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How same-day trades are merged

Before HMRC works out which shares you sold, it does a tidying-up step that surprises people: if you buy the same share several times on one day, those buys are combined into a single purchase. The same happens to multiple sales of it on one day. This guide explains that “merging” step on its own, because everything else builds on it.

The idea in one sentence

For Capital Gains Tax, all acquisitions of the same share by the same person on the same day are treated as one acquisition, and all disposals of it that day as one disposal. Several trades collapse into a single combined trade at their average price.

Merging vs. the same-day rule — what's the difference?

They sound alike but do different jobs. Merging (this page) just combines your same-day buys into one buy and your same-day sells into one sell. The same-day rule is the next step: it matches a combined same-day sale against a combined same-day purchase before anything else. Merging always happens; the same-day rule only matters when you both buy and sell on the same day.

Our worked example

Our investor buys BP twice on the same morning at different prices, then sells 300 shares that same day. The sale triggers the same-day rule, which matches it against the merged same-day purchase — so merging directly affects the cost and gain. You can load this example into the calculator at the bottom of the page.

DateTypeSymbolQtyPriceFeesCcyFX RateGBP Value
12 May 2026BuyBP200£5.00£0.00GBP1.00£1,000.00
12 May 2026BuyBP200£6.00£0.00GBP1.00£1,200.00
12 May 2026SellBP300£6.50£0.00GBP1.00£1,950.00
Try this example in the calculator

Working it out, step by step

Two buys on one day

On 12 May 2026 our investor buys 200 BP at £5.00 (£1,000.00) and then another 200 at £6.00 (£1,200.00). To them these feel like two separate trades — but HMRC merges them before anything else happens.

Merge the buys into one purchase

Because they're the same share on the same day and both buys, they combine into a single purchase of 400 shares at the average price.

Combined cost
£1,000.00 + £1,200.00 = £2,200.00
Average cost per share
£2,200.00 ÷ 400 shares = £5.50

The same-day sell is matched against the merged buy

On the same day, our investor sells 300 BP at £6.50 (£1,950.00). The same-day rule matches this sale against the merged purchase — and because of merging, the cost per share is the blended £5.50, not specifically £5.00 or £6.

Proceeds
300 shares × £6.50 = £1,950.00
Allowable cost (300 shares at the merged £5.50)
300 shares × £5.50 = £1,650.00
Gain
£1,950.00 − £1,650.00 = £300.00

Remaining shares go to the pool

400 shares were bought and only 300 were matched by the same-day rule; the remaining 100 enter the Section 104 pool at the same £5.50 average.

Section 104 pool now holds

100
shares
£550.00
total cost
£5.50
average per share

100 shares at £5.50 — ready for whenever they're eventually sold.

Why merging matters here

Without merging, the same-day rule would see two separate purchases (one at £5.00, one at £6.00) and you'd have to decide which the sale came from — or HMRC would have to pick. Merging removes that ambiguity: it gives the same-day rule a single, clear, average-priced purchase to match against. The gain is always £300.00 regardless of how you'd have split the individual buys.

The result

Proceeds
£1,950.00
Allowable cost
£1,650.00
Gain
£300.00
Try this example in the calculator

What to remember

  • Same-day, same-share buys merge into one purchase; same-day sells merge into one sale.
  • Merging uses the average price — the individual trade prices stop mattering once combined.
  • It applies to trades of the same type; a same-day buy and a same-day sell are handled by the same-day rule, not merged together.
  • Merging happens before the matching rules run, so every later step works with these tidy combined trades.

Related: the same-day rule, the 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.