Late filing
The official Self Assessment penalties page lists this schedule if you send the return late: an initial £100 penalty; after 3 months, additional daily penalties of £10 per day, up to a maximum of £900; after 6 months, a further penalty of 5% of the tax due or £300, whichever is greater; after 12 months, another 5% or £300, whichever is greater. This calculator follows that wording. It treats “after 3 months” as after 90 days, so the first £10 day is day 91, and it caps the daily total at £900. It treats 6 months as 181 days — the span from 31 January to 31 July in a year that is not a leap year — and 12 months as 365 days. If your deadline is a different date, or you need an exact calendar count, use HMRC’s own estimate-your-penalty service as well.
The initial £100 applies once the return is late, including by a single day on this arithmetic. The later percentage penalties use the tax due you enter. If that tax is low, the £300 minimum can be larger than 5%.
Late payment
The same official page says you will get penalties of 5% of the tax unpaid at 30 days, 6 months and 12 months, and that you will also be charged interest on the amount owed. This tool applies those three 5% steps using the same 181-day and 365-day conventions. It does not calculate interest in pounds, because the official late-payment interest rate is linked to the Bank of England base rate and is updated on HMRC’s interest-rates page. As of the 6 November 2026 update of that page, late payment interest is 7.75% from 9 January 2026 and 8.00% from 24 November 2026, set at base rate plus 4% from 6 April 2025. Use that page rather than a figure invented here.
Failure to notify
If you register after 5 October and do not pay all of your tax bill by 31 January, you may also get a “failure to notify” penalty. The official penalties page says this is based on the amount still left to pay and is received within 12 months after HMRC receives the return. The published factsheet CC/FS11 (updated 19 March 2026) explains that the percentage depends on whether the failure was delayed, deliberate, or deliberate and concealed, and whether the disclosure was prompted or unprompted. This calculator does not apply that schedule — it would be misleading to collapse it into a single box. Read the factsheet if that situation applies, and still send the return and pay as soon as you can.
HMRC may accept a reasonable excuse in some cases. This page cannot decide whether you have one. Pay and file as soon as possible to limit further daily penalties, and use the official appeal route if you disagree with a penalty notice.