Late Self Assessment penalty estimator

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Rules used (GOV.UK: penalties, checked 17 October 2026). Late filing: £100; after 3 months £10 a day up to £900; after 6 months 5% of the tax due or £300, whichever is greater; after 12 months the same again. Late payment: 5% of the tax unpaid at 30 days, 6 months and 12 months, plus interest.
This is a rough estimate. GOV.UK’s official estimator is the one to use. Like it, this tool ignores earlier penalties, credits and reasonable-excuse relief.

Interest rate: HMRC changes it, so we do not hard-code one. Look up the current late-payment rate on GOV.UK. Leave blank to skip interest.

Uncertain points

Date arithmetic is our reading of GOV.UK and HMRC manuals (SAM61390): penalty date is the day after the deadline; daily penalties start 3 months after it (about 1 May 2027 for a 31 January 2027 deadline) and are counted inclusively; the first payment penalty applies to tax still unpaid after day 30 (2 March 2027), the second 5 months and the third 11 months later. Confirm with HMRC’s estimator.
The 5% filing penalty is applied to the “tax due” you enter. Failure-to-notify penalties, payments on account and special cases (trusts, partnerships, late registration) are not modelled.

Sources

Last verified: 17 October 2026.