Free tool • 2025/26 bill due 31 January 2027 • Nothing stored

Self Assessment Penalty, Interest & Payment Plan Calculator

Late with your return or can’t pay by 31 January 2027? See the exact penalties and interest for your dates, what each extra month of waiting costs, the monthly figure for an HMRC payment plan, and whether your tax code could collect it instead.

By The HandySums teamLast checked:
Your datesPenalties and interest worked out day by day for the dates you enter.
Payment planAn estimated monthly figure for Time to Pay, including interest.
PrivateRuns in your browser. Nothing is sent or stored.
Example figures — replace with your own. Example figures are filled in so you can see how it works — replace them with your own before relying on the result shown.

1. Your 2025/26 return

The return for 6 April 2025 to 5 April 2026. The bill is due on 31 January 2027.

Late filing penalties start from the deadline for the way you file.
Use a date in the future if you have not filed yet.
The tax still owed for 2025/26 after any payments on account. Late payment penalties are worked out on this.
Also due on 31 January 2027. Charged interest if late, but no late payment penalty. Enter 0 if none.

2. How you will pay

Pay everything on one date, or agree a payment plan (Time to Pay) with HMRC.

Pay by 2 March 2027 and there is no late payment penalty, only interest.

3. Interest rate

HMRC’s late payment interest rate is the Bank of England base rate plus 4%. It changes when the base rate changes.

7.75% from 9 January 2026. Check GOV.UK for the current rate.

4. Optional: pay through your tax code?

If you also have a job or pension taxed through PAYE, HMRC may collect a small bill through your tax code over the next tax year instead.

Leave blank to skip this check.
Voluntary Class 2 cannot be collected through your tax code.

Why use this one

What is different about this calculator

  • It works out late filing, late payment and interest together, for the 2025/26 bill and your actual dates.
  • It shows what paying a day, a month or a year later costs, so you can see the 2 March 2027 cliff edge.
  • It estimates the monthly payment for an HMRC Time to Pay plan, including the interest that keeps building.
  • It checks whether you could pay through your tax code instead, and whether a plan can probably be set up online.
  • It includes your first payment on account, which gets interest but not a late payment penalty.
Guide

How to use the calculator

Enter how and when you filed your 2025/26 return, what you owe and how you plan to pay. The calculator adds up the late filing penalty, any late payment penalties and the interest, and shows what waiting another day, month or year would cost.

How it is worked out

  1. Late filing: £100 the day after the deadline, £10 a day for up to 90 days once the return is 3 months late, then 5% of the tax due or £300 (whichever is more) at 6 months and again at 12 months.
  2. Late payment: 5% of the balancing payment still unpaid after 30 days, 6 months and 12 months. Payments on account do not get these penalties.
  3. Interest: simple interest at HMRC’s late payment rate on everything paid after 31 January 2027, from 1 February 2027 until you pay.
  4. Payment plan: equal monthly payments that clear the tax and the interest that builds up while you pay, assuming the plan is kept to.

Key dates for the 2025/26 bill

DateWhat happens
30 December 2026Last day to file online and have a bill under £3,000 collected through your tax code
31 January 2027Online return and payment due. From 1 February, a £100 penalty if not filed and interest on anything unpaid
2 March 2027Last day to pay, or agree a payment plan, before the first 5% late payment penalty
1 April 2027About 60 days after the due date: the usual last day to set up a plan online
1 May 2027Online return 3 months late: £10 a day starts
1 August 2027Online return 6 months late: 5% of the tax due or £300
2 August 2027Last day to pay before the second 5% late payment penalty

What is not included

Interest on penalties, the second payment on account (due 31 July 2027), tax-geared penalties for deliberately withholding information, and the new points-based penalties for people in Making Tax Digital. If you can’t pay, read what to do if you can’t pay your tax bill, and see what a late filing penalty letter means.

Frequently asked questions

What is the penalty for filing my tax return late?

£100 as soon as it is late, even if you owe no tax. After 3 months HMRC adds £10 a day for up to 90 days (£900). After 6 months it adds 5% of the tax due or £300, whichever is more, and the same again after 12 months.

What is the penalty for paying my Self Assessment bill late?

5% of the tax unpaid 30 days after the deadline, another 5% of what is still unpaid at 6 months and another 5% at 12 months. For the 2025/26 bill, pay by 2 March 2027 and there is no late payment penalty, only interest.

Does a payment plan stop the penalties?

A Time to Pay plan agreed before a penalty date, and kept to, means that late payment penalty is not charged. Interest is still charged on the tax you have not paid yet. A plan does not cancel late filing penalties.

Can I set up a payment plan online?

Usually, if you have filed your return, you owe up to £30,000 and you have no other tax debts, plans or missing returns. HMRC’s online plans are for up to 12 months and must be set up within 60 days of the due date. Otherwise call HMRC, which can agree longer plans.

Can I pay through my tax code instead?

If you owe less than £3,000, already pay tax through PAYE and file online by 30 December 2026 (paper by 31 October 2026), HMRC can collect the 2025/26 bill in 12 monthly instalments through your tax code from April 2027.

Is interest charged on payments on account?

Yes. If your first payment on account for 2026/27 is paid after 31 January 2027, interest is charged on it, but late payment penalties apply only to the balancing payment.

Is anything I type stored?

No. The calculator runs in your browser. Nothing is sent to us or stored, and there is no sign-up.

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Sources and review

Last checked: 2 November 2026. The penalty amounts and trigger dates, interest rate, payment plan and tax code rules used by this calculator. Written and checked by the HandySums team against the GOV.UK pages below. HandySums is independent and not an accountancy firm — see how we check our sums.

Estimate only — not tax advice. Results are a guide and HMRC has the final say. This is a guide only: the penalty and plan rules may change, and HMRC decides whether a plan is agreed and how much you pay. The figures you enter stay in your browser and are not stored. Verify with HMRC or a qualified accountant.