Section 104 Pool for Crypto: Same-Day and 30-Day Rules

HMRC does not let you choose which bitcoin you sold. Instead, every type of token you own is treated as one section 104 pool with a single average cost, and each disposal is matched against your purchases in a fixed order: tokens bought on the same day first, then tokens bought in the 30 days after the sale, and only then the pool. These rules come from share taxation and apply to crypto in exactly the same way. This guide explains each rule with worked examples and shows what happens to the pool at every step, for 2025/26 (return due 31 January 2027) and earlier years alike.

The examples use the same figures our calculator's tests are built on, and the last section points to HMRC's own examples so you can check any tool against them.

What is a section 104 pool?

A section 104 pool is the total quantity of one type of token you hold and the total allowable cost of acquiring it, treated as a single asset. HMRC's Cryptoassets Manual (CRYPTO22200) explains that pooling under section 104 of the Taxation of Chargeable Gains Act 1992 applies to tokens because they are "dealt in without identifying the particular tokens disposed of or acquired". Each type of token has its own pool: someone holding bitcoin, ether and litecoin has three pools, each with its own pooled allowable cost.

Every time you acquire tokens (buying, receiving income or an airdrop, receiving a gift), you add the quantity and its sterling cost to the pool. Every time you dispose of tokens, you remove a proportionate share of the pooled cost: pooled cost multiplied by the quantity disposed of, divided by the quantity in the pool. What remains is the cost for future disposals.

GOV.UK's example. You buy 100 tokens for £2 each (£200), then 300 more for £1 each (£300). Your pool is 400 tokens with a cost of £500 — an average of £1.25. You sell 200 tokens. The allowable cost is £250 (£500 × 200 ÷ 400), leaving 200 tokens and £250 in the pool.

The pool is per token type, not per account, so bitcoin on Coinbase and bitcoin in a hardware wallet are one pool. Transfers between your own wallets never change it.

What is the same-day rule?

If you acquire and dispose of the same type of token on the same day, the tokens acquired are matched against the tokens disposed of first, and neither side touches the pool to the extent they match. HMRC's manual (CRYPTO22200, citing section 105) says all tokens acquired that day are treated as a single acquisition and all tokens disposed of that day as a single disposal, so you only ever need one computation per token per day.

If you bought more than you sold, the excess is then considered for the 30-day rule and, failing that, goes into the pool. If you sold more than you bought, the excess is considered for the 30-day rule and, failing that, comes out of the pool.

Example. You hold 1.5 BTC in your pool costing £52,500. On 10 June 2025 you buy 0.2 BTC for £16,000 and sell 0.5 BTC for £45,000. The same-day rule matches 0.2 BTC of the sale against the purchase: proceeds £18,000 (0.2 ÷ 0.5 of £45,000), cost £16,000, gain £2,000. The £16,000 never enters the pool. The remaining 0.3 BTC of the sale moves on to the next rule.

Day means the UK calendar day. A trade at 23:30 UTC in July happens at 00:30 British Summer Time the next day, so tools need to convert timestamps to UK time before matching.

What is the 30-day rule (bed and breakfasting)?

If you dispose of tokens and acquire the same type within the next 30 days, the acquisition is matched to the earlier disposal instead of going into the pool. HMRC's manual (CRYPTO22200, citing section 106A) sets out the order: apply the same-day rule first; then match acquisitions in the following 30 days to disposals, earliest disposal first; anything acquired beyond what was disposed of goes into the pool.

The rule exists to stop "bed and breakfasting" — selling an asset to realise a loss or use up the annual exempt amount, then buying it straight back. Under the rule, your gain or loss is the difference between the sale proceeds and the cost of the tokens you bought back, not the cost in your pool. The rule looks forward only: buying before you sell is just a normal pool acquisition. HS284, HMRC's helpsheet on shares, adds that the 30-day rule does not apply if you were not UK resident when you made the later acquisition.

Continuing the example. On 25 June 2025 you buy 0.1 BTC for £8,500 — within 30 days of the 10 June sale. So 0.1 BTC of the sale is matched to it: proceeds £9,000, cost £8,500, gain £500. The £8,500 never enters the pool either. That leaves 0.2 BTC of the sale unmatched.

How do the three rules combine?

Whatever is left after the same-day and 30-day rules comes out of the pool at average cost. Putting the June example together:

And the pool:

One disposal, three computations, one gain of £13,500 to report. The 25 June purchase is fully used up by the 30-day match and never affects your average cost.

If you had instead bought 0.4 BTC on 25 June, only 0.3 BTC would be matched to the sale (0.5 sold minus 0.2 same-day) and the other 0.1 BTC, with its share of the cost, would go into the pool — exactly what HMRC shows happening in example 3 of its manual (CRYPTO22253), where the unmatched 200 tokens and their £60 of cost go into Rachel's pool.

