SA108 Crypto 2024/25 — Boxes 13.1 to 13.8 and Box 51 Explained

The 2024/25 Self Assessment is the first year with dedicated crypto boxes on SA108. Boxes 13.1 through 13.8 are specifically for cryptoasset disposals — previously, crypto went into the general capital gains section alongside shares, property, and other assets. HMRC has given crypto its own section. That is not a coincidence.

2024/25 is also a split year for CGT rates. The Autumn Budget changed rates on 30 October 2024: from 10%/20% to 18%/24%. Every disposal must be checked against this date. If you have disposals on both sides of the boundary, Box 51 comes into play — and almost no one is explaining how it works.

This guide explains every SA108 crypto box with worked examples, covers the Box 51 split-year adjustment in full detail, and explains where crypto income goes (spoiler: not SA108).

What's new on SA108 for 2024/25

HMRC added boxes 13.1–13.8 to the Capital Gains supplementary page (SA108) specifically for "crypto assets and digital tokens." This is a direct response to the CARF reporting framework and the growth of crypto tax compliance as a priority for HMRC.

Previously, crypto disposals were reported in the general "Other capital gains" section alongside any other asset. The new dedicated section means HMRC can now automatically cross-reference your declared crypto gains against CARF data from exchanges. The days of crypto gains going unnoticed are over.

Box 13.1 — Number of disposals

This box asks for the total count of crypto disposals in the tax year. A disposal is any event where you give up ownership of a cryptoasset:

• Selling crypto for GBP (or any fiat)
• Swapping one token for another (including stablecoin swaps)
• Removing liquidity from an LP position
• Buying an NFT with crypto (disposal of the crypto spent)
• Selling an NFT for crypto or fiat
• Making a donation of crypto (disposal at market value)

The following are not disposals:

• Bridging between chains
• Wrapping/unwrapping (ETH→WETH)
• Depositing into Aave, Compound, or other lending protocols
• Transferring between your own wallets
• Buying crypto with GBP (this is an acquisition, not a disposal)

Box 13.2 — Total disposal proceeds

The sum of GBP market value at the time of every disposal. This is not what you received in fiat — it is the fair market value of what you disposed of, on the date of disposal.

Note that the USDC→ETH swap counts because it is a disposal of USDC. The proceeds are the GBP value of the USDC at the time of the swap.

Box 13.3 — Total allowable costs

This is not the total amount you originally paid for your crypto. It is the Section 104 pool cost basis allocated to each disposal, plus gas fees.

For each disposal, the allowable cost is determined by which HMRC matching rule applied:

• Same-day match: cost basis is the acquisition price on the same day
• B&B match: cost basis is the reacquisition price within 30 days
• Section 104: cost basis is the weighted average from the pool

Gas fees paid on each disposal are added to the allowable cost (they reduce your gain). Box 13.3 is the sum of all these costs across every disposal.

Box 13.4 — Total gains

The sum of all positive gains only. If you had three disposals with gains of £350, −£200, and £1,200, Box 13.4 is £350 + £1,200 = £1,550. The loss is reported separately.

Box 13.5 — Total losses

The sum of all negative gains, expressed as a positive number. From the example above, Box 13.5 = £200.

Losses can offset gains, reducing your CGT liability. But HMRC has specific rules about how losses are applied — they reduce your net gain down to the annual exempt amount, not below it. Unused losses carry forward to future years. For the full mechanics, see crypto losses and tax relief.

Box 13.6 — Net gains or losses

Box 13.4 minus Box 13.5, after loss offset. If total gains exceed total losses, this is your net gain. If losses exceed gains, this is your net loss (reported as a positive number with a loss indication).

The annual exempt amount (£3,000 for 2024/25) is applied after this netting. You only pay CGT on the amount above the exempt threshold.

Boxes 13.7 and 13.8 — Real-time reporting

Box 13.7 asks for gains already reported to HMRC via a real-time Capital Gains Tax service (the online CGT reporting tool). Box 13.8 asks for tax already paid on those gains. For most crypto investors, both boxes are zero. The real-time service is primarily used for property disposals.

Box 51 — the split-year CGT adjustment

This is the box that nobody is explaining. The Autumn Budget 2024 changed CGT rates on 30 October 2024:

If all your disposals fall on one side of 30 October, it is straightforward — one rate applies. But if you have disposals on both sides, Box 51 adjusts the computation so that each disposal is taxed at the rate applicable on its specific date.

How Box 51 works — worked example

What about unpriced disposals?

Some DeFi tokens have no reliable GBP price data — obscure governance tokens, LP positions, or tokens from small protocols. If ChainTax cannot find a price from DefiLlama or CoinGecko, the disposal is flagged with an amber notice and excluded from SA108 box totals.

The disposal still happened. You should either estimate the fair market value yourself (using exchange rates, DEX data, or other evidence) or discuss the valuation with your accountant. HMRC expects you to make a reasonable best estimate rather than omit the disposal entirely.

Where does crypto income go?

Not on SA108. This is one of the most common filing mistakes.

Crypto income — staking rewards, LP fee collections, airdrops, mining rewards, Coinbase Earn — is reported on your main SA100 return under "Other income" (or the Self Employment supplementary page if it qualifies as a trade). It is taxed at your marginal income tax rate: 20%, 40%, or 45%.

SA108 is exclusively for capital gains. Putting income in the CGT boxes means it gets taxed at the wrong rate and reported in the wrong section. For a complete breakdown, see DeFi income vs capital gains.

The annual exempt amount

For 2024/25, the annual exempt amount is £3,000. This is the amount of capital gains you can make before any CGT is due. It has been reduced significantly — it was £12,300 until 2022/23, then £6,000 in 2023/24.

The exempt amount is applied after netting gains against losses. If your net gain (Box 13.6) is below £3,000, no CGT is due. If it exceeds £3,000, you pay CGT on the excess.

Losses carried forward from previous years reduce your net gain down to the annual exempt amount — not below it. You cannot create a net loss position by over-applying carried forward losses. See crypto losses and tax relief for the full mechanics.

SA108 boxes at a glance

How ChainTax fills SA108 automatically

ChainTax computes every SA108 box directly from your transaction data. Here is what happens:

1. Classification: Every transaction is classified as a disposal, income, transfer, or cost basis change using 28 protocol-specific classifiers plus exchange CSV parsing.
2. HMRC matching: Same-day, B&B, and Section 104 rules are applied in the correct order to determine the cost basis for each disposal.
3. Box computation: Boxes 13.1–13.8 are derived from the matched disposals. Box 51 is computed for any year with a mid-year rate change.
4. Show Working: Every disposal has a detailed breakdown showing which matching rule was used, the S104 pool state before and after, the gain calculation, and the applicable CGT rate.

Unpriced disposals (where no reliable GBP price could be found) are excluded from the box totals and tracked separately. You can see exactly how many unpriced disposals exist and resolve them before filing.

For the full picture of how 2024/25 tax rules work, see the UK crypto tax 2024/25 complete guide.