One pool per token
Under the section 104 rules, every acquisition of the same token merges into a single pool holding two numbers: total units and total allowable cost, in sterling, fees included. Buy 1 BTC at £20,000 and later 1 BTC at £40,000, and you do not own a cheap coin and an expensive coin — you own a pool of 2 BTC at £60,000, average £30,000. A disposal takes its cost from the pool average, and reduces the pool proportionately.
If you learned crypto tax from American sources, unlearn the lot. There is no FIFO election, no specific identification, no per-lot holding clock and no long-term rate. The pool is the whole story — except for the two rules that come before it.
The matching order: same day, 30 days, then the pool
Every disposal must be matched in this exact order:
- Same-day rule. Tokens disposed of are first matched with tokens of the same type acquired the same day. All same-day trades in a token effectively composite into one.
- 30-day rule. Any remainder is matched with acquisitions of the same token in the following 30 days, earliest disposal first. The repurchase price — not the pool — becomes the cost basis for the matched portion.
- Section 104 pool. Only what survives both rules draws its cost from the pool average.
One disposal can be split across all three matching layers
Assume you dispose of 1.00 BTC. You acquire 0.20 BTC later the same day, then 0.30 BTC 14 days later. The remaining 0.50 BTC is matched to the Section 104 pool. The tax computation follows that matching order even if your exchange software labels the sale against a specific purchase lot.
Which matching rule owns the disposal?
| Fact | Treatment | Why it matters |
|---|---|---|
| Fungible tokens acquired the same day as disposal | Same-day rule first | Those units do not enter the Section 104 match for that disposal. |
| Same token acquired in following 30 days | 30-day rule after same-day matching | Repurchase cost can replace pooled cost for the matched quantity. |
| Quantity left after both rules | Section 104 pool | Uses the pooled allowable cost per unit. |
| Separately identifiable NFT | Not pooled under HMRC CRYPTO22200 | The share-style matching rules described here do not apply in the same way. |
Do not apply the pool mechanically until these facts are settled
- Whether the token is fungible or separately identifiable.
- Whether acquisitions occurred on the same day or within the following 30 days.
- Whether all wallets/platforms belonging to the same beneficial owner have been included in the token-level pool.
- Whether receipts such as staking rewards were brought into the computation at the correct sterling amount before later disposal.
Why the 30-day rule rewrites harvested losses
The rule exists to kill bed-and-breakfasting: selling to crystallise a loss and buying straight back. Where the US wash-sale statute currently misses digital assets (see our US §1091 note — the two systems could not differ more here), the UK rule catches them squarely. Sell 5 ETH from a £1,000-average pool at £800, and the £1,000 loss you expected exists only if you stay out of ETH for 30 days. Buy back on day 14 at £850 and the disposal is matched to that repurchase: your loss is £50 per coin against the buyback price, not £200 against the pool — and your pool cost quietly changes too.
The loss on your exchange screen is calculated by software that has never heard of section 104. The loss on your return is calculated by the matching order. They agree only by coincidence.
Where returns actually go wrong
Exchange CSVs and per-lot software. Tools built for US rules produce US answers. A UK computation needs pooling logic — per token, across every platform and wallet at once, because the pool is per asset, not per exchange.
Swaps forgotten as disposals. Token-for-token trades, stablecoin legs included, are disposals at sterling value on the day. Active traders generate hundreds of pool events they never mentally registered as "selling".
Income entering the pool wrong. Staking and mining rewards are income at sterling value on receipt — and that value is what enters the pool as cost. Skip the income step and the pool is wrong forever after.
Frequent trading around the 30-day window. High-frequency traders trigger cascades of same-day and 30-day matches; a manual spreadsheet almost never survives contact with them.
What this means for planning
Two honest consequences. First, loss harvesting works in the UK only with a genuine 30-day exit — the exposure gap is the price of the loss. Second, the pool average means a partial disposal always realises the blended history of every purchase, which cuts both ways and surprises people in both directions. Anyone with more than trivial activity should compute the pools before making disposal decisions in March, not discover them in January.
This page explains a computational rule. It is not advice on your return, and reading it creates no professional relationship.