Section 104 pooling explained
You cannot pick which specific coins you sold. HMRC pools every unit of the same crypto asset into one holding — the Section 104 pool — with a single running average cost. Here is how it works.
What a Section 104 pool is
A Section 104 pool (TCGA 1992 s.104) holds every unit of a single asset that has not been matched by the same-day or 30-day rules. The pool tracks two numbers: the total quantity of coins held and the total pooled cost, including dealing fees.
When you buy more of the same coin, both numbers go up and the average cost recalculates. When you sell, you remove a proportional slice of the pooled cost — never a specific purchase.
A worked example
Suppose you buy 1 ETH for £1,000, then later buy 1 ETH for £3,000. Your pool is 2 ETH at a total cost of £4,000 — an average of £2,000 each.
If you then sell 1 ETH for £2,500, your allowable cost is £2,000 (one unit’s share of the pool), so your gain is £500. The pool drops to 1 ETH and £2,000 cost remaining. It does not matter that you originally paid £1,000 or £3,000 for any particular coin.
Why fees matter
Acquisition fees increase the pooled cost and disposal fees reduce the proceeds, both lowering your gain (TCGA 1992 s.38). Leaving them out overstates your tax. Maneta folds reliably-recorded exchange and gas fees into the pool automatically.