About this calculator

Why it exists

Working out CGT on shares by hand is harder than it looks. HMRC does not let you assume you sold your oldest shares first — a disposal is matched against same-day purchases, then against anything bought back in the following 30 days, and only then against the pooled average cost of what remains. Once you add dealing fees, foreign-currency trades, RSU vests, stock splits and a spouse transfer or two, a spreadsheet stops being trustworthy.

The paid tools that solve this want an account and a subscription, and generally want your trade history uploaded to their servers. This one asks for neither. It was built to be the version its author wanted to exist: free, private, and open about how it gets its numbers.

Who publishes it

One developer, working on it in their own time — not a company, not a tax practice. That cuts both ways, and it is worth being straight about it. There is no professional indemnity behind these figures and no accountant signing them off, so the calculator is built to be checked rather than trusted: it shows every match, every pool movement and every gain, and the code that produces them is open source. For anything that affects what you actually file, check it against HMRC's guidance or a qualified adviser. The methodology page sets out exactly how the figures are produced and where they come from.

What you get

• HMRC's share matching rules — same day, bed & breakfast (30-day), and the Section 104 pool — applied in HMRC's order, for every tax year from 2008/09 to 2026/27.
• RSU vests, sell-to-cover, stock splits, spouse and civil-partner transfers, and multi-currency trades converted at HMRC's official monthly rates.
• An annual allowance planner, a PDF report for your records or your accountant, and CSV/Excel export so next year you can re-import instead of retyping.
• 11 plain-English guides and 38 answered questions, each ending with the HMRC pages the rules come from.

Privacy is the design, not a policy

Your trades are calculated in your browser. They are not uploaded, not logged and not stored — there is no database and no account, and closing the tab discards everything. The only requests that leave your device are the optional exchange-rate and stock-price lookups, which send a currency code or a ticker symbol and nothing else. There are no analytics, no advertising and no tracking cookies. The privacy page has the detail.

How it is funded

It isn't, really. There are no ads, no fees, no premium tier and nothing to buy; hosting is covered by whoever chooses to chip in. That matters for a reason beyond cost: nothing here is paid for by anyone with an interest in what the answer is.

Found a mistake?

Please report it. A short set of trades that reproduces the problem is the fastest route to a fix — the contact page is the quickest way, and corrections to the maths itself can go straight to the engine repository.