What you pay on a £20,000 gain
£20,000 is a common disposal for someone rebalancing a portfolio outside an ISA. It is also the point at which the 30-day share matching rules start to matter, because the obvious move — sell to use the allowance, buy the same shares straight back — does not work the way people assume.
Start with the £3,000 annual exempt amount: it comes straight off the gain, which leaves £17,000 actually chargeable — the allowance alone shelters 15% of a gain this size. Whatever is left is then laid on top of your taxable income. The slice that still fits inside the £37,700 basic-rate band is charged at 18% and anything above it at 24%, so the bill runs from £3,060 on a £20,000 income up to £4,080 once your income has already used the band. At this size the difference between the two, £1,020, is often smaller than the cost of getting the timing wrong.
CGT on £20,000 by your income
Income means your total income before tax for the year. The second column shows how much of the £37,700 basic-rate band your income leaves unused — on a gain this size that is usually more than enough to keep the whole thing at 18%, which is why the bill is flat across the lower rows.
| Your income | Basic band left | Gain at 18% | Gain at 24% | CGT | You keep |
|---|---|---|---|---|---|
| £20,000 | £30,270 | £17,000 | £0 | £3,060 | £16,940 |
| £30,000 | £20,270 | £17,000 | £0 | £3,060 | £16,940 |
| £40,000 | £10,270 | £10,270 | £6,730 | £3,464 | £16,536 |
| £45,000 | £5,270 | £5,270 | £11,730 | £3,764 | £16,236 |
| £50,000 | £270 | £270 | £16,730 | £4,064 | £15,936 |
| £60,000 or more | £0 | £0 | £17,000 | £4,080 | £15,920 |
2026/27 figures. Annual exempt amount £3,000; rates 18% and 24% for disposals on or after 6 April 2026 (GOV.UK); personal allowance £12,570; basic-rate band £37,700. Assumes the whole gain falls in one tax year with no losses or reliefs.
How the gain stacks on your income
Capital gains are taxed last, after income has taken its share of the allowances and bands. That ordering is what makes a gain this size cheap for some people and not others, and it is worth seeing on real numbers rather than as a rule.
Worked through on a £40,000 income: the personal allowance covers £12,570, leaving £27,430 of taxable income, so £10,270 of the basic-rate band is unused. Take the £3,000 exempt amount off the gain and £17,000 is chargeable — £10,270 of it at 18% (£1,849) and £6,730 at 24% (£1,615), £3,464 in total. The effective rate across the whole gain is 17.3%, which is neither of the headline numbers.
Shares, funds, crypto and property: one set of rates
Most gains this size come from shares, funds or crypto, and the rate on them changed recently enough that a lot of guidance is still wrong. They used to be taxed at 10% and 20% while residential property sat at 18% and 24%. For disposals on or after 30 October 2024 the main rates were raised to 18% and 24% to match the property rates, which were left alone. So for 2026/27 there is one set of rates for everything: £4,080 on this gain for a higher-rate taxpayer whether it came from a fund or a flat.
HMRC treats cryptoassets as chargeable assets like any other, and each disposal counts — including swapping one token for another, not just cashing out to sterling. Shares bought in tranches are pooled and worked out through the share matching rules, which decide which purchase your sale is matched against. Gains inside an ISA or pension are outside the charge altogether.
The £3,000 allowance does most of the work here
The annual exempt amount is £3,000 for 2026/27 — unchanged from 2025/26 and 2024/25, but down from £12,300 in 2022/23. That cut is the reason gains that were never worth reporting a few years ago now produce a bill. On £20,000 it removes 15% of the gain outright and saves £720 at 24%, or £540 at 18% — 15% of what the tax would otherwise be.
It cannot be carried forward. Unused on 5 April it is simply gone, which is the whole argument for realising modest gains deliberately every year rather than letting them build into one large disposal. For couples the number doubles: transfers between spouses and civil partners who live together fall outside Capital Gains Tax entirely, so moving part of a holding across before a sale brings a second £3,000 and a second set of basic-rate band into play — £6,000 of gains sheltered a year between you.
Reliefs that might apply
Most gains of this size are on investments, where the reliefs are thin: the £3,000 allowance, losses, and the fact that anything held inside an ISA or pension is outside the charge completely. If the asset is your own home, Private Residence Relief normally covers it and there is usually no tax at all.
