CryptoLens/Free tools
HMRC · Section 104 · Free

Section 104 Pool Calculator (UK)

HMRC requires Section 104 weighted-average pooling for crypto — not FIFO, not LIFO. Paste a wallet and we apply the same-day rule, the 30-day bed-and-breakfast rule, then build the pool, automatically.

Calculate my pool — free See an example report
703
Wallets tracked
7
Holders signed up
8
Chains supported
  • Same-day rule applied first
  • 30-day bed-and-breakfast rule applied second
  • Section 104 pool with weighted-average cost basis
  • Multi-wallet aggregation across 8 chains
  • £3,000 annual exempt amount applied
  • SA108-ready PDF with disposal-by-disposal breakdown

What is Section 104 pooling?

HMRC's Section 104 rule means you can't pick which units of a token you sold for tax purposes. Instead, every unit of the same crypto goes into a single pool with a weighted-average cost basis — and disposals are deducted from that pool at the average cost. The catch: before the pool is touched, two priority rules apply. First, any units bought and sold on the same day are matched against each other. Second, units sold and re-bought within 30 days are matched against the rebuy (the "bed and breakfasting" rule, which exists to stop people from booking artificial losses). Most traders get this wrong. They use FIFO out of habit, or "specific identification" because that's what their exchange shows them. HMRC will reject both. CryptoLens applies all three rules in the correct order across every wallet you connect, every token you've ever held, every chain. The output is the exact set of figures that go on your SA108: total proceeds, total allowable cost, gain/loss per disposal, and the pool's end-of-year position carrying forward.

Frequently asked questions

What's the difference between Section 104, same-day, and 30-day rules?▾

Three priority rules in order: (1) Same-day — any tokens bought and sold on the same calendar day are matched 1:1. (2) 30-day bed-and-breakfast — tokens sold and re-bought within 30 days are matched against the rebuy at the rebuy price. (3) Section 104 — everything else uses the weighted-average pool. CryptoLens applies them in this exact order automatically.

Do I need a separate pool per chain?▾

No. HMRC treats all of your ETH as one pool, regardless of which chain (Ethereum mainnet, Arbitrum, Base, etc.) it's currently on. Bridging is not a disposal — it doesn't affect the pool. CryptoLens aggregates across all 8 supported chains automatically.

Does this work for memecoins and small-cap tokens?▾

Yes — the engine treats every token equally, including DeFi LP tokens, memecoins, and stablecoins. The cost-basis logic is identical regardless of market cap.

Can I see the maths, not just the totals?▾

Yes. The Pro PDF shows every disposal line-by-line: which rule matched (same-day / 30-day / Section 104), the cost basis used, the proceeds received, and the gain or loss. You can hand it to an accountant for sanity-checking.

Is this safe — do you store my keys?▾

We never ask for keys, seed phrases, or signatures. We read public on-chain data only. Wallet addresses you scan stay private to your account; we never publish or sell wallet data.

How much does the calculator cost?▾

The calculator itself is free forever — paste a wallet, see your pool numbers immediately. The Pro plan (£4.99/mo) adds the SA108-ready PDF export, signed disclosure document for HMRC, and unlimited wallet history.

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Calculate my pool — free