HMRC Penalty & Interest Calculator United Kingdom

What the HMRC penalty and interest calculator does

The calculator estimates what filing or paying your Self Assessment tax late could cost you. Late tax usually brings more than one charge: a fixed penalty for the late return, a penalty based on the tax you owe once you pass 30 days, and interest every day the tax stays unpaid. The tool adds these together into one figure.

Knowing roughly what the charges come to helps you decide how quickly to act and how much to set aside. Interest keeps building until the tax is paid, so acting early is usually cheaper than waiting.

The fixed late-filing penalty

A Self Assessment return filed after the deadline brings an automatic £100 penalty, even if you owe no tax, which surprises people who assume a nil return cannot be penalised. The calculator adds it as soon as the return is late.

If the return is still outstanding after three months, HMRC can charge £10 a day for up to 90 days, and larger tax-geared penalties follow at six and twelve months. The calculator includes only the initial £100, so for a return more than three months late its figure is too low. Filing promptly, even when you cannot yet pay, stops the filing penalties from growing.

The late-payment penalty

Paying late is penalised separately from filing late. Once your payment is 30 days late, HMRC charges 5% of the tax still outstanding, so a larger unpaid balance means a larger penalty.

The calculator applies that first 5% once the payment is 30 or more days late. It does not include the further 5% penalties at six and twelve months, so for tax unpaid for longer, treat the result as a minimum. Paying before each milestone avoids the next penalty.

How interest builds

HMRC also charges interest on unpaid tax, daily from the due date until you pay, at the Bank of England base rate plus 4 percentage points. Unlike the penalties, it grows every day the tax is outstanding.

The calculator works out interest as the tax owed times the annual rate times the fraction of the year that has passed. For example, £3,000 paid 90 days late at a 7.75% interest rate brings £100 for the late return, a £150 late payment penalty and about £57 of interest: about £307 in total. Try different numbers of days to see how much a delay costs.

What you need to enter

Enter the tax that was outstanding, which drives both the late payment penalty and the interest, and the number of days late, from the deadline to the day you expect to pay. The calculator uses the same number of days for the filing and payment charges, so if you filed and paid on different dates, use the later one for a cautious estimate.

Use the amount actually unpaid rather than your whole bill, since tax paid on time does not attract late payment charges. If you are unsure of the exact days, round up; a slightly higher count gives a safer estimate to budget against.

When to use this tool

• To estimate the cost of a late return and late payment before HMRC issues the charge.
• To see how much interest builds up each week the tax stays unpaid.
• To decide how urgently to clear an overdue balance.
• To check a penalty notice against a rough independent figure.

HMRC sets the penalties and the interest rate, and the rate changes with the base rate, so recalculate when it moves. For the exact amount, and for reasonable excuse or Time to Pay options, check your HMRC online account or contact HMRC.

How to reduce what you owe

File the return promptly to stop the filing penalties growing, even if you cannot yet pay, because filing and payment are penalised separately. Pay as much as you can before each milestone: it reduces both the tax-geared penalty and the balance that interest is charged on, so partial payments are rarely wasted.

If you cannot pay in full, HMRC’s Time to Pay arrangement can spread the debt and may prevent further late payment penalties, though interest usually continues. Where a late return or payment was caused by something outside your control, a reasonable excuse can cancel a penalty, but interest is generally not waived. Act early, because every week adds interest and brings the next penalty closer.

This is general information only and not financial advice.

About This HMRC Penalty and Interest Calculator

Use this HMRC penalty and interest calculator to get an instant, accurate estimate. The figures follow official HMRC guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.