UK Capital Gains Tax Calculator

The UK Capital Gains Tax calculator prices a single disposal at the 2026-27 rates in about ten seconds. For the wider rulebook (what counts as a disposal, what is exempt, the Bed-and-ISA mechanics, the policy backdrop) read the full Capital Gains Tax guide. This page is the operator's manual for the calculator itself: the five inputs, a worked example, and the half dozen edge cases the model deliberately hides so the front-page bill stays honest for the 90% of disposals it actually fits.

Contents

• What the calculator does
• The five inputs, in plain English
• Worked example: £20,000 gain, basic-rate worker
• What the calculator does not model
• Using it strategically across the tax year
• When the headline number is not your final bill

What the calculator does

One disposal, one bill. You give it the sale proceeds, what you paid, your other taxable income for the year, and how much of the £3,000 annual exempt amount you have already used. It returns the CGT due, the effective rate on the gain, and a breakdown that shows you how much of the gain was sheltered, how much sat inside the basic-rate band at 18%, and how much spilled into the higher-rate band at 24%.

The point of stacking the gain on top of your income (rather than just asking "are you a higher-rate taxpayer?") is that most retail disposals straddle the £50,270 threshold. A worker on £40,000 with a £20,000 gain is partly a basic-rate payer on the gain and partly a higher-rate payer on it. The calculator prices both portions separately. Asking "what is your tax band?" and applying a single rate would be wrong by hundreds of pounds in plenty of common cases.

The 2026-27 rates baked in are 18% within the basic-rate band and 24% above it, applied uniformly to shares, crypto and residential property. That harmonisation came in on 30 October 2024. The old 10% / 20% rates for non-property assets are gone.

The five inputs, in plain English

Asset type. Shares, property or crypto. Since the October 2024 harmonisation the rates are identical, so the toggle changes the calculation by nothing. It is still there because the reporting deadline differs. Residential property is reportable and payable within 60 days of completion through the HMRC real-time service. Shares and crypto go on the Self Assessment return for the year.

Disposal proceeds. What you sold for, net of dealing fees and broker commission. If your platform shows £30,000 sale value but £30 of commission, use £29,970. For a property, use the sale price net of estate agent fees and conveyancing.

Base cost. What you paid, plus stamp duty, plus any allowable improvement costs (capital improvements to a property, not maintenance). For shares bought in chunks over years, the base cost is the weighted average from the Section 104 pool, not the price of your most recent purchase. This is the input people get wrong most often, see the edge case section below.

Your other taxable income. Salary, self-employment profit, taxable dividends, rental income, before any pension contribution. This is what sets whether the gain sits in the basic-rate band or spills into the higher one. Leave it at zero only if you genuinely have no other UK taxable income that year.

Allowance already used. Out of the £3,000 annual exempt amount. If you sold £4,000 of another holding in May and used £4,000 of allowance there (well, £3,000 of it), enter £3,000 here and the calculator will not double-count the shelter.

Worked example: £20,000 gain, basic-rate worker

Set asset type to shares. Salary £40,000, so other taxable income £40,000. Proceeds £30,000, base cost £10,000. You have not used any AEA yet.

• Raw gain: £30,000 - £10,000 = £20,000.
• Less the £3,000 annual exempt amount: £17,000 taxable gain.
• Taxable income after the £12,570 personal allowance: £27,430.
• Headroom in the basic-rate band: £50,270 - £12,570 - £27,430 = £10,270.
• £10,270 of the gain taxed at 18% = £1,848.60.
• £6,730 of the gain spills above £50,270 and is taxed at 24% = £1,615.20.
• CGT due: £3,463.80. Effective rate on the gain: 17.3%.

Now change the salary to £55,000. Same gain, same proceeds, but you are already a higher-rate taxpayer before the gain hits.

• Headroom in basic-rate band: zero.
• All £17,000 of the taxable gain is at 24% = £4,080.
• An extra £616 of tax for the same disposal. Same investor, same fund, same dates. That is what the spillover does.

For a higher-rate worker, timing matters more than the headline rate (24% is 24%). The salary sacrifice optimiser shows how pension contributions in the same tax year can reduce your taxable income, which in turn affects where a later gain stacks. Whether that works for your situation depends on your earnings, contribution headroom and other reliefs - this is information, not personal tax advice.

What the calculator does not model

The model trades coverage for clarity. The following sit outside it and need a manual adjustment to the inputs before the answer is right.

S104 share pooling. Multiple purchases of the same share over years pool into a single weighted-average cost. The calculator wants the right base cost, not your most recent buy price. Work out the pool average from your broker statements before you type it in.

Partial disposals. Selling half a holding requires you to apportion the base cost. If you bought £10,000 of a fund and sold half, the base cost going into the calculator is £5,000, not £10,000.

Spousal transfers. Gifts to your spouse or civil partner are not disposals. There is a powerful planning move here (transfer half the holding before sale to use both £3,000 allowances and both basic-rate bands), but the calculator only prices one person at a time. Run it twice with half the gain each side.

EIS / SEIS deferral, BADR, gift relief. Specialist reliefs are not modelled. If you are claiming any of these, the calculator's number is the upper bound before relief, not the bill you actually file.

Forex on US stocks. HMRC requires you to convert proceeds and cost into sterling at the spot rate on the day of the transaction. A flat dollar gain can become a sterling loss if the pound strengthened between purchase and sale. Convert before you enter the figures.

Main-residence elections. If you have two homes and have not formally elected which one is your main residence, the calculator cannot work out how much PRR applies. That is a specialist case.

For any of the above, treat the calculator's output as a sanity check and a qualified UK tax adviser as the source of truth.

Using it strategically across the tax year

Run the calculator twice with the same numbers but different dates and you see why the £3,000 allowance is a use-it-or-lose-it lever rather than a "deal with it at filing" annoyance.

A £6,000 gain realised entirely on 5 April uses one year's AEA. The taxable gain is £3,000, taxed at 18% in the basic-rate band: £540 of CGT. Split that £6,000 across two disposals, £3,000 on 5 April and £3,000 on 6 April, and you use two years' AEA. Taxable gain on each side: zero. CGT bill: nothing.

That mechanic is the reason annual Bed-and-ISA gets cited so often. The pattern: sell up to £3,000 of gain inside your General Investment Account, buy the same fund back inside your ISA. The sale uses the AEA, resets the cost base higher, and moves the asset into a wrapper where gains are not subject to CGT under current rules. The 30-day rule does not block this because the buyback happens in a different account.

Combine that with the Stocks and Shares ISA £20,000 contribution cap and, under current rules, a working couple can in principle shelter up to £40,000 a year of fresh contributions plus another £6,000 of GIA gains (£3,000 each via the AEA) outside CGT. Allowances and rules change - this reflects 2026-27 figures only.

When the headline number is not your final bill

The calculator returns the tax on this single disposal in isolation. Three things change the final number on your Self Assessment return.

Carried-forward losses. Capital losses from prior years (registered with HMRC within four years of the loss) offset future gains pound for pound, before the AEA is applied. If you have £4,000 of carried losses and the calculator says £20,000 of taxable gain, your real taxable gain is £16,000.

Current-year losses. A loss-making disposal in the same tax year nets off against the gain before the AEA is touched. The calculator only knows about the one disposal you entered.

Foreign tax credits. Property sold abroad may already have suffered local capital gains tax. Double taxation treaties usually let you credit that against the UK bill, but you have to claim it. The calculator does not.

If any of those apply, treat the result as the gross UK CGT on this disposal and adjust on your return. For anything that is more than a single share sale or a single property completion, the full CGT guide walks through the structure that keeps most retail investors at zero forever.