Capital Gains Pooling Calculator United Kingdom

When you sell part of a shareholding, you need to know which shares you sold and what they cost. This calculator works it out the way HMRC does: it matches the sale against same-day purchases, then against anything you bought back within the next 30 days, and finally against the Section 104 pool at its average cost per share. It then works out the gain, applies the annual exempt amount and calculates the Capital Gains Tax due.

What the capital gains pooling calculator is doing

UK tax law does not track individual share certificates. All shares of the same class in the same company are treated as one asset, the Section 104 holding, named after the section of the Taxation of Chargeable Gains Act 1992 that creates it. The pool has two running figures: the number of shares and the total cost of acquiring them. Divide one by the other and you have the average cost per share.

When you sell, the pool gives up shares and a proportional slice of its cost. Sell 400 shares from a pool of 1,000 that cost £8,000, and the allowable cost is 400 × £8 = £3,200. The pool carries forward 600 shares and £4,800 of cost for the next disposal. The calculator reports that carried-forward figure as well as the tax.

If those 400 shares sold for £25 each, the proceeds are £10,000 and the gain is £6,800. After the £3,000 annual exempt amount, £3,800 is taxable. For someone with £40,000 of other income, £10,270 of the basic rate band is unused, so all £3,800 is taxed at 18%, giving £684 of tax.

The matching rules come first

Two rules take priority over the pool.

The same-day rule matches a sale against any shares of the same class bought on the same day. Trades that cancel each other out within a day cannot create a gain or a loss.

The 30-day rule, usually called the bed and breakfasting rule, matches a sale against shares of the same class bought in the following 30 days. It targets the old practice of selling on 5 April to use the annual exempt amount and buying back on 6 April. Do that now and the sale is matched to the repurchase price instead of the pooled cost. In the example above, buying the 400 shares back at around £25 within 30 days would reduce the £6,800 gain to almost nothing.

Only what is left after both rules comes out of the Section 104 pool. Shares matched under either rule never enter the pool, which is why the pool balance in the results does not fall when a repurchase absorbs the whole sale.

Costs you are allowed to deduct

The allowable cost includes more than the purchase price. You can add the incidental costs of buying and selling: broker commission on both sides, stamp duty or stamp duty reserve tax paid on the purchase, and transfer or settlement fees. Enter those in the dealing and transfer costs field and they reduce the gain directly.

Interest on a loan used to buy the shares is not allowable. Neither are ongoing platform or custody fees, research subscriptions or the value of your own time. Dividends already taxed as income play no part in the capital gains calculation.

Losses and the annual exempt amount, in that order

Capital losses come off the gain first, and the annual exempt amount is applied to what remains. Losses from the same tax year must be used in full, even if that wastes part of the exempt amount. Losses brought forward from earlier years are used only down to the level of the exempt amount, so they are never wasted. The calculator takes one losses figure and applies it before the exemption, which matches the brought-forward treatment most people need.

What is left is the taxable gain. It is stacked on top of your income: the part that fits in your unused basic rate band is taxed at the lower rate for shares and the rest at the higher rate. Two people with the same gain can therefore face very different bills, which is why the calculator asks for your other taxable income.

Keeping the pool right over time

The pool is only as good as your records. Every purchase adds its full cost, including stamp duty and commission. Rights issues add shares and cost. Bonus issues and share splits add shares without adding cost, which lowers the average. Takeovers, mergers and share-for-share exchanges usually carry the old cost across to the new holding instead of triggering a disposal. Dividends reinvested through a DRIP are new purchases at the reinvestment price and must be added like any other purchase. They are easy to forget, and missing them understates the pooled cost for years.

Shares held in an ISA or a pension are outside Capital Gains Tax altogether, so there is no pool to keep.

Before you file on these numbers

The tool models one disposal from one pool, which covers most ordinary share sales. It does not handle share reorganisations, employee share schemes with their own base cost rules, holdings acquired before April 1982, negligible value claims, or jointly held shares where each owner has a separate pool. Non-residents, trusts and companies follow different rules.

Use the result to plan and to sanity-check a broker’s tax report. Confirm anything you intend to file with a qualified tax adviser or HMRC’s guidance. This is information, not financial or tax advice.

About This Capital Gains Pooling Calculator

Use this capital Gains pooling calculator to get an instant, accurate estimate. The figures follow official GOV.UK guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.