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Capital gains tax on shares and the 30 day trap

Fahad ZarCrypto tax accountant, licensed and regulated by the AAT. MSc Accounting and Finance, BPP University London. Six years reconciling wallet and exchange histories for UK filings.

Published 11 August 2026· Reviewed against gov.uk 25 August 2026· 4 min read·Capital gains tax

Sell shares for more than they cost you and the gain above your £3,000 allowance is taxed at 18% or 24%, depending on where you sit against the basic rate band. Simple enough. What is not simple is the phrase “more than they cost you”, because for shares, cost is defined by matching rules most investors have never heard of, and one of them, the 30 day rule, rewrites gains for anyone who sells and rebuys the same holding.

Which shares are even taxable

Start by excluding the shelters. Anything inside an ISA or a pension is outside capital gains tax entirely, no reporting, no allowance used. Gilts are exempt. What is left, the general investment account, the employee shares you kept, the inherited portfolio, is where the rules below live, and GOV.UK’s shares guidance is the official map.

The matching rules that define your cost

You do not choose which of your shares you sold. HMRC’s share identification rules impose the order: shares bought the same day are matched first, then shares bought in the following 30 days, then everything else at the average cost of your pooled holding, the section 104 pool.

The 30 day rule is the one with teeth. It exists to stop bed and breakfasting, the old practice of selling on Friday and rebuying on Monday to bank a loss while keeping the position. Sell at a loss and rebuy within 30 days and your sale is matched against the new purchase, not the pool, and the loss you intended usually shrinks or vanishes into the new holding’s cost. Investors doing routine rebalancing trip this constantly without knowing it, and monthly investment plans walk into it on schedule. Crypto runs on the same machinery, where we see the damage weekly, our crypto gains guide covers that side.

The pool itself carries its own trap for long holders. Your average cost includes every purchase, every reinvested dividend, every corporate action back to the beginning, and when brokers change or platforms migrate, that history is what goes missing. A pool rebuilt from partial records nearly always understates cost, which means overstated gains, which means overpaid tax.

Costs you can add, income you must not forget

Dealing fees and the stamp duty paid on purchases go into your cost. On the other side, accumulation fund units reinvest income that you were taxed on as income at the time, and that reinvested amount adds to your base cost too. Miss it and you pay tax twice on the same money, once as income when it accrued, again as gain when you sell. Anyone holding accumulation funds for a decade outside a wrapper almost certainly has a base cost higher than their platform reports.

Reporting

Gains above £3,000, or total proceeds above £50,000 even without taxable gains, go on the capital gains pages of your return. Losses want claiming in the same breath, they expire unclaimed after four years, as covered in yesterday’s post on losses. And the allowance planning that applies everywhere applies here, two spouses, two allowances, transfers between them at no gain, timing across 5 April.

Fahad Zar

Crypto tax accountantAAT licensed 1010475

• AAT Licensed Accountant and MAAT, licence 1010475
• MSc Accounting and Finance, BPP University London
• Six years inside digital asset accounting, across several crypto tax firms
• Practises through Zar Enterprises Ltd, ICO registration ZC225094
• Supervised for anti-money laundering by the AAT

What matters to me is that the number on the return is true, whatever it turns out to be.

Whether this is worth a conversation

Most people who read this page do not need an accountant. A few do.

• Your gains are near the £3,000 line
• You have swapped tokens and doubt the numbers
• You have old losses nobody ever claimed

If you sold one holding once and the figures are plain, file it yourself. Paying someone to check arithmetic you can do is not a service.

If one of these is you, the first look costs nothing.

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