Start with the account and the UK tax year
This guide is for an individual who is UK tax resident and sells shares outside an ISA or another exempt arrangement. Gains on shares held in an ISA are generally free of Capital Gains Tax, while employer plans, trusts, estates, non-residence, and the foreign income and gains regime need separate treatment.
The UK tax year runs from 6 April to 5 April. A sale on 5 April and a sale on 6 April belong to different tax years, so the disposal date determines which annual losses, exemption, income bands, and reporting deadline apply.
Calculate the gain before choosing a rate
Start with disposal proceeds and subtract the allowable cost assigned to the shares, plus deductible acquisition and disposal costs such as broker fees and Stamp Duty Reserve Tax paid on purchase. Tax applies to the gain, not to the full value of the sale.
Market value can replace the actual price in special cases, including some gifts and sales below market value. Inheritances, employee share schemes, reorganisations, takeovers, and earlier relief claims can also change the cost, so a broker app average is not always the tax cost.
Apply the UK share-matching order
For shares of the same class in the same company, UK rules generally match a disposal first with acquisitions on the same day, then with acquisitions in the following 30 days, and then with the Section 104 holding. The Section 104 pool uses an aggregated average cost rather than a freely selected purchase lot.
A pre-sale estimate therefore needs the pooled quantity and cost as well as purchases planned or completed around the sale date. Ignoring the same-day and 30-day rules can produce the wrong gain even when the total number of shares is correct.
Net gains and losses, then use the annual exemption
Combine chargeable gains and allowable losses for the whole tax year, include eligible losses brought forward under the applicable rules, and apply any reliefs before calculating the taxable amount. Losses must be claimed and cannot simply be inferred from a falling portfolio value.
For individuals, the annual exempt amount is £3,000 for both 2025 to 2026 and 2026 to 2027. It is one allowance for the tax year, not £3,000 per account or per stock, and it may be unavailable when certain foreign income and gains claims are made.
Split the taxable gain across the 18% and 24% bands
From 6 April 2026, the main individual Capital Gains Tax rates are 18% and 24%. Add the taxable gain to taxable income: the part that fits within the unused basic-rate band is generally charged at 18%, and the remainder is generally charged at 24%.
A calculator must ask for estimated taxable income instead of applying one headline rate to the whole gain. Reliefs and special asset categories can change the result, so the rate assumptions should remain visible beside the estimate.
Worked example: a pooled gain above the allowance
Assume gross proceeds of £15,000, selling fees of £100, and a matched Section 104 cost of £10,000. The gain is £4,900. If £900 of other allowable losses is available, net gains are £4,000; after the £3,000 annual exempt amount, the taxable gain is £1,000.
If the full £1,000 falls in the 18% band, estimated CGT is £180 and estimated cash from the sale after fees and that tax is £14,720. If it falls in the 24% band, estimated CGT is £240 and estimated after-tax sale cash is £14,660.
Keep enough information to report the disposal
Keep contract notes, dates, quantities, acquisition costs, fees, corporate-action records, Section 104 pool movements, same-day and 30-day purchases, and loss claims. The broker statement is evidence, but it may not reconstruct a pool spanning transfers or several platforms.
Reporting can depend on the size of proceeds, gains, losses, and whether Self Assessment already applies. Check the HMRC reporting guidance for the relevant tax year rather than treating the payment estimate as proof that no return or claim is needed.
What the Portle estimate should ask and show
Inputs should include tax residence, account type, disposal date, proceeds, fees, same-day and next-30-day acquisitions, Section 104 quantity and pooled cost, current-year gains and losses, brought-forward losses, reliefs, and estimated taxable income. Results should separate matched cost, gain, net annual gain, remaining exemption, the 18% and 24% portions, estimated CGT, and after-tax sale cash.
Official references: HMRC Tax when you sell shares (gov.uk/tax-sell-shares), Capital Gains Tax rates and allowances (gov.uk/guidance/capital-gains-tax-rates-and-allowances), and Helpsheet HS284. This is an educational estimate, not tax advice or a completed tax return.