HMRC does not let you choose which coin you sold. Under section 104 of the Taxation of Chargeable Gains Act 1992, every unit of a given token you own sits in one pool carrying one running cost, and each disposal removes a proportional slice of that cost. The Cryptoassets Manual puts it plainly: pooling applies to shares and securities and also to “any other assets where they are of a nature to be dealt in without identifying the particular assets disposed of or acquired”.
So if you hold bitcoin, ether and solana you have three pools, each with its own allowable cost. Non-fungible tokens sit outside the system — HMRC treats them as “separately identifiable and so are not pooled and no matching rules are applied”. The pool follows beneficial ownership, not location: coins across two exchanges and a hardware wallet are one pool, not three.
How the pool actually moves
Take someone who bought ether three times. Two units in March 2024 for £4,200, three more in September 2024 for £8,400, and one in January 2025 for £2,600. The pool is now six units at a total cost of £15,200, an average of £2,533.33 each. The three prices they actually paid have stopped mattering.
In August 2025 they sell 2.5 units for £9,500. The cost that comes out is the same fraction of the pool as the units disposed of: £15,200 × 2.5 ÷ 6, which is £6,333.33. The gain is £9,500 less £6,333.33, or £3,166.67. The pool is left holding 3.5 units at a cost of £8,866.67 — still the same £2,533.33 average, because a proportional withdrawal cannot move it.
The annual exempt amount for 2025/26 is £3,000, so £166.67 is taxable. A basic-rate taxpayer pays 18%, or £30.00; someone above the basic rate band pays 24%, or £40.00. Which rate you land on depends on how much of your basic rate band your income has already used, so a gain can straddle both.
A proportional withdrawal cannot change the average cost of the pool. If your calculation shows the average moving after an ordinary sale, the calculation is wrong.
Two rules come first
The pool is the last place HMRC looks, not the first. Disposals are matched in a fixed order: same-day acquisitions first, then acquisitions in the following 30 days, and only what remains touches the pool. An order, not a menu.
The 30-day rule is the one that surprises people, and it does not always work against you. Say the same holder sells one unit for £3,900 on 10 June 2026 — a new tax year, with its own allowance — and buys one back for £3,500 on 20 June. That repurchase is inside the window, so the disposal is matched to it rather than to the pool. The gain is £3,900 less £3,500, or £400. Had the pool been used, the cost would have been £2,533.33 and the gain £1,366.67 — £966.67 more. The pool itself is untouched at 3.5 units and £8,866.67.
Reverse the price movement and it bites the other way. The point is that you do not get to decide; the dates decide.
| Pool after three acquisitions | 6 units, £15,200 |
| Average pooled cost | £2,533.33 per unit |
| Disposal of 2.5 units | £9,500 proceeds |
| Cost removed from pool | £6,333.33 |
| Gain | £3,166.67 |
| Pool after disposal | 3.5 units, £8,866.67 |
| Annual exempt amount, 2025/26 | £3,000 |
| Taxable gain | £166.67 |
| Tax at 18% / at 24% | £30.00 / £40.00 |
| 30-day matched disposal, 2026/27 | gain £400, not £1,366.67 |
Where it goes wrong
A common failure is a tax tool built for the United States applying first-in-first-out. Pooling is mandatory for fungible tokens, so there is no method to choose, and FIFO gives a materially different answer. A separate calculation per exchange fails for the same reason: one pool per token, across everything you beneficially own.
The second is forgetting that receipts enter the pool. Staking and mining rewards are taxed as income when received, and that same value becomes their acquisition cost in the pool. Leave them out and you understate your cost and overstate every later gain.
The third is treating a move between your own wallets as a disposal. HMRC is explicit that there is no disposal where you retain beneficial ownership throughout, so nothing enters or leaves the pool. Bridging to a different asset on another chain has its own rule, and the two are easily conflated.
HMRC expects the token type, date, direction, number of units, sterling value on the day and a running cumulative total — the pool, kept as you go. Exchanges close and delete histories; the obligation stays with you.
Pooling rule, matching order and NFT treatment quoted from HMRC's Cryptoassets Manual CRYPTO22200; disposals and wallet transfers from CRYPTO22100; record-keeping from CRYPTO10400. Annual exempt amount of £3,000 for 2024/25, 2025/26 and 2026/27 and the 18% and 24% rates from GOV.UK Capital Gains Tax rates and allowances; those rates apply to disposals on or after 30 October 2024 and are confirmed at Capital Gains Manual CG10245. Personal allowance of £12,570 and basic rate band of £37,700 for 2026/27 from GOV.UK rates and thresholds. All retrieved 31 August 2026. The arithmetic was computed for this article and rounded to the penny; HMRC's published examples round differently in places. HMRC does not state in terms that it rejects FIFO — that follows from pooling being mandatory, not from a published prohibition. Income tax figures are for England, Northern Ireland and Wales; Scottish bands differ and change which capital gains rate applies. The 30-day illustration is placed in 2026/27 so that it does not share the 2025/26 allowance used earlier. Nothing here is tax advice, and a position spanning several years or exchanges is worth putting in front of an accountant.