Buy a token and sell the same token on the same day, and HMRC does not let those coins near your section 104 pool. The disposal is matched against that day’s acquisition first, at that day’s cost, and the pool sits untouched. It is the first of the matching rules, it applies before everything else, and it is the step that spreadsheets skip.
The order is set out at CG51555: same-day acquisitions first, then acquisitions in the 30 days following, then the section 104 pool, then — rarely — later acquisitions, earliest first. HMRC’s Cryptoassets Manual states the rule at CRYPTO22200, citing section 105 of the Taxation of Chargeable Gains Act 1992.
There is no election. CG51555 applies the rules “even if the particular shares were identified in some other way when they were disposed of”. You do not choose which coins you sold, or the method.
The whole day becomes one transaction
Section 105(1)(a) treats every acquisition of a token type on a day as a single acquisition, and every disposal of it that day as a single disposal. Both sides aggregate. The time of day is irrelevant: HMRC’s own worked example at CRYPTO22252 deliberately runs a morning sale, an afternoon purchase and an evening sale, and collapses all three into one computation.
HMRC frames it as administrative — one computation per token type per calendar day. It is a simplification that changes the tax.
You are not matching trades in time order. You are collapsing a day into one buy and one sell, then matching those.
What it does to the number
Suppose a pool of 40 units acquired for £56,000, an average of £1,400 each. On one day you buy 12 units for £21,600 and sell 12 units for £26,400. The prices here are illustrative, not market data.
The same-day rule matches all 12 disposed against all 12 acquired. The cost is the £21,600 you actually paid that day, so the gain is £4,800, and the pool finishes exactly where it started: 40 units, £56,000. Append the purchase to the pool instead, as most exported spreadsheets do, and you get a pool of 52 units at £77,600, a cost of £17,907.69 for the 12 sold, and a gain of £8,492.31.
| Pool before | 40 units, £56,000 |
| Same-day purchase | 12 units, £21,600 |
| Same-day disposal | 12 units, £26,400 |
| Gain, same-day rule applied | £4,800 |
| Gain, purchase wrongly pooled | £8,492.31 |
| Pool after, rule applied | 40 units, £56,000 |
| Pool after, wrongly pooled | 40 units, £59,692.31 |
| Annual exempt amount | £3,000 |
| CGT at £46,000 income, rule applied | £324.00 |
| CGT at £46,000 income, wrongly pooled | £1,061.95 |
On an income of £46,000 the personal allowance leaves £33,430 taxable, so £4,270 of the £37,700 basic rate band is unused. The correct gain less the £3,000 allowance is £1,800, all inside that headroom at 18 per cent: £324. The pooled version leaves £5,492.31 taxable — £4,270 at 18 per cent and £1,222.31 at 24 — or £1,061.95. Same trades, £737.95 of difference.
It cuts both ways
Nothing about this rule is a shelter. It matched a same-day cost that happened to be higher than the pool average, so it reduced the gain. Buy cheaper than your pool average on the day you sell and the rule works in the other direction, producing a larger gain than pooling would. There is no motive test in section 105 and no way to switch it off.
The pool error also does not wash out. In the example above the wrongly pooled version leaves £3,692.31 of extra cost sitting in the pool, which understates every future gain until the pool is emptied. You pay too much now and too little later, across years with different allowances.
What HMRC will not accept
Not FIFO, and not LIFO. Not choosing which coins you sold. Not a separate pool per exchange or per wallet — pooling is per person, per token type, so a buy on one venue matches a sale on another the same day. Not the gain figure from an exchange’s own report: GOV.UK says plainly that “they’re not tax calculations” and “will not keep track of your pooled costs”. And not for non-fungible tokens, where CRYPTO22200 says no matching rules apply at all.
One more: a same-day swap out of a token and back into it is a disposal and an acquisition even though no sterling moved. The rule fires.
Sources, all retrieved 7 September 2026: HMRC Cryptoassets Manual CRYPTO22200 and worked example CRYPTO22252; HMRC Capital Gains Manual CG51555 and CG51560; TCGA 1992 sections 105 and 1I via legislation.gov.uk; GOV.UK “Check if you need to pay tax when you sell cryptoassets”, Capital Gains Tax rates and Income Tax rates. Figures used, all 2026/27 and all fetched on the day: annual exempt amount £3,000, CGT 18 and 24 per cent, basic rate band £37,700, personal allowance £12,570. Token prices in the worked example are illustrative, not market data. Contested: helpsheet HS284 is labelled 2026 but states internally that it covers 2025/26, so it is cited only for the matching order, which is not year-sensitive; CRYPTO22200 omits both the fourth matching step and the residence condition CG51555 and CG51560 apply to the 30-day rule. Scottish taxpayers pay Scottish income tax rates, but GOV.UK applies the £37,700 UK band to the CGT split with no Scotland caveat and we found no page stating that in terms. Not a recommendation to buy, sell or hold anything, and not tax advice.