Crypto and the SA108 Capital Gains Summary

In the UK, crypto capital gains are reported on the SA108 Capital Gains Summary, the supplementary page that goes with your SA100 Self Assessment return. CryptaTax produces your SA108-ready figures from your transaction history.

General information, not tax advice. Verify against current GOV.UK / HMRC guidance or a tax professional.

What is the SA108?

The SA108 is the Capital Gains Summary, filed alongside the main SA100 Self Assessment return. It's where you report disposals of chargeable assets, including crypto, which HMRC treats as property, with a dedicated cryptoassets section. You enter your disposals, proceeds, costs, and the resulting gains or losses, and apply the annual exempt amount (£3,000 for 2025/26).

How crypto goes on the SA108

For each token, you work out gains using HMRC's Section 104 pooling (an averaged cost), together with the same-day and 30-day matching rules. Those gains and losses are then reported in the cryptoassets section of the SA108. Two reporting triggers to remember:

• You must report if your net gains exceed the £3,000 allowance, or
• if your total disposal proceeds exceed £50,000 in the tax year, even if no tax is due.

The exact SA108 cryptoassets box numbers change by tax year, CryptaTax maps your figures to the current year's boxes. → Crypto tax in the UK →

Generate your SA108 figures with CryptaTax

Import your exchanges and wallets, and CryptaTax applies Section 104 pooling plus the same-day and 30-day rules automatically, then produces SA108-ready capital gains figures, and flags the £3,000 and £50,000 thresholds, ready for your Self Assessment or your accountant.

→ Import your exchanges & wallets →

Where the SA108 sits in your Self Assessment

It helps to see the SA108 in context. It is a supplementary page, not a standalone return: it travels with the main SA100 Self Assessment, carrying the detail of your chargeable gains so the main return can account for them. HMRC treats crypto as property, which makes disposing of it a chargeable event in the same family as selling shares or other assets, and the cryptoassets section of the SA108 is where those disposals are summarised. Understanding that hierarchy matters: the SA108 is about the capital side of your crypto, and earned crypto income belongs elsewhere on the return entirely.

You will generally need an SA108 for crypto if you disposed of cryptoassets during the tax year and your situation crosses HMRC's reporting triggers, and a disposal, as always, is broader than cashing out to pounds. Selling for fiat, swapping one token for another, and spending crypto are all disposals. Simply buying and holding, or moving your own coins between your wallets, is not. For how the UK rules fit together overall, see crypto tax in the UK →.

How the SA108 figures are produced, pooling done right

The UK's defining feature is Section 104 pooling: rather than tracking each coin individually, holdings of the same token are merged into a pool with a single averaged cost, and disposals draw from that average. Layered on top are the same-day rule and the 30-day (bed-and-breakfasting) rule, which match certain disposals to acquisitions made around the same time before the pool is touched. Getting these interactions right by hand is genuinely difficult, and it is where most manual UK crypto calculations go wrong.

Accurate pooled figures depend on the same disciplined handling of your full history that any crypto reporting needs, applied through the UK's specific matching order.

• Complete history across every wallet and exchange, back to your first acquisition, the pool's averaged cost is only correct if every acquisition that fed it is included.
• Matched transfers, so moving coins between your own wallets is recognised as a transfer and does not disturb the pool or invent a disposal.
• Cost basis that follows the coins off exchanges and into self-custody, so a later disposal is measured against the right pooled cost rather than nothing.
• Correct application of the same-day and 30-day rules before the Section 104 pool, in the order HMRC requires, so the matched portions are calculated properly.

The exact boxes in the cryptoassets section, and the allowances and thresholds that decide whether and how you report, change from one tax year to the next. For those specifics, always rely on the current GOV.UK / HMRC guidance rather than last year's figures. Understand the underlying mechanics at cost basis →.

How the SA108 relates to your other reports

The SA108 is the UK destination for the capital side of your crypto, the same underlying disposals that a country-neutral capital gains report → describes. Think of the capital gains report as the raw, complete picture and the SA108 as its UK-specific presentation, with Section 104 pooling applied. Sitting apart from it is your crypto income, staking, mining, airdrops, rewards, which is taxed when received and reported in a different part of the Self Assessment, captured by the income report →. The US equivalent of the SA108's role is Schedule D →, if you are comparing across jurisdictions. Browse everything at crypto tax reports & forms →.

