Ever tried to make sense of Box 51 and felt like you were working with only half the instructions? The rules talk about splitting gains before and after last October 2024, yet it is not clear how to do it or what happens if you skip it.

HMRC does provide an adjustment calculator, but it will not split your transactions for you, you still need to work out the totals for the period before and the period after 30 October.

If you sold crypto in the UK during the 2024/25 tax year and your total gains are above the annual exempt amount (£3,000.00), the 30 October 2024 rate change affects how you report them. HMRC introduced higher main Capital Gains Tax (CGT) rates for certain chargeable assets, including crypto, from that date. When you have disposals in both periods, you must calculate the gains for each, then use them in the HMRC calculator to get your adjustment. That figure goes in Box 51 of the SA108 (or in the “Adjustment to CGT” field if filing online).

Box 51 has been part of the SA108 for years, but for many crypto investors it only became relevant after this change. The Self Assessment calculation does not apply the new main rates automatically, so without the adjustment your tax bill will be wrong. CryptoBooks can give you the pre- and post- October totals instantly, ready to feed into the calculator and avoid that problem.

Why HMRC uses Box 51 for crypto disposals

Before last October 2024, gains on most crypto disposals were taxed at 10% for basic rate taxpayers and 20% for higher or additional rate taxpayers. The Autumn Budget 2024 increased these main CGT rates to 18% and 24% for disposals made on or after 30 October 2024.

This change does not affect every type of asset. Gains on residential property, carried interest and disposals that qualify for Business Asset Disposal Relief or Investors’ Relief (within the lifetime limit) remain on their existing rates.

Box 51 has long been part of the SA108 form to deal with situations where rates change partway through a tax year. In 2024/25, it is used to enter the adjustment that makes sure the correct rate is applied to each part of your gains, based on when the disposal happened. Without it, the Self Assessment system would apply one rate to the whole total, which would be wrong.

When Box 51 applies to your crypto

You will need to complete Box 51 if you made one or more crypto disposals on or after 30 October 2024, your overall gains for the year are above the annual exempt amount £3,000.00, and the HMRC adjustment calculator gives you a figure to enter.

This will typically happen if you:

• sold crypto both before and after 30 October 2024;

• sold only after 30 October 2024, and your gains still exceed the annual exempt amount (although in this case the adjustment will usually be zero if there are no pre-30 October gains).

You will not need a rate-change adjustment in Box 51 if all your chargeable crypto disposals in 2024/25 took place before 30 October. Always run the HMRC calculator to confirm and only enter an adjustment if it gives you a non-zero figure.

How to calculate the crypto disposals amounts for Box 51

The HMRC adjustment calculator asks for your total gains on or after 30 October 2024 before deducting the annual exempt amount and any unrestricted losses. These totals must exclude:

• gains on residential property;

• carried interest;

• gains eligible for Business Asset Disposal Relief or Investors’ Relief within the lifetime limit.

For crypto, this means you first need to:

• identify all your taxable crypto disposals in 2024/25;

• separate them into two periods: before 30 October and on or after 30 October;

• apply HMRC’s matching rules (same-day, 30-day, Section 104 pool) to each period;

• convert acquisition costs and disposal proceeds into GBP using the correct date rates;

• deduct allowable costs and any unrestricted losses;

• report your crypto gains in the SA108 as usual, then enter the adjustment amount from the HMRC calculator in Box 51 (or the “Adjustment to CGT” field if filing online);

If you disposed of crypto to a connected person, check the clogged losses rules to see if any additional entries are needed.

Example of how to split your crypto CGT

Imagine you sold crypto twice in the 2024/25 tax year:

• on 15 September 2024 you sold 1 BTC, making a gain of £4,000. This falls into the pre-30 October period.

• on 20 November 2024 you sold 2 ETH, making a gain of £2,000. This falls into the post-30 October period.

You enter both disposals in the SA108 form as normal. Then, you take the totals for each period (£4,000 and £2,000) and put them into the HMRC adjustment calculator. The calculator works out how much extra (or less) tax is due because the post-30 October gain is taxed at the new main CGT rates. That adjustment figure is what you enter in Box 51 of the SA108 (or in the “Adjustment to CGT” field if filing online).

Why you must enter the Box 51 adjustment yourself

If you enter only a single combined gain for 2024/25, HMRC’s system will apply one CGT rate to the entire amount instead of splitting it between gains made before and after 30 October 2024. This means your tax could be underpaid and when HMRC corrects it you can face interest charges and potential penalties.

Because the rate change is still recent, many taxpayers and even some advisers are missing this Box 51 adjustment step. Using the HMRC calculator and entering the result in Box 51 ensures your tax is calculated correctly.

From transactions to Box 51: how CryptoBooks does the heavy lifting

Separating and calculating crypto gains for Box 51 means splitting every taxable disposal in 2024/25 into before and after 30 October and applying HMRC’s matching rules to each period. This can be complex if you trade across multiple wallets, exchanges, or blockchains.

CryptoBooks automates the process by:

• importing your complete transaction history from all connected wallets and exchanges;

• applying HMRC’s official matching rules to calculate gains correctly;

• splitting your totals automatically into pre- and post- 30 October 2024 periods;

• preparing your SA108 with the correct Box 51 adjustment (paper return) or the “Adjustment to CGT” entry (online).

You can start by creating a free account, connect your sources and review your entire crypto history. When you are ready to file, you can upgrade your account to a paid plan and export an HMRC-compliant Self Assessment report knowing your Box 51 adjustment is accurate and the correct tax rates will be applied.