Section 104, Same-Day & 30-Day Rules for IBKR Shares
Updated: 21/10/2026
How HMRC decides which IBKR shares you sold: same-day, 30-day and section 104 pool rules, with a worked example and the mistakes that cost money.
HMRC does not use FIFO or the tax lots your broker picks. When you sell shares held at Interactive Brokers, the shares sold are matched first with any of the same shares bought on the same day, then with any bought in the following 30 days, and only then with your section 104 pool at average cost in sterling.
Rule 1: shares bought on the same day
All purchases of the same share on the same day are treated as one acquisition, and all sales that day as one disposal. The sale is matched with that day's purchases first. If you trade intraday on IBKR, most of your round trips are settled by this rule, and the cost is simply what you paid that day plus commission.
Rule 2: shares bought in the next 30 days
Next, the sale is matched with shares of the same company bought in the 30 days after the sale, earliest purchase first. This is the bed and breakfasting rule. It stops you selling at a loss on 5 April and buying straight back on 6 April to bank the loss while keeping the shares.
Two details are often missed. The rule looks forward only: shares bought ten days before a sale are already in the pool, not matched under this rule. And it works both ways, so it can increase a gain as well as cancel a loss. It applies when you are UK resident at the time you buy back.
Rule 3: the section 104 pool
Everything left over comes from the section 104 pool. Each class of shares you own has a single pool holding the total number of shares and their total allowable cost: the sterling price on each purchase date plus commissions and, for UK shares, stamp duty. When you sell from the pool, you take out a proportionate share of the cost, so the average cost per share stays the same.
Corporate actions change the pool. A share split or consolidation changes the number of shares but not the total cost. Takeovers and mergers can carry the cost over into the new shares, so check IBKR's corporate actions section for any holding you have kept for a while.
How to apply the rules to your IBKR trades
- Export every trade in each holding since you first bought it, not just the current tax year, using Activity statements from the Client Portal.
- Convert each purchase and sale to sterling at the exchange rate on its trade date, including the commission.
- Group the trades by share class, so the same company bought on different days lands in one pool.
- For each sale date, match first with that day's purchases, then with purchases in the next 30 days, then with the pool.
- Update the pool after each step and carry its closing number and cost into the next tax year.
- Add up proceeds, allowable costs, gains and losses for the year and enter them on SA108.
Worked example
You bought 200 shares for £2,000 plus £5 commission in 2024, and 100 more for £1,600 plus £5. Your pool holds 300 shares at a total cost of £3,610.
On 10 February 2026 (tax year 2025/26) you sell 150 shares at £20, paying £6 commission, so net proceeds are £2,994, or £19.96 per share. The same day you buy 50 shares for £1,000 plus £5. On 20 February you buy 40 more for £760 plus £5.
- Same-day match, 50 shares: proceeds £998, cost £1,005, loss £7.
- 30-day match, 40 shares: proceeds £798.40, cost £765, gain £33.40.
- Pool match, 60 shares: proceeds £1,197.60, cost £3,610 × 60/300 = £722, gain £475.60.
Your overall gain is £502, which you can check as £2,994 minus total costs of £2,492. The pool now holds 240 shares at £2,888, still about £12.03 each. The 90 shares bought in February never enter the pool, because they were used up by the matching.
Common mistakes
- Taking IBKR's realised profit and loss at face value. It reflects the lot-matching method set in your account (FIFO, specific lot and so on), which is irrelevant for HMRC.
- Building the pool only from the current year's statement, which gives the wrong average cost.
- Calculating gains in dollars and converting the result at the end, instead of converting each trade on its own date.
- Forgetting that a repurchase in the next 30 days also affects winning trades.
- Missing a split or reverse split, so the pool shows the wrong number of shares.
Selling at a loss without falling foul of the rule
You can still realise a loss legitimately. Buying the shares back inside a stocks and shares ISA (bed and ISA) is not matched, because ISA holdings sit outside capital gains tax. A spouse or civil partner can buy the shares instead of you. Or you can switch into a similar but not identical investment, such as a different fund tracking the same market.
One missed 30-day match can turn a loss you were counting on into nothing, and HMRC can go back years to correct it. Upload your IBKR statement to Brokers.tax and see every sale matched under the same-day, 30-day and section 104 rules, in sterling, before you commit figures to your return.