Self Assessment Late Filing & Late Payment Penalty Calculator

Two clocks, two sets of penalties, plus interest on top. The 2025/26 tax return is due online by 31 January 2027 (31 October 2026 on paper) — and the balancing payment is due 31 January 2027 either way. Miss the filing deadline and HMRC charges a filing penalty that grows the longer the return stays outstanding; miss the payment deadline and a separate late-payment penalty and daily interest both start running on the unpaid tax. This calculator works out both penalty tracks and the interest, from the dates you enter. Self Assessment deadlines and penalties are set by HMRC and apply the same way across England, Scotland, Wales and Northern Ireland — unlike the tax itself, where Scottish income tax has its own bands, filing and penalty administration is not devolved.

Late filing: how the penalty builds

• Immediately late: a flat £100 penalty, even if there's no tax to pay.
• More than 3 months late: daily penalties of £10 a day, capped at 90 days (£900) — on top of the £100.
• More than 6 months late: a further 5% of the tax due, or £300, whichever is greater.
• More than 12 months late: another 5% or £300, whichever is greater — in the standard case. Where HMRC can show the taxpayer deliberately withheld the information needed to assess the tax, the law allows a much higher 12-month penalty instead: up to 70% of the tax liability if deliberate but not concealed, or up to 100% if deliberate and concealed (Finance Act 2009, Sch 55, para 6). This calculator always uses the standard 5%-or-£300 figure — HMRC decides deliberateness on the facts of the case, not from a date calculation.

Late payment: a separate penalty, charged on the tax itself

Whether the return was filed on time or not, unpaid tax after the 31 January payment deadline attracts its own penalty: 5% of the tax still unpaid at each of three points — 30 days late, 6 months late, and 12 months late — up to three separate 5% charges if the tax stays unpaid that long. This is the penalty regime that currently applies to every Self Assessment return; the newer points-based penalty system is being phased in for Making Tax Digital quarterly updates first, and HMRC's Autumn Budget 2025 announcement confirmed it will extend to all Self Assessment taxpayers only from the 2027/28 tax year — so it doesn't touch the 2025/26 return this calculator covers.

Interest runs on top of both

Separately from either penalty, HMRC charges daily interest on tax paid late, from the day after the payment deadline until the day it's paid. The rate is set in law at the Bank of England base rate plus a fixed premium — 4 percentage points since 6 April 2025 (it was 2.5 points before that) — and moves whenever the base rate does. The rate in force is 7.75% a year, effective from 9 January 2026. This calculator applies that current rate as simple daily interest across the whole period you enter; HMRC compounds daily and the rate itself can change mid-period, so treat the figure as a close estimate, not a bill.

Appealing a penalty

A penalty can be cancelled or reduced if there's a reasonable excuse — HMRC's examples include a partner's death shortly before the deadline, an unexpected stay in hospital, or a failure of HMRC's own online service. The appeal goes in on form SA370, through the HMRC online account, or by phone, within 30 days of the date on the penalty notice — not 30 days from when the letter was opened. "I forgot" or "I couldn't afford it" are not accepted as reasonable excuses on their own.

Where this fits with the rest of the return

This tool only covers the return itself. If the tax owed includes payments on account, the payments on account calculator works out the 31 January and 31 July split. If the return is disclosing rental income HMRC didn't already know about, the penalties are different — see the undeclared rental income guide and the Let Property Campaign penalty calculator. And the Self Assessment deadlines page lays out every date in the year, not just the filing and payment ones used here.

Deadlines and penalties: gov.uk "Self Assessment tax returns" (deadlines and penalties pages), read 13 Sep 2026. Deliberate-withholding 12-month penalty: Finance Act 2009, Sch 55, para 6, legislation.gov.uk, read 13 Sep 2026. Interest rate: gov.uk "Rates and allowances: HMRC interest rates for late and early payments", read 13 Sep 2026 — 7.75% from 9 January 2026, base rate plus 4 percentage points since 6 April 2025. MTD points regime timing: gov.uk "Penalties for Making Tax Digital for Income Tax" and Autumn Budget 2025 coverage, read 13 Sep 2026. Appeals: gov.uk "Appeal a Self Assessment penalty for late filing or late payment" (form SA370), read 13 Sep 2026. General information, not advice — HMRC decides reasonable-excuse appeals and any deliberate-withholding penalty on the facts of the individual case.