About the UK Capital Gains Tax Calculator
Capital gains tax on shares, a second property, a business or crypto, at 18% and 24%.
The question people ask is "which rate am I on?" — and it has no answer until you know something that has nothing to do with the gain:
How much of your basic rate band your income has already used up.
The band is shared between income and gains, and income goes first. So the rate on your gain is decided by your salary, and the arithmetic produces a number most people find surprising.
How to Use the UK Capital Gains Tax Calculator
Enter the gain — proceeds less what you paid and the costs of buying and selling.
Enter your income for the year. This is the input that decides the rate, and leaving it out is why single-rate estimates go wrong.
The exempt amount, personal allowance, band width and both rates are editable, because all of them have moved recently and several have moved a long way.
Step-by-Step Example
A £20,000 gain on a £35,000 salary.
Gain: 20,000
Less annual exempt amount: 3,000
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Taxable gain: 17,000
Income 35,000 − 12,570 allowance = 22,430 taxable
Basic band 37,700 − 22,430 = 15,270 left
15,270 at 18% = 2,748.60
1,730 at 24% = 415.20
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Tax: 3,163.80
Kept: 16,836.20Effective rate 15.82%. Marginal rate 24%.
Charging the whole £17,000 at 24% would have given £4,080 — £916.20 too much. Charging it all at 18% would have been too little. The gain straddles.
The £270 Problem
Now run the same £20,000 gain on a £50,000 salary.
50,000 − 12,570 allowance = 37,430 taxable income
37,700 − 37,430 = 270 of band leftTwo hundred and seventy pounds.
Someone on £50,000 is comfortably a basic rate taxpayer as far as income tax goes. They have almost none of the band left for a capital gain, so £16,730 of the £17,000 is taxed at 24%.
This is the single most common miscalculation in UK capital gains tax. People identify as a basic rate taxpayer, assume 18%, and are out by thousands. The band is a shared resource, and your salary has already spent it.
| Income | Basic band left | £20,000 gain taxed |
|---|---|---|
| £35,000 | £15,270 | £3,163.80 |
| £50,000 | £270 | £4,063.80 |
| £80,000 | £0 | £4,080.00 |
The Exempt Amount Has Been Cut by Three Quarters
| Tax year | Annual exempt amount |
|---|---|
| 2022/23 | £12,300 |
| 2023/24 | £6,000 |
| 2024/25 | £3,000 |
| 2025/26 | £3,000 |
A £10,000 gain was entirely covered in 2022/23. Today it is taxable.
That cut is why people who have never filed a self assessment return suddenly have to. Nothing about their investing changed; the threshold moved underneath them. Any guidance written before 2023 will understate the tax badly, and a good deal of it is still online.
The allowance also cannot be carried forward. Unused, it is gone at the end of the tax year — which makes spreading disposals across two tax years a genuine and entirely ordinary piece of planning.
One Way the UK Differs Sharply From the US
Worth stating because people increasingly hold assets under both systems, and the intuitions do not transfer.
A UK capital gain does not reduce your personal allowance. The taper above £100,000 is measured on adjusted net income, and a capital gain is not income for that purpose.
A US capital gain does raise MAGI, and can push you over the threshold for the 3.8% net investment income tax.
Same instrument, opposite interaction. Reasoning from one while filing in the other produces confident, wrong answers in both directions.
There is a second structural difference: the US has a genuine 0% capital gains band that can cover tens of thousands of pounds of gain. The UK has no such band. Once the £3,000 exempt amount is used, everything above it is taxed, even with no other income at all.
Losses Are Worth Reporting Even in a Quiet Year
A capital loss is set against gains in the same tax year, and any excess is carried forward indefinitely.
The catch: a loss generally has to be reported to HMRC to be usable later. People take a loss in a year with no gains, decide there is nothing to report, and discover years later that the relief is not available when they finally need it.
The calculator refuses to answer for a negative gain rather than pretending a loss is a gain in reverse, because the rules genuinely are different.
Understanding Your Result
After tax is what you keep from the gain.
Tax due is the capital gains tax itself.
How the gain was taxed shows the exempt amount coming off and the split between the two rates.
Your remaining basic rate band is the number that decided everything — how much of the band your income left over.
Effective and marginal gives the rate across the whole gain and the rate on the next pound.
Worth knowing flags the straddle, the exhausted band, or the unused allowance.
When Should You Use This Calculator?
Before selling. Particularly if the gain is large relative to your income.
When planning across tax years. Two exempt amounts and two sets of basic rate band beat one.
After a pay rise. More salary means less band left, so the same gain costs more.
To sanity-check an estimate. If someone has applied one rate to the whole gain, it is probably wrong.
Common Mistakes
Assuming your income tax band is your capital gains rate. A £50,000 salary leaves £270 of basic band.
Applying one rate to the whole gain. Most gains straddle both.
Using an old exempt amount. It was £12,300 three years ago and is £3,000 now.
Forgetting the allowance does not carry forward. Unused, it is lost at the year end.
Not reporting a loss. It may not be available when you need it.
Assuming UK and US rules work alike. A gain reduces neither your personal allowance here nor your bands, but it does raise MAGI there.
Rates, allowances and thresholds are set annually and have changed repeatedly in recent years. Reliefs for a main residence, business asset disposal and other situations sit outside this calculation. Every figure here is an estimate for planning, not tax advice — check with HMRC or an accountant before relying on it.