Capital Gains Tax Calculator for UK Shares

When you sell shares in a company you bought at different times, HMRC decides which purchase you sold. You don't get to pick, and it isn't first in, first out. Each sale is matched in this order:

1. Same day. Shares of the same company bought on the day of the sale are matched first.
2. The next 30 days. Then shares bought in the 30 days after the sale, earliest first. This is the bed and breakfast rule, so the gain or loss on those shares uses what you paid to buy them back.
3. The Section 104 pool. Whatever is left comes out of the pool of everything else you bought, at its average cost.

In the example loaded above, you sell 3,500 shares on 2 June 2025 for £18,900.00. Here is how they are matched:

• 1,000 shares, Same day. Bought the same day, so they cost £5,361.95 with their dealing costs.
• 1,500 shares, 30-day rule. Bought back on 20 June 2025, inside 30 days, for £7,586.95. These count as the shares you sold even though you bought them later.
• 1,000 shares, Section 104 pool. Taken from the pool, which held 7,500 shares at an average of £4.2699, so they cost £4,269.85.

The allowable cost is £17,230.70, the shares plus your dealing costs on the sale, and the gain is £1,669.30. The pool only shrinks by the 1,000 shares it supplied, and its average cost per share stays the same.

Every share you buy that isn't matched by the same-day or 30-day rule goes into one pool per company, or per fund share class. The pool keeps two numbers: how many shares it holds and what they cost in total, dealing costs included. Buying adds to both. Selling from the pool takes out a slice of the cost in proportion to the shares sold, so the average cost per share only moves when you buy.

The pool runs for as long as you hold the shares, across tax years. That is why a sale this year can depend on a purchase from ten years ago, and why the full trade history matters.

Add up the gains and losses on all your sales in the tax year, which runs from 6 April to 5 April. The first £3,000 of net gains is covered by the annual exempt amount, the figure for 2024-25 to 2026-27. This estimate includes losses for the selected tax year only.

What remains is taxed at 18% where it fits inside your unused basic rate band and 24% above it, for sales from 30 October 2024. Before that date the rates were 10% and 20%, and the calculator applies the right ones by sale date. The basic rate band is £37,700. Your gains sit on top of your taxable income. Enter it to see an estimated split between the two rates.

Listed shares and funds go on the capital gains pages of Self Assessment, form SA108: the number of disposals, the proceeds, the allowable costs, the gains and the losses. You have to report when your gains are over the annual exempt amount, and also when you are registered for Self Assessment and your sale proceeds for the year are over £50,000.

Gains inside an ISA or a pension are tax free and stay off the return. A spouse or civil partner has their own pool and their own allowance.