Where the penalties come from
At six months, two kinds of charges have built up: penalties for filing late and penalties plus interest for paying late. The calculator is set for a 2025/26 return filed at the end of August 2027 with £5,000 of tax, paid on 19 August 2027 — 200 days after the deadline.
Filing late
- Initial penalty: £100
- Daily penalties: the full 90 days × £10 = £900
- 6-month penalty: 5% of £5,000 is £250, but the minimum is £300, so £300
Filing penalties: £1,300.
Paying late
- 5% of the unpaid tax at 30 days (unpaid on 3 March 2027): £250
- Another 5% at 6 months (unpaid on 3 August 2027): £250
- Interest: £5,000 × 7.75% × 200 ÷ 365 = £212.33
Altogether: £2,012.33 in penalties and interest, so £7,012.33 to HMRC including the tax.
When the 6-month penalty is more than £300
The 6-month penalty is 5% of the tax due or £300, whichever is higher. It only goes above £300 when the tax is over £6,000:
| Tax on the return | 6-month penalty | All filing penalties if filed in August 2027 |
|---|---|---|
| £0 | £300 | £1,300 |
| £2,000 | £300 | £1,300 |
| £6,000 | £300 | £1,300 |
| £8,000 | £400 | £1,400 |
| £20,000 | £1,000 | £2,000 |
The 6-month filing penalty is charged even if you have already paid all the tax. That is why, when a return is very late, filing it — even with a few estimated figures — is the single most useful thing you can do.
And at 12 months?
If the return is still not filed by 1 February 2028, there is another penalty of 5% or £300, whichever is higher. HMRC can charge much more — up to 100% of the tax — if it decides information was deliberately withheld. A third 5% late payment penalty also applies to tax still unpaid at 12 months.
What to do now
- File the return — this stops further filing penalties. Use estimates if you need to and amend later.
- Pay, or set up Time to Pay. A payment plan stops further late payment penalties as long as you keep to it.
- Consider an appeal if you had a reasonable excuse, such as serious illness. HMRC looks at each penalty separately.
- Check your online account — it lists each penalty and the interest so far, so nothing comes as a surprise. Unpaid penalties also attract interest after 30 days.
- Ask for help if things feel overwhelming. Free help is available from charities such as TaxAid and Tax Help for Older People.
FAQ
How much is the penalty if my tax return is 6 months late?
At least £1,300 in filing penalties: £100, plus £900 of daily penalties, plus the higher of £300 or 5% of the tax due.
What will I owe on £5,000 of tax filed in August and paid 200 days late?
£2,012.33: £1,300 filing penalties, two 5% late payment penalties of £250 each, and £212.33 interest at 7.75%.
When is the 6-month penalty more than £300?
When 5% of the tax due is more than £300 — that is, when the tax is over £6,000. On £8,000 it is £400; on £20,000 it is £1,000.
Sources (checked 5 November 2026): GOV.UK “Self Assessment tax returns: penalties” (gov.uk/self-assessment-tax-returns/penalties); GOV.UK “Penalties for Making Tax Digital for Income Tax” (current penalties apply to the 2025 to 2026 return due 31 January 2027); GOV.UK “HMRC interest rates for late and early payments” (7.75% from 9 January 2026); Finance Act 2009 Schedules 55 and 56; ICAS “Self-assessment penalties: which regime applies for 2025-26 and 2026-27”.