Business Asset Disposal Relief is the exception worth knowing. If the disposal is of a business, or of shares in a personal trading company where you are an employee or officer, the rate falls to 18% up to a £1 million lifetime limit — on £20,000 that would be £3,060 instead of £4,080. The rate has changed twice recently (10% up to 5 April 2025, 14% for 2025/26, 18% from 6 April 2026), so check the tax year the disposal actually falls in. Small as this gain is, spending part of a once-in-a-lifetime limit on it deserves a moment's thought.
Reporting and paying
You must report if your total taxable gains for the year are above the £3,000 allowance — on £20,000 they are, so this one has to be declared. Separately, anyone already registered for Self Assessment must report disposals whenever the total they sold assets for exceeds £50,000 in the tax year, even where the gain is below the allowance. That test is on proceeds, not profit.
There are two routes for gains on anything other than UK residential property. HMRC's 'real time' Capital Gains Tax service lets you report by 31 December in the tax year after the gain and pay by 31 January, which suits people not otherwise in Self Assessment. Otherwise it goes on your tax return in the normal way. UK residential property is the exception: report and pay within 60 days of completion.
Bringing a £20,000 gain down
At this size the bill can often be removed entirely rather than merely reduced. Use the allowance every year: £3,000 unused on 5 April is gone, so realising gains in instalments beats one large disposal. Bring in a spouse: transfers between partners who live together are not disposals, so half the holding moved across doubles the shelter to £6,000. Split the sale across two tax years: two allowances instead of one, and the second one costs nothing but patience.
Then get the asset out of the charge for good. Gains inside an ISA or pension are not chargeable at all, and selling a holding to buy it straight back inside an ISA converts a taxable position into a sheltered one — see bed and ISA explained. Do not simply repurchase in a general account: the share matching rules match a repurchase within 30 days against your sale and the gain never crystallises. And claim your losses — see claiming capital losses.
The 30-day rule that stops the obvious trick
Sell shares and buy the same shares back the next morning and HMRC does not accept that you disposed of anything meaningful. The share matching rules take your sale and match it, in order, against shares acquired on the same day, then against any acquired in the following 30 days, and only then against the pooled holding. Repurchase inside 30 days and the £20,000 gain you were trying to realise largely evaporates, along with the allowance you were trying to use.
The rules bite hardest on regular investors — monthly contributions into the same fund count as acquisitions, so a sale can be matched against a purchase you did not think of as a trade at all. Our guide to which shares you actually sold works through the ordering. Get it wrong on a £20,000 disposal and the difference between the £3,060 and £4,080 columns above is not the issue — the gain simply lands in a different year.
Work out your own gain, costs and income
Capital gains tax calculator →Frequently asked questions
How much capital gains tax will I pay on a £20,000 gain?
A higher-rate taxpayer pays £4,080 in 2026/27: the £3,000 annual exempt amount comes off first, leaving £17,000 taxable, charged at 24%. A basic-rate taxpayer on a £20,000 income pays £3,060, because £17,000 of the gain still fits inside the unused basic-rate band at 18%. Your income decides the split.
Is the rate different for property and for shares?
No — not any more. Since 30 October 2024 the main rates for other chargeable assets rose from 10% and 20% to 18% and 24%, matching the residential property rates, which were left unchanged. For 2026/27 both are 18% and 24%. What still differs is the deadline: UK residential property must be reported and paid within 60 days of completion, while other gains go through Self Assessment or the real time service.
How does the gain interact with my salary?
The gain is stacked on top of your taxable income. Whatever is left of the £37,700 basic-rate band after your income is charged at 18%, and anything above it at 24%. On a £40,000 income, £10,270 of band is unused, so £10,270 of this gain is taxed at 18% and £6,730 at 24% — £3,464 in total, an effective 17.3% on the whole gain.
Can Business Asset Disposal Relief reduce this?
If the disposal qualifies, yes: BADR charges 18% for disposals on or after 6 April 2026, up to a £1 million lifetime limit. On £20,000 that is £3,060 instead of £4,080, a saving of £1,020. It applies to disposals of a business or of shares in a personal trading company, with conditions that must be met before the sale — check eligibility with an accountant in advance, not afterwards.