UK HMRC Late Payment Interest & Penalties Calculator (2026).

Use the formula: unpaid tax × annual interest rate × days late ÷ 365. At 7.75%, £5,000 paid 30 days late gives £31.85 of simple interest. If HMRC changes its rate during the overdue period, split the calculation into separate date ranges and use the rate that applied to each range. Penalties are calculated separately.

HMRC charges 7.75% per annum simple interest from the payment due date. The rate is reviewed periodically by HMRC.

Yes, if you have a reasonable excuse such as serious illness, bereavement or HMRC delays. Countify can prepare and submit a penalty appeal.

The Self Assessment late filing penalty is £100 immediately, plus £10 per day after 3 months up to £900, plus 5% of tax or £300 after 6 and 12 months.

Penalties are fixed amounts at set milestones. Interest is charged at a simple daily rate on the outstanding tax. They do not compound on each other.

For most tax payments, interest starts on the payment due date and ends on the date HMRC receives the payment. Special rules can apply to some liabilities, and interest continues on any balance left unpaid after a part-payment or Time to Pay arrangement.

As soon as possible. Every day adds to the interest charge, and even a part-payment reduces the amount on which interest accrues.

HMRC charges £100 per 50 employees for each month (or part month) a P11D or P11D(b) is filed late, plus interest on any Class 1A National Insurance paid late — calculated the same way as the late payment interest above. The filing deadline is 6 July after the tax year end, with payment due by 22 July (19 July if paying by cheque).

Yes. Interest on tax settled through a disclosure facility (such as the Worldwide Disclosure Facility) runs from each year's original due date to the date of payment, using the same simple daily-rate method as any other late payment — it just typically covers several tax years at once, so the total can be substantial even at a modest annual rate.