Calculating Interest and Penalties on Tax

If you've paid tax late or need to work out what you owe HMRC for a previous tax year, you'll need to calculate both interest charges and any penalties that apply. HMRC provides free interactive calculators that work out these amounts for you, covering tax years from 2005/06 t...

Introduction

Which calculator do you need?

HMRC offers two different calculators depending on which tax year you're dealing with and what you need to calculate:

Interest and penalty calculator – This covers tax years ending 5 April 2006 through to 5 April 2024 (tax years 2005/06 to 2023/24). Use this calculator when you know how much tax you owe and need to work out the interest and penalties on top of that amount.

Tax, interest and penalty calculator – This covers tax years ending 5 April 2010 through to 5 April 2024 (tax years 2009/10 to 2023/24). This more comprehensive calculator helps you work out the tax itself as well as any interest and penalties.

If you're dealing with a tax year from 2009/10 onwards and need to calculate everything including the tax amount, use the second calculator. For earlier years (2005/06 to 2008/09), only the interest and penalty calculator is available.

How the calculators work

Both calculators are interactive PDF forms that you complete on screen. They automatically work out the interest charges based on HMRC's rates and apply the correct penalties according to how late your payment is.

The calculators take into account:

The amount of tax you owe
When the tax was originally due
When you actually paid (or intend to pay)
The various interest rates that applied during the period
Which penalties apply based on the length of delay

This saves you from having to manually calculate interest across different periods or work out which penalty rates apply to your situation.

How to use the calculators

To complete either calculator, you'll need Adobe Reader installed on your computer. The forms won't work properly if you try to open them directly in your web browser.

Follow these steps:

1. Download the calculator – Right-click on the form link and select 'Save target as' or 'Save link as' (Windows) or 'Save linked file as' (Mac). Save it somewhere you can easily find it on your computer.

2. Open in Adobe Reader – Locate the saved file and open it using Adobe Reader. If you don't have Adobe Reader, download the latest free version from Adobe's website.

3. Complete on screen – Fill in the requested information, such as the tax year, the amount of tax owed, and the relevant dates. The calculator will automatically work out the interest and penalties.

4. Review the results – The calculator will show you a breakdown of what you owe, including the original tax, interest charges, and any penalties.

If you have problems

The most common issue is trying to open the calculator in a web browser rather than Adobe Reader. If the form doesn't load or work properly:

Check your computer settings to make Adobe Reader the default program for opening PDF documents
Save a copy of the form to your computer first, then open it directly in Adobe Reader
Make sure you're using the latest version of Adobe Reader

If you're still having difficulties, you can contact HMRC's online services support for help.

Accessible formats

If you use assistive technology such as a screen reader and need the calculator in a more accessible format, you can email HMRC at different.format@hmrc.gov.uk. Include details of what format you need and which assistive technology you use.

The calculators are also available in Welsh. Contact HMRC's Welsh language helpline to request a Welsh version.

When to use these calculators

These calculators are specifically for historical tax years up to 2023/24. They're useful if you're:

Settling an old tax bill and need to know the full amount including interest and penalties
Checking whether HMRC has calculated your interest and penalties correctly
Working out what you'll owe if you pay by a certain date
Dealing with a tax investigation or disclosure for earlier years

For the current tax year (2025/26) or very recent years, HMRC will calculate interest and penalties for you and include them on your tax statement. However, these calculators remain available for checking older liabilities.

What happens next

Once you've calculated what you owe, you'll need to pay HMRC. The calculator helps you understand the total amount, but it doesn't automatically notify HMRC or set up a payment for you.

If you're unable to pay the full amount immediately, contact HMRC as soon as possible. They may be able to arrange a payment plan, which can prevent further penalties from accumulating. Interest will continue to accrue on any unpaid amounts until you've paid in full.

Sources

This article provides general guidance based on current HMRC rules. For advice specific to your situation, speak to your accountant.