Reporting capital gains tax for 2024/25? Make sure you’re not caught out

Chancellor Rachel Reeves increased the main rates of capital gains tax (CGT) to 18% and 24% in her maiden budget on 30 October 2024. The rate for trustees and personal representatives also rose to 24%.

But the mid-year change means some people filing self-assessment tax returns for 2024/25 risk underreporting and underpaying any tax they owe.

That’s because HMRC’s tax software can’t cope with different tax rates across a tax year and will simply apply the rates in force before 30 October to all chargeable gains on investments like shares or investment funds. Calculations for property gains and carried interest are not affected.

Read more about CGT and how tax is calculated

HMRC adjustment calculator

To help taxpayers who’ll rely on HMRC’s systems for their 2024/25 returns — perhaps because they complete them without the help of commercial software or an accountant — the taxman has developed an online calculator. The 2024/25 calculator can help individuals, trustees or the personal representatives of someone who has died.

If you use commercial software to calculate and submit your tax return, this might have been updated to use the correct rates, but you should still double check.

The calculator will work out an adjustment to include on the return so that the correct amount of tax is declared and paid.

This figure should be inputted into box 51 of the capital gains tax summary pages of your return, and you should also save the online results page and submit it as an attachment at the same time.

Using the calculator

You’ll need to use the calculator if you have total gains in excess of your annual tax free exemption for the year (relating to your AJ Bell Dealing account, or a different provider), and some of those gains were made on or after 30 October 2024.

To use it, you’ll need:

• The date you sold or transferred (disposed) of the investment(s)
• The amount of gain
• Your taxable income for 2024/25
• Details of any losses
• The amount you paid into pensions (gross)
• Details of any charity donations that qualify for gift aid (gross)

Any disposals you’ve made in your AJ Bell Dealing account will be shown in your annual tax summary in date order. You can find this by logging into your account and selecting the ‘Documents’ section of the Dealing Account menu.

If you have gains both before and after the tax rates changed on 30 October 2024, the HMRC calculator will set your annual exempt amount against gains arising at the new (increased) rates of CGT first.

Deadlines and penalties

If you’re completing an online return for the 2024/25 tax year, this must be filed, and any tax paid by 31 January 2026. Paper returns are due by 31 October 2025.

If you pay the wrong amount of tax and still owe money after 30 days, a 5% fine applies, with an extra 5% after six and 12 months. You’ll also be charged interest on late payments. The rate will be base rate plus 4% from the due date until the payment date.