How the Section 104 pool works
When you sell UK shares you cannot pick which ones you sold. HMRC pools all shares of the same company into a single Section 104 holdingHMRC's share-pooling rule: all shares of the same class in the same company sit in one pool at their average cost, so you cannot choose which specific shares you sold. and averages the cost: (total cost of all shares owned ÷ total number of shares) = average cost per share. This calculator builds that pool for you.
The order HMRC matches shares
Before the pool is used, two anti-avoidance rules are checked first, in order:
- Same-day rule: shares bought on the day of the sale are matched first.
- Bed & breakfastSelling shares to use your CGT allowance then quickly rebuying them. HMRC blocks it - shares rebought within 30 days match to the sale, not the cheaper pool, so no gain is crystallised. (30 days): shares bought in the 30 days after the sale are matched next - this stops you selling to use your allowance and instantly buying back.
- Section 104 pool: if neither applies, the pool average is used. This calculator assumes the pool.
How your gain is actually taxed
This is where most calculators mislead you. CGT is not simply "18% if you are a basic-rate taxpayer". Your taxable gain - after the £3,000 annual exempt amount - is stacked on top of your income. The slice that still fits inside the basic-rate band (up to £50,270) is taxed at 18%; anything above is taxed at 24%. A single gain can be split across both rates.
Worked example: income £40,000, gain £20,000. After the £3,000 allowance, £17,000 is taxable. Around £10,270 fills the rest of the basic-rate band at 18%, and the remaining £6,730 is taxed at 24%.
Using capital losses
Losses on other share disposals are deducted from your gains in the same year first. If losses remain, they carry forward indefinitely - but you must claim them within 4 years of the end of the tax year in which they arose, so report a loss on your tax return even in a year you owe nothing. Brought-forward losses are only used down to the level of the annual exempt amount, never below it, so the allowance is not wasted.
Crystallising a gain without breaking the 30-day rule
Selling to use your £3,000 allowance and rebuying the same shares within 30 days does not work - the bed & breakfast rule matches the repurchase to the sale. The legitimate alternatives that keep you invested are to rebuy through your spouse (bed-and-spouse), rebuy the holding inside an ISA (bed-and-ISA), or buy a similar but not identical fund.
Deductible costs
You can reduce the gain by the incidental costs of buying and selling: broker fees and commission, and Stamp Duty Reserve Tax (SDRT, usually 0.5%) paid on purchase.
Reporting and paying
Share gains are not reported through the 60-day service - that is only for UK residential property. Report shares on your Self Assessment return, or HMRC's real-time CGT service. You must report if your gains exceed £3,000 or your total proceeds exceed £50,000, even when the gain is within the allowance.
The 2026/27 rates
Share CGT is 18% and 24% from April 2026, now aligned with residential-property rates, with a £3,000 annual exempt amount. Selling a second property too? Use the property CGT calculator.
Hold income funds? Reinvested income in accumulation unitsFund or ETF units that reinvest income instead of paying it out. The reinvested income is still taxed each year and must be added to your Section 104 cost, or you overpay CGT on sale. is taxed as income but also raises your pool cost - our guide to CGT on shares covers this, plus RSU and foreign-share cost basis, in full.