Self Assessment Penalty Calculator
Last updated: October 2026 · Penalty rules per GOV.UK · Interest rate 7.75%
Missing the Self Assessment deadline can cost far more than people expect, and the penalties build up in stages. This calculator estimates the late filing penalties, the late payment penalties and the interest on unpaid tax from the dates you enter. Nothing you type is stored or sent anywhere; the calculation runs in your browser.
How Self Assessment penalties work
There are two separate sets of penalties: one for filing the return late and one for paying the tax late. You can face both at once, and interest is charged on top of the tax you owe. For an online return for the 2025/26 tax year, both the filing deadline and the payment deadline are 31 January 2027.
| When | Late filing penalty | Late payment penalty |
|---|---|---|
| Day after the deadline | £100 fixed penalty, even if no tax is due | Interest starts to build up |
| 30 days after | Nothing further | 5% of the tax still unpaid |
| 3 months after | £10 a day for up to 90 days (maximum £900) | Nothing further |
| 6 months after | 5% of the tax due or £300, whichever is greater | A further 5% of the tax still unpaid |
| 12 months after | Another 5% of the tax due or £300, whichever is greater | A further 5% of the tax still unpaid |
In serious cases involving deliberate concealment, the 12-month filing penalty can be higher, up to 100% of the tax due. Penalties for late filing apply even when you owe no tax or are due a refund, which is why filing on time matters even if you cannot yet pay.
Worked examples
Return filed three weeks late, nothing owed. A return for 2025/26 filed on 20 February 2027 with no tax due costs a flat £100 penalty. No daily penalties apply because the 3-month point has not been reached.
Filed and paid four months late, £5,000 due. Filing and paying on 31 May 2027 means £100 plus 31 days of £10 daily penalties (£310) for filing, so £410 in total, plus a 5% late payment penalty of £250. Late payment interest at 7.75% adds about £127. The total is roughly £787 on top of the tax itself.
Filed and paid twelve months late, £5,000 due. Filing and paying on 31 January 2028 means £100 plus £900 in daily penalties, then £300 at 6 months and £300 at 12 months (5% of £5,000 is only £250, so the £300 minimum applies), making £1,600 for filing. Three 5% late payment penalties add £750, and a year of interest adds about £388. The total is about £2,738 on top of the £5,000 tax bill.
Interest on unpaid tax
HMRC charges interest on tax paid after the deadline, from the day after the deadline until the day you pay. Since April 2025 the rate has been the Bank of England base rate plus 4%, and it was 7.75% a year when this page was last updated. The rate changes whenever the base rate changes, so check the current figure on GOV.UK and update the rate in the calculator. HMRC also charges interest on late payment penalties, which this estimate does not include.
Appealing a penalty and paying in instalments
If you had a reasonable excuse for filing or paying late, such as serious illness, a bereavement, or a failure of HMRC's online service, you can appeal. You normally have 30 days from the date of the penalty notice to appeal. If you cannot pay your bill in full, contact HMRC before the deadline to set up a Time to Pay arrangement. Paying what you can by the deadline reduces the penalties and interest that follow.
Making Tax Digital and the newer penalty rules
The figures above apply to the standard Self Assessment return. Taxpayers who have joined Making Tax Digital for Income Tax, which started in April 2026 for people with qualifying income over £50,000, follow a points-based system for late quarterly updates and different late payment rules. See our MTD readiness checker to find out whether you are affected, and our tax deadlines page for the key dates.
Frequently asked questions
Do I get a penalty if I owe no tax?
Yes. The £100 fixed late filing penalty applies even when no tax is due, and the later penalties can apply too. The 5% element is calculated on the tax due, so the 6-month and 12-month penalties are at least £300 each.
What is the maximum penalty for filing late?
For an ordinary late return the filing penalties can reach £100 plus £900 plus £300 plus £300, which is £1,600, before any late payment penalties and interest. Deliberate concealment can lead to higher penalties.
When does interest start?
Interest on unpaid tax runs from the day after the payment deadline until the tax is paid, at the Bank of England base rate plus 4%.
Can I get a penalty cancelled?
You can appeal if you had a reasonable excuse, normally within 30 days of the penalty notice. HMRC may also cancel penalties where it made an error.
Sources: GOV.UK: Self Assessment penalties and GOV.UK: HMRC interest rates for late and early payments. This tool provides estimates, not tax advice; confirm any penalty with HMRC or a qualified adviser.