Common mistakes UK crypto filers make

• Assuming nothing to report because they never cashed out to pounds. Crypto-to-crypto swaps and spending are disposals, and the proceeds-based reporting trigger can apply even with no tax due.
• Pooling incorrectly, or not at all. Treating each coin as a separate lot, or ignoring the same-day and 30-day rules, produces gains that do not match HMRC's method.
• Counting wallet transfers as disposals. Without matched records, moving your own coins can be misread as a sale, creating a gain that is not real.
• Forgetting losses. Reporting allowable losses can reduce your gains and be carried forward; leaving them off discards relief you are entitled to.
• Dropping allowable costs. Transaction fees and certain costs adjust the gain; ignoring them overstates what is chargeable.
• Rebuilding the year from incomplete exports. Reconstructing pooled history after an exchange has closed is where errors multiply, keep records as you go.

Record-keeping for the SA108

HMRC expects you to be able to substantiate your figures, so keep, for every token, the date and sterling cost of each acquisition including fees, the date and sterling proceeds of each disposal, and the wallet addresses and accounts involved so transfers can be matched rather than counted as sales. Because pooling averages across acquisitions, complete records back to your first purchase are what make the pooled cost correct. Organise them by venue and keep platform statements, so the same-day and 30-day matching can be reconstructed if asked.

Amending a prior year

If you realise a previous Self Assessment understated or omitted crypto gains, a missing exchange, an unreported swap, a transfer treated as a sale, it should be put right. There are time limits and a defined process for correcting a UK return, and these are set out in current GOV.UK / HMRC guidance, so check there for the specifics that apply to your year. The crypto essential is to rebuild the affected year from a complete history, with Section 104 pooling and the matching rules applied correctly, so the revised SA108 figures are right. Re-running your full history in CryptaTax for that year produces those corrected figures.

How CryptaTax produces your SA108 figures

CryptaTax does the UK-specific heavy lifting. Connect your exchanges and wallets, and it ingests your full history, links self-transfers, carries cost basis with the coins, and applies Section 104 pooling together with the same-day and 30-day rules in the order HMRC requires, then produces SA108-ready capital gains figures for the current tax year, with the transactions visible behind every number. You can hand the result to your accountant or use it for your own Self Assessment, and trace any figure back to the disposals that produced it. Connect your accounts at integrations →.

The matching rules in plain terms

The part of UK crypto reporting that trips people up most is the order in which disposals are matched to acquisitions. Rather than going straight to the Section 104 pool, HMRC's method first looks at acquisitions made on the same day as a disposal, then at acquisitions made in the following short window covered by the 30-day rule, and only then draws from the averaged pool for whatever remains. The 30-day rule exists to stop a particular manoeuvre, selling to crystallise a loss and immediately buying back, from working as neatly as it otherwise would, by matching the repurchase to the sale instead of to the pool.

For an active crypto trader, these rules interact constantly: a single day can contain buys and sells of the same token that have to be matched in the right sequence before the pool is even consulted. Doing this by hand across a year of activity is where manual calculations most often go wrong, and the errors compound because each mis-match shifts the pooled cost for everything that follows. CryptaTax applies the same-day rule, the 30-day rule, and Section 104 pooling in HMRC's required order automatically, so the matched portions and the pooled portions are each calculated correctly. The precise wording and any year-specific detail should always be taken from current GOV.UK / HMRC guidance.

More questions about crypto and the SA108

Do crypto-to-crypto swaps go on the SA108?

Yes, HMRC treats swapping one token for another as a disposal of the first at its sterling value, so the resulting gain or loss belongs in the cryptoassets section, even though no pounds changed hands. The report treats swaps as disposals accordingly. Confirm against current HMRC guidance for your year.

What is the point of Section 104 pooling?

It is HMRC's method for working out the cost of crypto you dispose of when you hold several acquisitions of the same token. Instead of identifying specific coins, you average the cost across the pool and draw from that average on disposal, subject first to the same-day and 30-day matching rules. CryptaTax applies all of this automatically.

Can I file the SA108 myself with these figures?

Yes. The SA108-ready figures are designed to drop into your Self Assessment, and they come with the full transaction detail behind them so you can review the working first. Many filers complete their own return; others pass the figures to an accountant. The exact boxes for the current year should be taken from the current form and HMRC guidance.

Do reporting losses on the SA108 help me?

Reporting allowable losses can reduce your chargeable gains and, where the rules allow, be carried forward to later years. Leaving losses off the return forfeits that relief. How losses are claimed and carried forward is set out in current HMRC guidance, so check there for the specifics.

Getting the figures behind your SA108 right

Whatever the form asks for, the figures behind it come from the same place: a reconciled record of every disposal, with matched transfers, consistent cost basis and the relevant matching rules applied in order. Get that underlying record right and filling in the form is mechanical; get it wrong and no amount of care with the boxes will rescue the totals. CryptaTax produces the reconciled figures so you can complete the form with confidence, see the cost basis guide → and UK crypto tax → for the rules that sit underneath.