Estimate HMRC Late Payment Interest
Use this calculator to estimate interest on overdue UK tax. It uses a simple daily interest method for a single annual rate across the late period.
Important: This is an estimate tool only. HMRC rates can change during a late period. For exact values, use HMRC statements or split calculations by rate period.
What is HMRC late payment interest?
HMRC charges late payment interest when tax is paid after the due date. The interest is designed to reflect the cost of paying late rather than being a punishment by itself. It applies to many UK taxes, including Self Assessment, VAT, PAYE, and Corporation Tax, depending on your circumstances.
The key idea is simple: the longer the tax remains unpaid, the more interest builds up each day.
How this calculator works
This calculator uses a standard daily simple-interest method with one annual rate over the full overdue period:
- Daily rate = annual interest rate ÷ 365
- Days late = payment date minus due date
- Estimated interest = unpaid tax × daily rate × days late
If your payment date is on or before the due date, the estimated interest is £0.00.
Example
| Item | Value |
|---|---|
| Unpaid tax | £5,000.00 |
| Annual interest rate | 8.50% |
| Days late | 30 |
| Estimated interest | £34.93 |
How to use the calculator correctly
1) Enter the unpaid amount
Use the tax still outstanding during the late period. If you made part-payments, this simple calculator may overstate interest because HMRC usually calculates interest on the reducing balance.
2) Add the due date and payment date
Choose the original due date and the date paid (or planned payment date). Interest generally starts once the amount is overdue and increases for each day late.
3) Enter the annual rate
Use the rate that applies to your late period. HMRC can change rates, so for long overdue periods you may need to split the calculation into multiple chunks and add the results together.
Late payment interest vs late filing penalties
People often mix these up:
- Late payment interest grows daily while tax remains unpaid.
- Late filing penalties can apply if you file a return after the deadline.
You can sometimes face both at once, so it is worth checking your filing date and payment date separately.
Tips to reduce what you owe
- Pay as much as possible as early as possible.
- Contact HMRC quickly if you cannot pay on time.
- Ask about a Time to Pay arrangement where appropriate.
- Keep records of payment dates, bank references, and correspondence.
Even a partial payment can reduce future daily interest because the outstanding balance falls.
Frequently asked questions
Does HMRC compound late payment interest daily?
In most practical personal calculations, people estimate with simple daily interest. Official calculations can include specific treatment depending on tax type and system rules, so this tool should be treated as an estimate.
What if rates changed while I was late?
Split the overdue period into date ranges and run separate calculations for each rate, then total the interest amounts.
Can I rely on this as my final tax figure?
No. Use it for planning and budgeting. Your final payable amount is what HMRC confirms on your account or statement.
Final word
An HMRC late payment interest calculator gives you a quick forecast so there are no surprises. The sooner you pay, the lower the interest. If your case is complex (multiple rates, partial payments, or long arrears), treat this as a starting point and compare with HMRC's official numbers.