The same-day rule, established by TCGA 1992 s.105 and explained in HMRC's Capital Gains Manual at CG51550, is the first of three share matching rules that HMRC requires you to apply when calculating capital gains tax on share disposals. If you sell shares and also buy shares of the same class in the same company on the same day, the sale must be matched against the same-day purchase before any other matching rule is considered.
What the same-day rule does
When you dispose of shares, HMRC needs to determine the cost basis - that is, which shares you are deemed to have sold and what you paid for them. As set out in the share identification rules under TCGA 1992 s.104, s.105, and s.106A (and summarised in HMRC's Capital Gains Manual at CG51560), HMRC mandates a strict priority order for matching disposals against acquisitions:
- Same-day rule - shares bought on the same day as the disposal are matched first.
- 30-day rule - shares bought in the 30 days following the disposal are matched next.
- Section 104 pool - any remaining shares are matched against the average-cost pool.
The same-day rule sits at the top of this priority order. It must be applied before the 30-day rule or the Section 104 pool.
Why this rule exists
Without the same-day rule, an investor could sell shares in the morning to crystallise a gain or loss at the Section 104 pool cost, and then immediately rebuy the same shares to reset the cost basis. The same-day rule prevents this by ensuring that the actual purchase price on that day is used.
How the matching works in practice
When you sell shares and have also purchased shares of the same class on the same day:
- If you bought the same number or more shares as you sold, all sold shares are matched against the same-day purchase. The cost basis for the matched shares is the actual purchase price paid on that day.
- If you bought fewer shares than you sold, only the number purchased is matched under the same-day rule. The remaining sold shares move down to the next matching rule (30-day, then Section 104 pool).
Worked example
David holds 500 shares in TechCo plc through his Section 104 pool. The pool has an average cost of £10.00 per share (total cost: £5,000). On 14 November 2024:
- At 9:30am, David sells 200 shares at £12.00 each (proceeds: £2,400).
- At 2:00pm, David buys 100 shares at £11.00 each (cost: £1,100).
Time of day does not matter
Despite the fact that the sale happened before the purchase, the same-day rule still applies. As confirmed in HMRC Helpsheet HS284 (Shares and Capital Gains Tax), all same-day acquisitions are matched against same-day disposals regardless of timing within the day.
Step 1 - Apply the same-day rule
100 of the 200 shares sold are matched against the 100 shares bought on the same day.
- Proceeds: 100 x £12.00 = £1,200
- Cost: 100 x £11.00 = £1,100
- Gain: £1,200 - £1,100 = £100
Step 2 - Match remaining shares
The remaining 100 shares are not matched by the same-day rule. There are no purchases in the following 30 days (for the purposes of this example), so they fall to the Section 104 pool:
- Proceeds: 100 x £12.00 = £1,200
- Cost from pool: 100 x £10.00 = £1,000
- Gain: £1,200 - £1,000 = £200
Total gain on the disposal
David's total gain is £100 + £200 = £300.
Without the same-day rule, all 200 shares would have been matched against the Section 104 pool at £10.00 each, producing a gain of £400. The same-day rule reduced the gain because the actual same-day purchase price of £11.00 was higher than the pool average of £10.00.
Updating the Section 104 pool
After this disposal, David's Section 104 pool needs to be updated:
- Before: 500 shares at £5,000 total cost.
- 100 shares were sold from the pool (the same-day matched shares do not affect the pool).
- After: 400 shares at £4,000 total cost (average cost remains £10.00 per share).
The 100 shares bought on the same day were entirely consumed by the same-day matching - they never entered the Section 104 pool. If David had bought 150 shares instead of 100, only 100 would be matched against the sale, and the remaining 50 would be added to the Section 104 pool.
Multiple same-day transactions
If you make multiple purchases on the same day as a sale, all the purchases are pooled together for the purposes of the same-day rule. For example:
- You sell 300 shares at £15.00.
- You buy 100 shares at £14.50 in the morning.
- You buy 150 shares at £14.80 in the afternoon.
The total same-day acquisitions are 250 shares at a combined cost of £3,670 (£1,450 + £2,220), giving a weighted average cost of £14.68 per share. The same-day rule matches 250 shares at this average cost, and the remaining 50 shares fall to the next matching rule.
Same-day rule with multiple sales
Similarly, if you make multiple sales on the same day, they are aggregated. If you sell 100 shares in the morning and 200 shares in the afternoon, the total disposal is 300 shares. If you also buy shares on the same day, the same-day rule matches against the total of 300 shares sold.
Key points to remember
- The same-day rule has the highest priority of the three share matching rules.
- The time of day does not matter - all transactions on the same calendar day are treated together.
- The "same day" refers to the trade date, not the settlement date.
- The rule applies separately for each class of share in each company.
- Shares matched under the same-day rule are excluded from both the 30-day rule and the Section 104 pool.
- Dealing fees on same-day purchases and sales should be included in the cost and proceeds respectively.
How FiscalFox applies the same-day rule
When you upload your transaction history to FiscalFox, the platform automatically detects same-day buy and sell transactions for the same security and applies the same-day matching rule. The detailed computation for each disposal shows clearly which shares were matched under the same-day rule, the 30-day rule, or the Section 104 pool, so you have a complete audit trail for HMRC purposes.