UK crypto CGT worked example — pooling and a swap explained
Quick answer: Example: buy 1 BTC at £20,000 and 1 BTC at £40,000 (pool average £30,000). Sell 1 BTC for £50,000 → gain £20,000 before allowance. Swapping BTC for ETH is a disposal at sterling market value — same maths. Deduct the £3,000 annual exempt amount if no other gains.
This worked example shows how HMRC pooling applies across multiple buys and a token swap. Numbers are simplified for teaching — use your own records for filing.
Example 1 — Two purchases, one sale
April 2025: buy 0.5 BTC for £15,000 (£30,000 per BTC). August 2025: buy 0.5 BTC for £25,000 (£50,000 per BTC). Pool: 1 BTC costing £40,000 — average £40,000 per BTC. March 2026: sell 1 BTC for £55,000. Gain = £55,000 − £40,000 = £15,000. After £3,000 annual exempt amount, taxable gain £12,000. At 18% basic / 24% higher rate on crypto gains depending on your income band.
Example 2 — Swap BTC for ETH
You dispose of BTC at sterling value on swap day. If 0.2 BTC worth £12,000 is swapped for ETH, BTC pool cost is deducted (pro-rata from pool). You acquire a new ETH pool at £12,000. No cash changes hands but CGT still applies on any BTC gain.
Example 3 — 30-day rule
You sell 1 ETH on 1 June for £3,000 (pool cost £2,000, gain £1,000). You buy 1 ETH back on 15 June for £2,800. The 30-day rule matches the sale to the rebuy — adjusting gain calculation before the pool. HMRC applies same-day rule first, then 30-day, then Section 104 pool.
Putting figures on SA108
Sum all disposal proceeds for the tax year → box 13.2. Sum allowable costs → 13.3. Net gains → 13.4. Losses → 13.5. See our SA108 guide for the full checklist.
Frequently asked questions
Do I use the price on the exchange or HMRC rate?+
Use a consistent sterling market price at transaction time. Many filers use exchange prices; HMRC publishes rates for some conversions.