Section 104
pool calculator
Enter your buys and sells. This works out the Section 104 pool, applies HMRC's matching order, and shows the chargeable gain on each disposal with the arithmetic behind it.
Input — your trades
| Date | Security | Buy / sell | Quantity | Amount (£) | Fees (£) | Remove |
|---|
Amount is the value of the trade before fees. Fees are dealing costs — commission, stamp duty or SDRT, the PTM levy.
Result
Enter your trades and press Calculate. If you want to see what it does first, load HMRC’s own worked example from HS284 — the figures it produces are the ones HMRC publishes.
The calculation runs inside this browser tab, in the same engine the desktop app uses, compiled to WebAssembly. Your figures are not sent anywhere — there is no server here that receives them, and the page loads nothing from any other domain.
What the Section 104 pool is
When you buy the same share more than once, HMRC does not let you choose which ones you sold. Almost all of your holding goes into a single pool — the Section 104 holding — which carries one running total of shares and one running total of allowable cost. Sell part of it and you take out a proportional slice of that cost.
This is why "I bought at £2 and sold at £5" is rarely the answer HMRC wants. If you also bought at £4.10 in between, both purchases are in the pool, and the cost you can deduct is the pool's average — not the price of whichever shares you had in mind.
The matching order, which comes first
Before anything reaches the pool, two rules take precedence, in this order:
- Same day — shares bought on the same day as the sale are matched against it first, at that day's average cost (TCGA 1992 s.105).
- The following 30 days — shares bought in the 30 days after the sale are matched next, earliest first. This is the "bed and breakfast" rule (s.106A), and it exists to stop a disposal being made purely to realise a loss.
- The Section 104 pool — whatever is left is matched against the pool at its average cost.
The calculator applies all three. Where a disposal is split across more than one rule, each slice is shown separately, because that is how the working has to read if anyone is going to check it.
A worked example you can check
This is HMRC's own Example 3 from helpsheet HS284. Load it with the button above and you will get these numbers.
HMRC publishes £329 and £300. So does this calculator, and so does the desktop app — the same engine computes all three, and how that is tested is written down.
What this calculator does not do
- It does not apply the annual exempt amount, or your income, or losses brought forward. It gives you the gain, not the tax.
- It handles buys and sells only. Share splits, consolidations, accumulation-fund cost adjustments and negligible-value claims all change a pool, and all of them are in the desktop app.
- It pools per security, per this page. HMRC pools a share across everything you hold of it — so if the same holding sits at two brokers, both sides belong in one calculation. That is what GainPool is for.
- It is not tax advice, and it is not a filing service.
Doing this for a real tax return? Typing a year of trades into a web form is not how anyone should spend an evening, and the moment you hold the same share at two brokers, this page cannot give you the right answer at all.
Your whole year,
from the files you already have.
Multiple brokers merged into one pool per security, SA108 box numbers, and a computation pack that shows every figure's working. £12, once.