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Worked example
Real numbers from this tool, line by line.
A UK sole trader with £7,400 of 2025-26 Income Tax and Class 4 NI, who already paid £5,000 in payments on account, owes £6,100 on 31 January 2027, not £2,400.
| Line | Amount |
|---|---|
| Balancing payment for 2025-26 | £2,400.00 |
| First 2026-27 payment on account | £3,700.00 |
| Total to pay by 31 January 2027 | £6,100.00 |
| Second payment on account, 31 July 2027 | £3,700.00 |
That is the whole surprise. The balancing payment is what is left of last year: £7,400 minus the £5,000 already paid. On top of it, HMRC asks for the first payment on account for 2026-27, which is half of the £7,400, so £3,700. The other half follows on 31 July 2027.
When payments on account do not apply
You do not make payments on account if your Self Assessment tax for the last year was under £1,000, or if more than 80% of your tax was already taken at source, for example through PAYE. Capital Gains Tax and student loan repayments are paid with the balancing payment, but they are left out of payments on account. A landlord with a job who owes £900 through Self Assessment pays £900 in January and nothing in July.
What the deadline costs if you miss it
gov.uk lists the penalties for a late online return:
- £100 as soon as the return is late;
- 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, another 5% or £300.
Paying late is charged separately: 5% of the tax still unpaid at 30 days, at 6 months and at 12 months, plus late payment interest. HMRC's rate is 7.75% from 9 January 2026 (base rate plus 4%).
For the same sole trader, filing and paying 120 days late (31 May 2027) adds:
- late filing: £100 + 31 days at £10 = £410.00;
- late payment: 5% of the £2,400 balancing payment = £120.00;
- interest: £6,100 × 7.75% × 120/365 = £155.42.
Total extra: £685.42, so £6,785.42 in all.
At 182 days late the extra cost is £1,845.73, because the 6-month penalties land: 5% of £7,400 is £370, which is more than £300, and a second 5% of the unpaid £2,400 is added.
Two dates before 31 January
The paper return deadline is 31 October 2026. If you pay tax through PAYE and owe less than £3,000, filing online by 30 December 2026 lets HMRC collect the bill through your tax code instead of one January payment.
If you joined Making Tax Digital for Income Tax from 6 April 2026, HMRC says the current penalties still apply to this 2025-26 return. The new points-based penalties start with your first MTD tax year.
Why HMRC's estimator is not enough
HMRC's own penalty estimator says it does not take account of payments you have already made, and it does not work out payments on account. It answers "what is the penalty", not "how much do I need on 31 January".
Check your own figures
The free calculator takes four figures from your 2025-26 tax calculation and the number of days late, and shows all ten lines above with your numbers. The workbook adds the same bill priced at 10 HMRC penalty dates, a 25-person register for a household or a bookkeeper's clients, and a monthly set-aside plan up to 31 July 2027.
15 seconds — what it actually does
Questions people ask
What does this Self Assessment deadline calculator work out?
It works out what you must pay HMRC on 31 January 2027 for the 2025-26 tax year: the balancing payment plus the first payment on account for 2026-27, and the second payment due on 31 July 2027. If you file or pay late, it adds the £100 and £10-a-day filing penalties, the 5% late payment penalties and HMRC interest at 7.75%.
Who is it for?
UK sole traders, landlords and people with side income who file an online Self Assessment return for 6 April 2025 to 5 April 2026. It suits anyone whose January bill came as a shock last year because the first payment on account was added to it, and anyone who is already late.
Why not just use HMRC's free penalty estimator?
HMRC's estimator says itself that it does not take account of payments you have made towards your bill, and it does not work out payments on account. This calculator starts from your own 2025-26 tax, deducts what you already paid in January and July 2026, adds the 2026-27 payment on account, and only then prices each day late.
What is free and what is in the paid workbook?
Free in the browser: your full 31 January 2027 bill and the cost of filing or paying late, for one person and one late date. The paid .xlsx keeps it: a 25-person register for a household or bookkeeping practice, the cost at every HMRC penalty date side by side, and a monthly set-aside plan up to 31 July 2027.
How does the price compare with the alternatives?
FreeAgent charges sole traders £19 a month excluding VAT for its accounting software, and landlords £10 a month. This workbook is a one-off payment of $21, not a subscription. It does not file your return; it tells you the amount and the deadline cost before you file, using your own figures.
When do these rules change next?
If you join Making Tax Digital for Income Tax from 6 April 2026, HMRC's new points-based penalties apply from the 2026-27 tax year. HMRC says the current penalties still apply to the 2025-26 return due on 31 January 2027. HMRC interest moves with the Bank of England base rate.
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