The UK rules, section 104, same-day, and the 30-day rule

Checked against app build 2026-08-26-01-41 on 2026-08-26.

Every UK crypto disposal answers one question, which purchase is this sale matched against. The answer decides your cost basis, so it decides your gain, and UK law answers it with three rules applied in strict order.

What order do the UK crypto matching rules apply in?

Same-day first, the following 30 days second, the section 104 pool last. The order is not a preference, it is the order the legislation sets, and each rule only sees the units the rule above it did not already consume.

Work a disposal through it like this. Take the quantity you sold. Match as much of it as you can against acquisitions on the same day, section 105 TCGA 1992. Match whatever is left against acquisitions in the 30 days after the disposal, section 106A. Whatever remains after both comes out of the section 104 pool at its average cost, section 104. One disposal can split across two or even three of the rules, and when it does, each part carries its own cost, which is why the app shows the parts separately rather than blending them into one average.

The rules apply per asset. Your ETH pool and your BTC pool never touch, and a same-day buy of one asset does nothing to a disposal of another.

What is the section 104 pool and how is its cost calculated?

The section 104 pool is a single running holding per asset, carrying a total quantity and a total cost, from which every unmatched disposal takes its cost at the pool’s average.

Acquisitions add to both figures. A disposal removes units in proportion, the cost coming out is the pool’s total cost multiplied by the fraction of the pool’s units disposed of, and the pool’s average cost per unit is unchanged by the disposal itself. This is the rule that does the everyday work, most disposals for most people are pure pool disposals, and it is why a partial import produces confidently wrong numbers, a pool missing its earliest acquisitions prices every later sale too high.

Two consequences catch people out. The pool is per owner, not per wallet or per exchange, so coins bought on Coinbase and sold on Kraken sit in the same pool, which is why per-wallet balancing is a bookkeeping check rather than a tax rule. And the pool has no memory of individual lots, so there is no choosing which coins you sold, the average is the answer whether it flatters you or not.

The long version, with worked arithmetic, is in the section 104 pool post.

What is the same-day rule for crypto disposals?

Anything you acquired on the same calendar day as the disposal is matched against it first, ahead of everything else, under section 105 TCGA 1992.

All acquisitions of that asset on that day are treated as one, so an active day of many small buys behaves as a single acquisition at the day’s aggregate cost, and the disposal matches against that before the pool is touched. Day traders meet this rule constantly without noticing, because buying and selling the same asset inside one day means the pool often never enters the calculation at all. If the day’s disposals exceed the day’s acquisitions, only the excess falls through to the next rule.

Note that a day here is a calendar day in the UK, not a rolling 24 hours, and it is the day of the disposal that matters, not the day the exchange settled or reported it.

What is the 30-day rule, and when is a rebuy outside the window?

Anything you buy back within the 30 days after a disposal is matched against that disposal, before the pool, under section 106A TCGA 1992. It exists to stop the bed and breakfast trade, selling purely to crystallise a loss and buying straight back into the same position.

The boundary is precise, and it is the part people get wrong. The first clear day is day 31. Sell on 13 August and a rebuy is only outside the window from 13 September. A rebuy on 12 September is matched against the August sale, and the loss you were counting on does not arise, it goes into the cost of the units you just bought instead. Acquisitions inside the window match in date order, earliest first, and only the quantity actually reacquired is affected, the rest of the disposal carries on to the pool.

This rule is also the one active traders trigger by accident, because a crypto-to-crypto swap is a disposal of one asset and an acquisition of the other, both sides count. Swap ETH out and back over a fortnight and you have triggered section 106A without ever touching sterling.

The full worked example, including what happens to the loss you were trying to bank, is in the bed and breakfast rule post, and the bed and breakfast rule calculator will date the window for a disposal you are considering.

How do I check gains.tax applied the right rule?

Every disposal in your reports names which of the three rules matched it, with the section cited, and you read that block in reading citations.

Beyond the per-disposal working, each rule’s implementation is tested against HMRC’s own worked examples, and those tests are published with their results on the verification page. If you want a figure before you commit to a trade, the capital gains tax calculator runs the same matching order on a handful of transactions.