You sold some shares at a profit. Or maybe crypto. Or a second property. Now a question you never considered looms: do you owe tax on the gain — and if so, how much, and to whom? Capital Gains Tax (CGT) is the tax most UK investors meet late and understand poorly, which is exactly why it produces the nastiest surprises. The good news: the core concepts take about fifteen minutes to grasp.

This guide explains CGT on investments from absolute zero: what it taxes, what it does not, how the calculation works, the allowances that shrink the bill, and the mistakes that inflate it.

What Capital Gains Tax Actually Is

CGT is a tax on the profit when you sell (or “dispose of”) an asset that has increased in value — not on the total sale price. Buy shares for £5,000, sell for £8,000: the gain is £3,000, and CGT applies to that £3,000 (minus allowances and costs). It covers shares, investment funds, cryptoassets, second properties, and valuable personal possessions sold for over £6,000 — but not your main home (covered by Private Residence Relief), ISAs and pensions (tax-sheltered), or everyday possessions.

The key word is dispose: selling is the obvious trigger, but so is gifting assets (except to a spouse), swapping one crypto for another, or receiving compensation for a lost asset. Many investors discover disposals they never thought of as sales. The gov.uk CGT rates and allowances pages set out the current figures — check them yearly, because rates and the annual exempt amount have changed significantly in recent years.

The Annual Exempt Amount: Your Tax-Free Slice

Everyone gets an annual CGT exempt amount — gains below it are tax-free and (below certain thresholds) need no reporting. This is the single most important number in CGT planning, and it has been cut sharply in recent years, dragging far more ordinary investors into CGT than before. Do not rely on remembered figures from old articles; check the current amount on gov.uk before making any disposal decision.

Practical implication: timing disposals across tax years (6 April–5 April) can double your exempt amount usage. Selling £20,000 of gains in March and £20,000 in April uses two years’ allowances — entirely legal, entirely sensible. This “use it or lose it” allowance is also why not selling can be costly: letting gains accumulate for a decade then selling all at once wastes years of allowances you could have harvested gradually.

Bed-and-ISA: the free CGT eraser

Selling investments and immediately rebuying them inside an ISA (“bed-and-ISA”) uses your £20,000 annual ISA allowance to shelter future gains permanently. It crystallises the gain now (using this year’s exempt amount) while protecting all future growth. For investors with unsheltered holdings, this is the highest-value routine in UK investing.

How the Calculation Works (Without Tears)

The basic formula: sale proceeds minus purchase cost minus allowable expenses (broker fees, stamp duty on shares) equals the gain. Sum your gains for the tax year, subtract allowable losses (more below), subtract the annual exempt amount, and apply the CGT rate to the remainder. Rates depend on your income tax band and the asset type — higher-rate taxpayers pay more, and residential property has its own rates and a 60-day reporting rule.

Two complications beginners meet. Pooling: for shares and crypto, HMRC averages your purchase costs across all buys (“Section 104 holding”) rather than matching specific shares to sales — you cannot cherry-pick the most expensive lot to minimise gains. The 30-day rule: if you sell shares at a loss and rebuy within 30 days, the loss is disallowed for tax (the “bed and breakfast” rule) — it matches the sale against the repurchase instead. Our Self Assessment deadlines and penalties works through pooling in the crypto context, where the same rules apply.

Losses: The Silver Lining

Capital losses are valuable: they offset gains in the same year, and excess losses carry forward indefinitely. But you must claim them — HMRC does not do it automatically, and unclaimed losses expire after four years. The strategic move is “tax-loss harvesting”: before year-end, review your portfolio and crystallise losses on hopeless positions to shelter gains elsewhere. Just respect the 30-day rule — selling and instantly rebuying to manufacture a paper loss does not work.

Keep a running losses log alongside your gains records. Many investors meticulously track winners and forget losers, then discover at filing time that they cannot evidence the losses that would have cut their bill. Losses are assets; treat them accordingly.

Common Mistakes That Inflate the Bill

Forgetting crypto-to-crypto swaps are disposals: swapping Bitcoin for Ethereum triggers CGT on the Bitcoin — a surprise for DeFi-active investors. Gifting without thinking: giving shares to your adult child is a disposal at market value, even though no money changed hands (spouses are exempt). Ignoring the property 60-day rule: UK residential property sales (beyond your main home) must be reported and the tax paid within 60 days — missing this brings penalties on top of the tax. Not using ISAs: every year of unsheltered investing is a year of future CGT you chose to pay. Guessing instead of recording: reconstructed cost bases are where HMRC enquiries begin; contemporaneous records are your armour.

When in doubt, the MoneyHelper tax resources offer free guidance, and an accountant’s hour before a large disposal routinely saves multiples of its cost.

Reporting: When and How

Report CGT via Self Assessment by 31 January following the tax year — unless the property 60-day rule applies. You may need to file even when no tax is due, if total disposals exceed the reporting threshold (check current figures). Register for Self Assessment in advance if you are not already filing; our How can i find what is on freeview without buying a tv guide covers the penalties for lateness. The discipline is the same as ever: records through the year, calculation in autumn, filing well before January.

Conclusion

Capital Gains Tax is a tax on profit above your annual exempt amount — manageable once you understand disposals, pooling, losses and the ISA shelter. The strategies that matter are simple: use allowances yearly, harvest losses deliberately, bed-and-ISA routinely, and keep records that would satisfy an enquiry. CGT rewards the organised and punishes the forgetful; fifteen minutes of understanding now saves thousands later. Start with this year’s exempt amount — it expires whether you use it or not.

Frequently Asked Questions

Do I pay CGT when I sell shares at a profit?

Yes, on gains above the annual exempt amount. Gains within ISAs and pensions are exempt, as is your main home.

What is the CGT annual exempt amount?

It changes — check gov.uk for the current figure, as it has been reduced significantly in recent years. Do not rely on old numbers.

Are crypto-to-crypto swaps taxable?

Yes. HMRC treats each swap as a disposal of the crypto you gave up, potentially triggering CGT even though no pounds changed hands.

Can I offset investment losses against gains?

Yes — capital losses offset gains in the same year and excess losses carry forward. You must claim them within four years.

Do I need to file a tax return for CGT?

Usually via Self Assessment by 31 January, though the 60-day rule applies to UK residential property. Check reporting thresholds yearly.