Section 104 calculator
A free UK Capital Gains Tax calculator that applies HMRC's share identification rules properly — same-day, the 30-day bed & breakfast rule, then the Section 104 pool — and shows you which rule produced each figure. It runs entirely in your browser; nothing you type is uploaded or stored.
It's the same calculation engine inside Pooled, tested against HMRC's own worked examples.
How the matching works
- Same-day rule. Sales are first matched against purchases of the same asset on the same day, all same-day buys treated as one acquisition.
- 30-day (bed & breakfast) rule. Anything left is matched against purchases made in the 30 days after the sale, earliest first. This is why selling to realise a loss and rebuying two weeks later usually doesn't work.
- Section 104 pool. Whatever remains draws its cost from your pooled average: pool cost × (quantity sold ÷ pool quantity).
Try the “Load bed & breakfast example” button above to watch rule 2 change an expected £760 loss into a much smaller one.
Questions
What is a Section 104 holding?
For CGT, HMRC treats all your shares (or tokens) of the same class as a single pool — the Section 104 holding — with one combined allowable cost. When you sell, cost leaves the pool in proportion to the quantity sold.
What order are the share identification rules applied in?
Strictly: first the same-day rule, then acquisitions in the 30 days after the disposal (the bed-and-breakfast rule, matched first-in first-out), and only then the Section 104 pool.
Why did my loss shrink when I rebought within 30 days?
The 30-day rule matches your sale to the repurchase instead of your original pool, so the cost used is the rebuy price — this is exactly the sell-and-rebuy loss harvesting the rule was designed to defeat.
Do fees and stamp duty count?
Yes. Purchase fees and stamp duty add to allowable cost; sale fees reduce proceeds. This calculator accepts a fees column; the full product also parses stamp duty from broker CSVs.
Does this work for crypto?
The same pooling rules apply to cryptoassets per token under HMRC’s Cryptoassets Manual. This free calculator handles any asset you enter manually; Pooled itself imports exchange CSVs and treats swaps as disposals correctly.
This calculator forgets. Pooled remembers.
Persistent Section 104 pools across tax years, broker and exchange CSV import, crypto in the same pool set, ISA allowance tracking and Self Assessment figures.