UK capital gains tax calculator

Successful calls are billed at $0.002; invalid input is not charged.

What UK capital gains tax is and how the tax owed is formed

Subtract the allowance from the gain and floor the result at zero; that remainder is taxable_gain. Multiply taxable_gain by the rate that belongs to the pair of income tax band and asset type, and you have tax, the amount this endpoint returns as the primary result and again as amount so a dashboard can bind either key. Other assets — shares, crypto, unit trusts, and most personal possessions above the chattel limit — are taxed at eighteen percent when income_tax_band is basic and at twenty-four percent when the band is higher or additional. When the allowance is larger than the gain, taxable_gain is zero and the tax owed is zero: that is how a disposal inside the annual exempt amount is represented, not as an error. A negative gain is not a capital-loss relief claim and is not a repayment: it is rejected as invalid_input so a typo cannot emit a signed bill that a downstream ledger would post as a credit. The formula string in the response restates tax equals the maximum of zero and gain minus annual_exempt_allowance, times rate_percent over one hundred, so a reviewer can print the identity next to the number without reconstructing it.

Input contract, response shape, and error behaviour

Example: gain fifty thousand, allowance three thousand, higher band, residential asset yields forty-seven thousand taxable at twenty-four percent and tax of eleven thousand two hundred eighty. Prefer bare numbers over formatted strings; currency marks, thousands separators, and trailing unit labels are not parsed. Pair the result with your own Self Assessment step if you need a filed figure. POST a JSON body with two required finite numbers and two required enumerated strings, all in pounds sterling for a single tax year. gain is the realised capital gain after incidental costs of sale and allowable expenditure, before the annual exempt allowance, and must be greater than or equal to zero; aliases amount, capital_gain, gain_amount, realised_gain, and realized_gain are accepted so a broker export does not need a rename step. A negative gain fails with invalid_input naming gain and is not billed. annual_exempt_allowance is the annual exempt amount still available for the year and must be greater than or equal to zero; aliases aea, allowance, annual_exemption, exempt_amount, and annual_exempt_amount are accepted. income_tax_band must be one of basic, higher, or additional; aliases tax_band, band, and income_band are accepted. asset_type must be one of other, residential, or carried_interest; aliases asset and asset_class are accepted, and shares, crypto, property, and carried map onto the canonical values. Each money field may arrive as a JSON number or as a numeric string that matches a strict finite decimal pattern. Missing required fields, empty strings, non-finite values, unknown enumerations, or amounts above one trillion fail with invalid_input naming the field. On success the body echoes rounded gain, annual_exempt_allowance, income_tax_band, and asset_type, returns taxable_gain as the floored remainder, allowance_used as the lesser of gain and the allowance, rate_percent as the looked-up UK CGT rate, and tax and amount as the same half-up two-decimal product, plus currency GBP and a formula string you can print next to the numbers. This endpoint does not split a gain across unused basic-rate band, apply Business Asset Disposal Relief, Private Residence Relief, or losses brought forward, and it does not file a return. Pricing for a successful item is a flat $0.002.

Where a UK capital gains tax calculator earns its keep

They want the tax owed from a known gain, the annual exempt allowance still unused, an income tax band, and an asset class, without rebuilding the same subtraction and table lookup in a spreadsheet cell that drifts when someone forgets which rate applied after 30 October 2024. Individuals drafting a Self Assessment worksheet from a broker statement need a number they can paste beside the disposal. Classroom and bookkeeping courses that teach UK CGT need an oracle that rejects a negative gain instead of silently emitting a signed total a grader might treat as a repayment. Failed validation does not consume balance. Pair this capability with a separate relief or loss tool when you need those layers; keep this surface focused on the UK CGT tax owed that people mean when they search that phrase. Accountants reviewing a client pack before year end need the same identity a finance script will later call over the API. Property investors disposing of a buy-to-let that is not the main residence, and carry holders estimating a carried-interest charge, need the residential and carried_interest rows of the table rather than a single blended rate. Keep requests small, pin tax and taxable_gain in tests, and you get a reproducible sterling amount for planning notes, year-end packs, and lightweight tax notebooks without standing up a full Self Assessment system.