Methodology

Which rules are implemented

Disposals are matched to acquisitions in HMRC's fixed order, and nothing else:

1. Same day — acquisitions on the date of disposal (TCGA 1992 s105(1)). All same-day buys of a share are merged into one acquisition, and all same-day sells into one disposal, before matching.
2. Bed & breakfast, the following 30 days — acquisitions in the 30 days after disposal, earliest first (TCGA 1992 s106A).
3. Section 104 pool — the balance, at the pooled average cost of everything still held (TCGA 1992 s104).

Transfers to a spouse or civil partner are treated as no gain, no loss (TCGA 1992 s58): the shares move at the transferor's cost, and the transfer itself produces no gain. Stock splits restate the pooled quantity while leaving the pooled cost untouched. Allowable expenditure (dealing fees, stamp duty) is added to acquisition cost and deducted from disposal proceeds; on a part disposal it is apportioned with the pool.

Where the rates and allowances come from

Rates, annual exempt amounts and reporting thresholds are configuration data inside the calculation engine, one entry per tax year, taken from GOV.UK's published figures. The site never hard-codes a rate in a page: the 19 supported tax years from 2008/09 to 2026/27 are read from the engine at build time, so the rates page, the calculator and this page cannot disagree. Where a tax year contains a rate change part-way through — as 2024/25 does, on 30 October 2024 — that year holds two rate periods and each disposal is taxed at the rate in force on its date.

For 2026/27 that means an annual exempt amount of £3,000 and a reporting threshold of £50,000 in disposal proceeds. Current figures are on the rates and allowances guide.

How the arithmetic is verified

• One engine, open to inspection. The matching rules live in the capital-gains-calculator library, which is open source and enforces 100% test coverage at build time. You — or your accountant — can read the code that produces your numbers. The website that wraps it is not open source; the part that does the maths is.
• The worked examples are produced by the engine. Every figure in the guides and on the examples page comes from the same engine that runs in the calculator, so a lesson and the tool cannot drift apart.
• Deterministic and auditable. The same trades always produce the same result, and the output shows how each disposal was matched, the pool before and after, and the gain or loss per disposal — so you can check any single line by hand.
• Tested at the boundary too. The website has its own test suite covering how it feeds trades into the engine, the exchange-rate lookups, the date handling, and the allowance planner, and the whole suite has to pass before anything is built or deployed.

How the guidance is reviewed and dated

Every content page shows when it was published and when it was last reviewed, and those dates are the same ones used in the page's structured data and in the sitemap — they come from a single record, not from a deploy timestamp. A date is only moved when the substance changes: copy, rates, tax-year data, structured data or links. An unrelated redeploy never changes it.

Each guide ends with the HMRC and GOV.UK pages its rules and figures come from, and those links are held in one shared list so a page cannot cite a source another page contradicts.

Limits — what this does not do

• It does not carry losses forward between tax years. Each year is calculated on its own; bring earlier losses forward yourself on your return.
• It does not apply Business Asset Disposal Relief, Indexation Allowance, Taper Relief or Change of Status.
• It covers shares, securities, ETFs and cryptoassets — not residential property, and not other asset classes such as art or collectibles.
• It covers tax years from 2008/09 onwards. For earlier holdings, work out the Section 104 pool up to 6 April 2008 and enter it as a single acquisition.
• The allowance optimiser is a planning aid: it works from the pooled average cost and deliberately does not model the same-day or 30-day rules, so re-run the full calculation after adding a suggested disposal.
• It assumes you are an individual UK taxpayer. Trusts, companies, non-residents and temporary non-residence are out of scope.

Corrections

If a figure here looks wrong, please say so — a small set of trades that reproduces the problem is the fastest route to a fix. Use the contact page, or raise it against the engine repository if it is the maths rather than the interface. This site is independent of HMRC and does not provide tax advice; see the terms.