Capital Gains Tax UK Calculator

This calculator estimates UK Capital Gains Tax (CGT) on a taxable gain after the annual exempt amount. It is designed for a specific planning question: if you already know the gain on shares, crypto, another chargeable asset, or taxable residential property, how much of that gain might be payable to HMRC and how much profit remains after tax? The tool asks for the tax year, asset type, gain, annual income, and whether the gain belongs to a trust. It then applies the stored allowance and rates for that year, splits the taxable gain between the basic-rate and higher-rate portions when appropriate, and reports estimated CGT, profit after tax, taxable gain, allowance, rate split, and effective tax rate.

This is informational, not tax advice. UK tax rules change, HMRC guidance can be updated, and a filed return may depend on acquisition costs, allowable losses, connected-party rules, residence, reliefs, and reporting deadlines that this estimator does not model.

How to use the calculator

Enter the capital gain, not the sale proceeds. If shares were bought for £30,000 and sold for £70,000, the starting gain is £40,000 before any allowable losses or reliefs. Choose shares, crypto, or other assets for most non-property gains. Choose residential property only when the gain is on taxable residential property; do not use it for a main residence that is fully covered by private residence relief.

Next, enter total annual taxable income. The calculator uses income only to estimate how much basic-rate band remains before the higher-rate threshold stored in the selected tax year. If income is already above that threshold, none of the taxable gain is treated as basic-rate gain. If income is below it, taxable gains fill the remaining band first. Switch on assets are in a trust only for trust gains. The trust setting uses the trust allowance and, puts the taxable gain into the higher-rate treatment.

For investment context, pair the result with the capital gains yield calculator, the ROI calculator, and the compound interest calculator. Those tools answer performance questions; this one focuses on the UK tax layer.

What the calculator does

The calculation begins by selecting the embedded rule set for the chosen tax year. For 2025/26, the calculator uses a £3,000 annual exempt amount for individuals, £1,500 for trusts, a £50,270 threshold for estimating the basic-rate boundary, and CGT rates of 18% and 24%. For older years, it uses the stored year-specific values, including property-specific rates where present. Do not treat those values as permanent; they are year-specific settings that need updating when the law changes.

After the allowance, the remaining gain is taxable. Individuals can have some taxable gain charged at the basic CGT rate if their taxable income leaves space below the threshold. Any taxable gain beyond that space is charged at the higher CGT rate. Trust gains in this model use the trust annual exempt amount and no remaining basic band. The result is an estimate of tax on the gain, not a full self assessment calculation.

Formula

$\text{taxable gain} = \max(\text{capital gain} - \text{annual exempt amount}, 0)$

$\text{basic-rate gain} = \min(\text{taxable gain}, \max(\text{threshold} - \text{income}, 0))$

$\text{CGT} = \text{basic-rate gain} \times \text{basic rate} + \text{higher-rate gain} \times \text{higher rate}$

$\text{profit after tax} = \text{capital gain} - \text{CGT}$

The displayed effective tax rate is the estimated CGT divided by the original gain. In KaTeX terms, the percentage relationship is:

$\text{effective rate} = \frac{\text{CGT}}{\text{capital gain}} \times 100\%$

Worked example

Using the default inputs, choose 2025/26, shares, crypto, or other assets, a £40,000 gain, £45,000 annual income, and no trust. The embedded annual exempt amount is £3,000, so the taxable gain is £37,000. The threshold is £50,270, leaving £5,270 of basic-rate band after the £45,000 income. That means £5,270 of gain is taxed at 18%, producing £948.60 of tax. The remaining £31,730 is taxed at 24%, producing £7,615.20. Estimated CGT is therefore £8,563.80, and profit after tax is £31,436.20. The effective tax rate on the original gain is 21.41%.

The calculation applies those figures in this order: allowance first, basic band second, higher-rate remainder last. If the same gain is placed in a trust, the allowance becomes £1,500 and the basic band is set to zero, so the estimate changes materially.

Who this affects

This calculator is most useful for UK individuals selling taxable investments, cryptoassets, second homes, buy-to-let property, or other chargeable assets where the gain is not fully covered by relief. It can also help trustees build a quick planning estimate before a more formal calculation. It is less suitable when the hard work is determining the gain itself, such as inherited assets, part disposals, foreign assets, reorganizations, or property with periods of main residence relief.

CGT planning often interacts with timing. A disposal just before or after a tax-year boundary can change which annual exempt amount and rates apply. Income also matters because the same gain can produce a different CGT result for a basic-rate taxpayer than for someone whose income already fills the basic-rate band. Use the estimate as a discussion aid, then confirm the final position with HMRC guidance or a qualified adviser.

Sources

• HMRC, Capital Gains Tax rates — official rate guidance for chargeable gains and asset categories.
• HMRC, Capital Gains Tax allowances — annual exempt amount guidance and trust allowance context.
• HMRC, Tax when you sell property — official reminder that property disposals can have separate reliefs and reporting rules.