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UK Self Assessment Late-Filing Penalty Calculator

2025-26 (UK) figures · Last reviewed by the US UK Tax Accountants tax team

Enter how late the return is and what tax remains unpaid. The calculator stacks HMRC's fixed, daily and percentage penalties exactly as the rules do — so you can see what waiting costs, and what acting today saves.

Your details

Counted from the filing deadline — 31 January for online returns.

days

The Self Assessment bill for the year. Enter 0 if nothing is owed.

£

Your result

Total penalties so far
£100
Late-filing penalties
£100
Late-payment penalties
£0
  • Interest runs on unpaid tax and unpaid penalties on top of these figures, at HMRC's current rate.
  • Penalties apply even when no tax is due — the £100 and daily charges do not depend on the bill.
  • A reasonable excuse — serious illness, bereavement, HMRC system failure — can cancel penalties on appeal.

This is an estimate for guidance only, not advice. Get your numbers checked by a specialist before acting on them.

Get this checked by a specialist
Assumptions behind this calculator
  • Figures follow HMRC's standard Self Assessment penalty ladder; interest on unpaid tax and penalties comes on top and is not included.
  • The daily £10 charge is modelled from day 91 for up to 90 days, per the standard rules.
  • Six- and twelve-month penalties use the greater of £300 or 5% of the tax due.
  • Late-payment penalties assume the tax stayed unpaid for the same period as the return.
  • A successful reasonable-excuse appeal can cancel penalties entirely — the calculator shows the default position.
  • Figures are based on 2025-26 (UK) rates from GOV.UK — Self Assessment penalties, GOV.UK — Self Assessment deadlines.

How HMRC's penalty ladder works

Self Assessment penalties stack in stages rather than arriving all at once. Miss the deadline by a day and £100 lands automatically, whether or not you owe tax. From three months late, £10 a day starts running for up to 90 days. At six months, and again at twelve, HMRC adds the greater of £300 or 5% of the tax due.

Late payment runs on its own parallel track. Tax unpaid 30 days after the deadline picks up a 5% charge, with further 5% charges at six and twelve months. Interest then accrues on everything — tax and penalties alike — which is why old returns grow so much faster than people expect.

Why does the £100 apply when I owe nothing?

Because the fixed and daily penalties punish the late return itself, not the unpaid bill. A nil return filed a year late can still carry £1,600 of filing penalties. Only the percentage-based charges scale with the tax due, which surprises almost everyone the first time.

What should you do right now?

File, even before you can pay. Filing stops the daily clock and the later filing penalties, while payment plans can handle the bill itself. If the lateness had a genuine cause — serious illness, bereavement, HMRC system failure — appeal with a reasonable excuse, because these appeals regularly succeed.

New to the system entirely? The step before any of this is getting registered — our guide to registering for Self Assessment covers the 5 October deadline that keeps you off this page altogether. And when a late UK return sits alongside missed US filings, sequence matters: our UK Self Assessment service handles both sides in one pass. Ask us about your position — the first conversation is free.

Last reviewed 8 September 2026 by the US UK Tax Accountants Tax Team. This tool is general information, not personal tax advice — speak to a qualified US/UK tax adviser about your own position.

This page is general information, not personal tax advice. Cross-border positions turn on individual facts — speak to a qualified US/UK tax adviser before acting. Ask us a question.

Related reading
Register for Self Assessment by 5 October — or invite penalties you never owedThe American in the UK: what you actually have to file

Questions, Answered.

Common questions about this tool

Is the £100 penalty automatic?
Yes. It applies the day after the deadline passes, regardless of whether any tax is due, and HMRC's systems issue it without human review. The only routes out are a reasonable-excuse appeal or showing the return was not actually required for that year.
When do the £10 daily penalties start?
Once the return is three months late, and they run for up to 90 days — a maximum of £900 on top of the initial £100. Filing the return stops the daily clock immediately, which is why filing now beats waiting until you can also pay.
Do these penalties apply if HMRC owes me a refund?
The fixed £100 and daily charges can still apply to a late return even in refund years, because they attach to the filing, not the bill. The percentage penalties fall to zero when no tax is due. Filing promptly protects the refund and stops the fixed charges growing.
What counts as a reasonable excuse?
Something unexpected and outside your control that genuinely prevented filing: serious illness, the death of a close relative near the deadline, or HMRC's own systems failing. Running out of time, finding the form difficult, or relying on a friend generally fail. Appeal quickly once the obstacle passes.
Can I appeal after paying a penalty?
Yes. Paying does not concede the point, and appealing within 30 days of the penalty notice keeps every option open. Set out the excuse, the dates it applied, and evidence where it exists. HMRC cancels a meaningful share of penalties at the first review stage.
What about late paper returns?
Paper returns carry an earlier deadline — 31 October instead of 31 January — so the same ladder starts three months sooner. Almost everyone is better off switching to online filing, which buys the later deadline and instant submission receipts.