pensioners who suspect their PAYE code was wrong, not for Self Assessment filers who reconcile through their return instead.
Working a refund on £40,000
Take the defaults: £40,000 of taxable income with £6,500 of tax deducted. The correct liability uses the £12,570 personal allowance, then 20% on the rest up to the basic-rate limit. Income of £40,000 leaves £27,430 taxable, all within the basic band, so the tax due is £5,486. You paid £6,500, so HMRC owes you the difference.
An overpayment of £1,014 like this is typical after an emergency tax code or a mid-year job change, where too much was deducted before the system caught up. The chart sets the tax paid against the tax due so you can see the overpaid slice at a glance.
Why your PAYE so often misses
PAYE is designed to get you close across a full year of steady employment, but several things throw it off. Changing jobs can leave you on an emergency code that ignores your allowance. Two jobs at once can apply the personal allowance twice or not at all. A taxable benefit reported late distorts the code. Unclaimed reliefs, such as pension contributions made from net pay or professional subscriptions, mean too much was collected. This tool also handles the high-earner case: above £100,000 the personal allowance tapers by £1 for every £2 of income and disappears entirely by £125,140, which is exactly where coding errors balloon. Enter an income above £100,000 and the calculator reduces the allowance automatically.
Claiming, and a deliberate simplification
If HMRC owes you, the P800 will either invite you to claim online, with the money usually arriving in about five working days, or it will post a cheque within a couple of weeks. If you owe, HMRC normally collects it by adjusting next year's tax code rather than demanding a lump sum, unless the amount is large. One honest caveat about this tool: it estimates Income Tax only and ignores National Insurance, which is calculated separately and is not part of a P800 reconciliation. It also assumes all your income is taxed the same way as employment, so it will not perfectly match a case mixing dividends or savings interest, which carry their own allowances and rates. A practical tip: you have four years from the end of the tax year to reclaim overpaid tax, so an old emergency-code overpayment you never noticed may still be recoverable.
Things people want to know
How does Scotland affect my P800?
Scottish taxpayers are reconciled against the Scottish rates and bands, which include a 19% starter rate, a 21% intermediate rate, and higher and advanced rates that begin at lower incomes than in the rest of the UK. This calculator uses the England, Wales, and Northern Ireland bands, so a Scottish taxpayer should treat its output as indicative only and check the figure against the Scottish bands.
I never received a P800. Should I have?
Not everyone gets one. HMRC only issues a P800 where its automatic reconciliation finds a mismatch, and Self Assessment taxpayers are excluded because they settle through their return. If you believe you overpaid but no P800 arrived, you can prompt a review through your personal tax account rather than waiting for the post.