All three rules in a single sale — the full walkthrough
This is where it all clicks. We'll take one sale of 3,000 shares and watch HMRC split it across all three matching rules in strict order — same-day first, then bed & breakfast, then the Section 104 pool — and finish with a real tax bill. Each rule here is covered in depth in its own guide (the same-day, bed & breakfast, and Section 104 pool guides); this one shows them working together.
The matching order (always the same)
When you sell, HMRC works through three rules in this fixed priority:
- Same-day — shares of the same kind bought on the day of the sale.
- Bed & breakfast — shares bought back in the 30 days after the sale.
- Section 104 pool — everything left over, at the pooled average cost.
A sale is filled from the top down: rule 1 takes as much as it can, rule 2 takes the next slice, and the pool mops up the rest.
Our example
Our investor holds 2,500 Shell (SHEL) shares bought last year at £10.00. On 15 May 2026 they sell 3,000 shares at £15.00 — more than the 2,500 they had been holding — having bought 500 that same day at £14.00. Three weeks later they buy 500 back at £14.50. That one sale will touch all three rules.
| Date | Type | Symbol | Qty | Price | Fees | Ccy | FX Rate | GBP Value |
|---|---|---|---|---|---|---|---|---|
| 1 Sep 2025 | Buy | SHEL | 2,500 | £10.00 | £0.00 | GBP | 1.00 | £25,000.00 |
| 15 May 2026 | Buy | SHEL | 500 | £14.00 | £0.00 | GBP | 1.00 | £7,000.00 |
| 15 May 2026 | Sell | SHEL | 3,000 | £15.00 | £0.00 | GBP | 1.00 | £45,000.00 |
| 5 Jun 2026 | Buy | SHEL | 500 | £14.50 | £0.00 | GBP | 1.00 | £7,250.00 |
How can they sell 3,000 when the pool only has 2,500?
Because of the 500 bought the same day: 2,500 (pool) + 500 (same-day) = 3,000 available to sell on 15 May. You can never sell more shares than you actually hold on the day — the same-day purchase is what makes the numbers add up here.
Working it out, step by step
The starting pool
The September 2025 purchase gives a pool of 2,500 shares at £10.00 — £25,000.00 of cost. This is the holding the pool slice of the sale will eventually draw on.
Section 104 pool now holds
2,500 shares at a £10.00 average.
Rule 1 — same-day: match 500 shares
500 SHEL were bought on 15 May 2026 at £14.00, the same day as the sale. The same-day rule claims the first 500 shares of the sale and costs them at that £14.00 purchase price.
500 of the 3,000 shares are now accounted for. 2,500 still to match.
Rule 2 — bed & breakfast: match another 500
On 5 June 2026 — 21 days after the sale, inside the 30-day window — they bought 500 SHEL back at £14.50. The bed & breakfast rule claims the next 500 shares of the sale and costs them at that £14.50 buy-back price.
1,000 shares matched so far. 2,000 left — and no more special purchases — so the pool takes the rest.
Rule 3 — the pool: match the final 2,000
The remaining 2,000 shares come out of the Section 104 pool at its £10.00 average cost.
Section 104 pool now holds
The pool now holds 500 of the original £10.00 shares (£5,000.00). The 500 bought back on 5 June are real and still owned — but the bed & breakfast rule already matched them against this sale, so adding them to the pool as well would count the same 500 shares twice. They stay out of the £10.00 pool and carry their own £14.50 cost for a future sale. So the pool's running count is 500.
Add the three slices together
The sale's total gain is just the three slices added up. Notice each slice used a different cost — £14.00, £14.50, and £10.00 — which is exactly why the matching order matters.
| Matched under | Shares | Allowable cost | Gain | Why |
|---|---|---|---|---|
| Same-day | 500 | £7,000.00 | £500.00 | Bought the same morning at £14.00. |
| Bed & breakfast | 500 | £7,250.00 | £250.00 | Bought back 21 days later at £14.50. |
| Section 104 pool | 2,000 | £20,000.00 | £10,000.00 | The rest, at the £10.00 pool average. |
| Total disposal | £34,250.00 | £10,750.00 | Proceeds £45,000.00 − cost £34,250.00 |
The result
What to remember
- One sale can be split across all three rules. They're always applied in order: same-day → 30-day → pool.
- Each slice can have a different cost, so the order genuinely changes the gain.
- Add the slices for the total gain, take off the Annual Exempt Amount, then apply your CGT rate.
- Your rate (18% or 24% for 2026/27) depends on how much spare basic-rate income band you have.
Go deeper on any single rule: the Section 104 pool, the same-day rule, or the bed & breakfast rule. Or start from 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 CG51550 — Outline of the share identification (matching) rules from 6 April 2008.
- 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 CG51575 — The Section 104 holding (pool) in detail: pooled cost and part-disposal apportionment.
- GOV.UK — Work out if you need to pay Capital Gains Tax — When you must report: gains above the allowance, or total proceeds over £50,000 (from 2023/24).