How does a crypto-to-crypto swap affect the pools?

A swap is a disposal from one pool and an acquisition into another, both at the sterling value of the transaction. HMRC's example 7 (CRYPTO22257) notes that "a transaction may simultaneously affect two section 104 pools" and that "every acquisition will involve a corresponding disposal that needs to be computed".

Example. After the June transactions your pool is 1.3 BTC costing £45,500. On 1 August 2025 you swap 0.3 BTC for 10 ETH worth £27,000. There are no same-day or 30-day BTC purchases, so the disposal comes from the pool: cost £45,500 × 0.3 ÷ 1.3 = £10,500, gain £16,500. Your BTC pool is now 1.0 BTC costing £35,000. Your ETH pool opens with 10 ETH costing £27,000.

HMRC's manual (CRYPTO23000) requires a consistent method for the sterling value when neither side of the swap is in pounds. Our approach is to value the transaction once — preferring the asset with the more reliable sterling price — and use that figure for both the disposal and the acquisition, so the two pools always agree.

How do fees fit in?

Fees paid in pounds are simple: on a purchase they add to the cost going into the pool; on a sale they reduce the proceeds. HMRC's list of allowable costs (CRYPTO22150) includes transaction fees, and for a crypto-to-crypto swap it accepts splitting the fee 50/50 between the disposal and the acquisition.

Fees paid in crypto are a little more involved. HMRC's manual (CRYPTO22280) treats the fee tokens as a disposal in their own right at market value, with the same value counting as an allowable cost of the main transaction. Because the fee and the sale usually happen on the same day in the same token, the same-day rule combines them into one computation. HMRC says it will accept a simplified approach as long as the resulting gain or loss is correct, which is what most calculators do.

What goes into the pool besides purchases?

Anything you acquire. Staking, mining and lending rewards enter the pool at their sterling value when received — the same value you have already paid Income Tax on — so the gain on a later disposal is only the growth since receipt. Airdropped tokens go into their own pool, or into your existing pool of that token if you already hold some (CRYPTO22350). Tokens received as a gift from your spouse or civil partner come in at their original cost; gifts from anyone else come in at market value.

How can you check a calculator against HMRC's examples?

HMRC publishes seven worked examples in its Cryptoassets Manual (CRYPTO22251 to CRYPTO22257), covering a basic pool disposal, the same-day rule, the 30-day rule, each rule's interaction with the pool, all three together, and a crypto-to-crypto exchange. Each example lists the transactions and the expected gains and pool balances. If a tool cannot reproduce them, it is not applying UK rules.

Example 6 (CRYPTO22256) is the best single test. Gulferaz holds 100,000 token F costing £300,000; over a week he buys 10,000 for £45,000 and sells 30,000 for £150,000 on 31 July, sells 20,000 for £100,000 on 5 August, buys 50,000 for £225,000 on 6 August and sells 100,000 for £150,000 on 7 August. HMRC's answers: a £15,000 gain on 31 July (same-day plus 30-day matching), a £10,000 gain on 5 August (30-day), 10,000 unmatched tokens entering the pool on 6 August, and a £163,637 loss on 7 August from the pool. Our engine's test suite includes every one of these examples, and every disposal in your results shows which rule produced it so you can check the workings yourself.

What if your records do not go back far enough?

The pool only works if it starts from zero. If your first export shows a sale of bitcoin you bought on an exchange that has since closed, the pool goes negative and the calculation is wrong. HMRC expects you to keep records of every acquisition (CRYPTO10400), so the fix is to find the missing history — old emails, bank statements, wallet activity — or, failing that, to add an opening balance with your best evidence of the quantity and cost and note the assumption. Our import review flags negative balances and lets you add an opening balance rather than silently guessing.

Working through several back years is common for people responding to an HMRC letter. The letter helper will tell you which years to calculate.

Sources

• Check if you need to pay tax when you sell cryptoassets — GOV.UK
• CRYPTO22200 — Pooling — HMRC Cryptoassets Manual
• CRYPTO22250 — Pooling examples: contents — HMRC Cryptoassets Manual
• CRYPTO22253 — Example 3: application of the 30 day rule — HMRC Cryptoassets Manual
• CRYPTO22256 — Example 6: same day rule, 30 day rule and section 104 pool — HMRC Cryptoassets Manual
• CRYPTO22257 — Example 7: crypto-to-crypto exchange — HMRC Cryptoassets Manual
• CRYPTO22280 — Fees satisfied in tokens — HMRC Cryptoassets Manual
• CRYPTO22150 — Allowable expenses — HMRC Cryptoassets Manual
• HS284 Shares and Capital Gains Tax (2026) — GOV.UK
• CRYPTO22350 — Airdrops (Capital Gains Tax) — HMRC Cryptoassets Manual