The same-day rule, explained
Before your pool (all your shares of one company, blended into a single average cost) ever gets a look-in, HMRC applies two priority rules. The first — and simplest — is the same-day rule: if you buy and sell the same share on the same calendar day, those trades are matched to each other first. This guide works through a simple case — one buy and one sell on the same day — so you can see clearly how the rule changes which cost is used.
When you sell shares, HMRC has to decide which of your purchases the sold shares came from — because that sets the cost. It always works through three rules in this fixed order, top to bottom, until the whole sale is accounted for:
- Same-day rule — this page — shares of the same company you bought on the very day you sold.
- Bed & breakfast (30-day) rule — shares you bought back in the 30 days after the sale.
- Section 104 pool — everything left over, at your blended average cost.
The idea in one sentence
Shares bought and sold on the same day are paired off against each other — not against your older pooled shares — so the cost of the sale is what you paid that day, not your historical average. (Your pool is all the shares of one company you've bought, blended into a single average cost per share — the subject of the Section 104 guide.)
What if there are several same-day trades?
HMRC treats all of one day's buys of a share as a single combined purchase (at their average price), and all of that day's sells as a single combined sale. Then it matches the two. So multiple buys and multiple sells on the same day collapse into one tidy “same-day” match — which is exactly the case we'll work through. This combining step is called merging, and it always happens before matching.
Our worked example
Our investor already holds 500 Tesco (TSCO) shares from last year at £3.00. On 15 May 2026 they buy 200 more at £4.00 and sell 200 at £4.50 — both on the same day. Watch which shares the sale gets matched against. You can load this example into the calculator at the bottom of the page.
| Date | Type | Symbol | Qty | Price | Fees | Ccy | FX Rate | GBP Value |
|---|---|---|---|---|---|---|---|---|
| 10 Jan 2025 | Buy | TSCO | 500 | £3.00 | £0.00 | GBP | 1.00 | £1,500.00 |
| 15 May 2026 | Buy | TSCO | 200 | £4.00 | £0.00 | GBP | 1.00 | £800.00 |
| 15 May 2026 | Sell | TSCO | 200 | £4.50 | £0.00 | GBP | 1.00 | £900.00 |
Working it out, step by step
Start with the existing pool
Back on 10 January 2025 our investor bought 500 TSCO at £3.00 (£1,500.00). Those shares form a Section 104 pool: 500 shares at £3.00 average. Keep an eye on it — we'll check at the end whether the same-day trades disturbed it.
Section 104 pool now holds
The long-term holding: 500 shares at a £3.00 average.
Same-day buy and sell on 15 May
On 15 May 2026 two things happen: a buy of 200 TSCO at £4.00 (£800.00) and a sell of 200 TSCO at £4.50 (£900.00). Because they're the same share on the same day, the same-day rule matches the sale against that day's purchase — not the older £3.00 pool shares.
Why not use the cheap £3.00 pool shares?
Matching against the pool would show a much bigger gain (200 × £4.50 − 200 × £3.00 = £300.00). HMRC's ordering forbids that while a same-day purchase exists: the same-day buy has first claim. The pool is only reached by whatever the same-day rule doesn't consume.
Work out the gain
The 200 shares sold are costed at the same-day purchase price of £4.00 — not the £3.00 pool average.
Confirm the pool is unchanged
The same-day buy and sell cancelled each other out — 200 in, 200 out — so neither touched the pool. The original 500 shares at £3.00 are exactly where we left them.
Section 104 pool now holds
Untouched. The same-day round-trip is self-contained: the pool's average cost stays protected.
The result
The matching is what matters
The lesson here isn't the size of the gain — it's which shares the sale was matched against. The same-day rule matched the sale to the same-day buy (£4.00 cost), keeping the cheap £3.00 pool shares — and a much larger potential gain — untouched for when they're eventually sold.
What to remember
- Shares bought and sold on the same day are matched to each other first, before the pool.
- “Same day” means the same calendar date — the order or time of the trades doesn't matter.
- If you buy more than you sell that day, the leftover shares fall into the pool. If you sell more than you bought, the excess comes from the pool (or the B&B rule, if a buy-back follows within 30 days).
- If there are several buys or several sells on one day, they're first merged into a single combined buy/sell — then the same-day rule matches the two. did); if you sell more than you buy, the excess is matched by the next rules.
- This is the first of two rules applied before your pool. The second is the bed & breakfast rule. See them combined in the all-three-rules walkthrough.
Related: bed & breakfast (30-day) rule, Section 104 pool, or getting started with CGT.
Sources
The rules and figures in this guide come from HMRC and GOV.UK. This site is independent and not affiliated with HMRC.
- HMRC Capital Gains Manual CG51560 — The same-day rule (TCGA92/S105) and the 30-day 'bed and breakfast' rule (TCGA92/S106A), with worked examples.
- HMRC Capital Gains Manual CG51550 — Outline of the share identification (matching) rules from 6 April 2008.
- GOV.UK — Tax when you sell shares — When CGT applies to selling shares and units, and which disposals are exempt.