Quick answer
HMRC charges interest on tax paid late, calculated daily from the due date at the current rate.
Updated October 2026 · Figures based on 2026/27 rates
Paying your taxes on time is very important, but sometimes delays happen. If you miss the deadline to pay your tax bill in the UK, HMRC will charge interest on your unpaid amount. Understanding how to calculate interest on self-assessment tax can help you manage your finances better and avoid unnecessary penalties. In this guide, we break down everything you need to know about how to calculate interest on self assessment tax simply.
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What is Self Assessment Tax?
Self-assessment tax is the system HMRC uses to collect income tax from the self-employed, company directors, and people with additional sources of income. If you fall into these categories, you must complete your self-assessment tax return each year and pay any tax before the deadline.
How Does HMRC Charge Interest On Late Self-Assessment Tax?
When you miss the deadline to pay your tax payment to HMRC, HMRC charges interest on the outstanding amount. The interest rate is set by HMRC and it can change over time. The interest is applied daily from the day after the deadline until the tax is fully paid.
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How to Calculate Interest on Self Assessment Tax?
The calculation of self-assessment tax interest requires a step-by-step process, which includes the following points:
Find the Outstanding Amount: The outstanding amount represents the self-assessment tax which HMRC expects from you following the deadline.
Check the Current Interest Rate: The interest rate which HMRC updates on a regular basis remains available for review. The up-to-date interest rates are available on HMRC’s website.
Multiply by the Interest Rate: You can obtain the daily interest rate by dividing the annual interest rate by 365 days.
Multiply by the Number of Days Late: Track the delay duration between when payments become due to when your payment is planned.
How to Avoid Paying Interest on Self-Assessment Tax?
Pay On Time: Meet your deadlines for 31 January and 31 July to ensure timely payment.
Set Up a Direct Debit: Direct Debit automation helps you prevent payment delays effectively.
Make Payments in Advance: An early payment of funds decreases your potential interest costs.
Use HMRC’s Budget Payment Plan: You can utilise HMRC’s Budget Payment Plan to distribute your next tax bill payments monthly.
Check Your Tax Bill Early: Finishing your tax return in advance gives you enough time to organise your payments before the deadline.
What Happens If You Don’t Pay at All?
When you postpone your tax payment, HMRC will charge late penalties besides charging interest. There are three late payment penalties:
- A penalty of 5% for any unpaid tax will apply 30 days after the due date.
- A penalty of 5% will be assessed against unpaid taxes starting five months after the penalty date.
- Taxpayers who wait 11 months past the penalty date face a 5% fee on their outstanding payments.
The delay in payment adds increased expense to your total bill, so you should always make payments as soon as possible.
Need Help with Your Self-Assessment Tax?
If your tax calculations are proving difficult and late payment penalties are creating problems, our professional team can assist with self-assessment tax preparation, interest computation and payment arrangement planning. Are you unsure how to calculate interest on self assessment tax? We can help you with this process. Call us to get compliant and prevent unnecessary fees. Contact us today!
When interest starts and when it stops
Interest on Self Assessment is not a penalty. It is HMRC’s charge for the time it has been without money you owed. It runs day by day from the date the payment was due until the day the payment reaches HMRC. For a balancing payment, that normally means from 31 January after the end of the tax year. For payments on account, it runs from 31 January and 31 July. The rate is set by reference to the Bank Rate plus 4%, so it moves when the Bank Rate moves. Always check current HMRC guidance for the rate that applies to your dates.
Because interest is compensation rather than punishment, a reasonable excuse will not normally remove it, although it can be relevant to penalties. Interest also keeps building if you agree a Time to Pay arrangement, so spreading the bill helps with cash flow but does not make it free. Our guide to paying Self Assessment tax in instalments explains how that works.
Worked example: a balancing payment paid two months late
The rate below is illustrative only, to show the arithmetic. It assumes interest at 8% a year, which is Bank Rate plus 4% if the Bank Rate were 4%. Your actual figure will depend on the rate in force.
| Step | Working | Result |
|---|---|---|
| Tax due on 31 January | Balancing payment | £4,000.00 |
| Date paid | 31 March (59 days late) | 59 days |
| Interest | £4,000 x 8% x 59 / 365 | £51.73 |
| Late payment penalty | 5% of £4,000 (still unpaid after 30 days) | £200.00 |
| Total extra cost | £51.73 + £200.00 | £251.73 |
The example shows something many people miss. The interest is the smaller part. The 5% late payment penalty, which applies once the tax is more than 30 days overdue, is the bigger cost. If the same £4,000 stayed unpaid for six months, a second 5% charge of £200 would apply as well, on top of the interest. See our guide to late payment penalties explained for the full timetable.
Common mistakes
- Assuming interest only matters on the January balancing payment, and forgetting that late payments on account attract interest too.
- Paying the tax but not the penalty, which leaves a separate balance that can itself start to build interest.
- Waiting for a bill to arrive. Interest runs from the due date whether or not HMRC has written to you.
- Paying a rough estimate and never reconciling it once the return is filed.
- Treating a Time to Pay plan as interest-free.
- Ignoring a refund position. If you overpay, HMRC pays interest on the repayment, but normally at a lower rate than it charges, so over-paying is not a saving strategy.
Practical checklist
- Log in to your HMRC online account and check the amount due and the due date for each payment.
- Work out the interest yourself using tax due x annual rate x days late / 365, using the current rate from HMRC.
- Pay as much as you can as early as you can, because interest stops accruing on whatever has been paid.
- If you cannot pay in full, contact HMRC before the deadline about a payment plan.
- Set aside a share of every receipt during the year. Our guide to paying your tax bill and the notes on reducing payments on account can help you plan.
- Keep proof of payment dates, as the date funds reach HMRC is what counts.
How SATR can help
If you are already behind, our qualified UK accountants can file the return online, calculate what you owe including interest, and explain your options, from a fixed fee from £199 + VAT. See our late Self Assessment tax return service to get started.
This is general information, not personal tax advice. Check current HMRC guidance for the interest rate that applies to you.
Frequently asked questions
What is the rate?
Base rate plus 4%, updated periodically.
Can I reduce interest?
Pay as much as you can as soon as possible.
Can I get interest on late Self Assessment tax removed?
Interest is a charge for the time HMRC has been without the money, so it is not normally cancelled for a reasonable excuse in the way a penalty can be. HMRC can sometimes reduce or remove it in exceptional circumstances, such as an HMRC error, so it is worth asking.
Does HMRC charge interest on penalties?
Penalties can themselves attract interest if they remain unpaid after the date they are due. Check current HMRC guidance for the exact dates that apply to your penalty notices.
Do I pay interest if I am on a Time to Pay arrangement?
Yes. Interest continues to build on the unpaid balance while you pay in instalments, so clearing the debt sooner reduces the total cost.
Related guides
- How Accountants Manage Self Assessment Deadlines
- How to Pay Your Self Assessment Tax Bill 2026/27
- Can You Pay Self Assessment Tax in Instalments?
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The Bottom Line
Interest is the smaller half of the problem: at Bank Rate plus 4% it adds up slowly, but the 5% late payment penalties at 30 days, six months and twelve months are what really sting. Pay on time, pay early if you can, and if you cannot pay, agree a plan with HMRC before the deadline.
Facing interest and penalties? SATR’s qualified UK accountants will file your return, calculate what you owe including interest, and sort your options for a fixed fee from £199 + VAT. Start your tax return today or get an instant quote.
