Tax-Loss Harvesting: Turning a Capital Loss Into a Tax Asset
Selling an investment that has fallen turns a paper loss into a realised one, which can be set against gains and reduce a Capital Gains Tax bill. In the UK there are two rules that decide whether that is worth doing at all: the annual exempt amount, which means many people owe nothing anyway, and the share identification rules, which constrain buying the holding back.
- The answer:: Realised losses are set against realised gains in the same tax year, before the annual exempt amount is applied. Anything unused is carried forward indefinitely, provided the loss is reported to HMRC.
- The trap:: If your gains are already inside the annual exempt amount you owe no CGT, so realising a loss saves nothing now and simply uses up a loss you could have banked. The exempt amount cannot be carried forward, but losses can.
- The recommendation:: Report losses even in years they do nothing, because carrying them forward requires it. And where the aim is to hold the same asset in a shelter, a Bed and ISA does the job the identification rules block a straight repurchase from doing.
Where the AI summary above gets this wrong
"Sell your losing shares before the end of the tax year to cut your Capital Gains Tax bill."
That's surface-true. Here's what it misses:
- Most people owe no CGT anyway — Gains within the annual exempt amount are not taxed. If your realised gains are below it, realising a loss reduces your bill from nothing to nothing, and consumes a loss that could have been carried forward instead.
- Losses are applied before the exemption — Losses of the same year reduce gains first, and the annual exempt amount is applied to what remains. That ordering is what makes harvesting pointless below the threshold — the exemption was going to cover the gain regardless.
- You cannot just buy it straight back — The share identification rules match a disposal against an acquisition of the same shares within the following 30 days. Repurchasing immediately means the sale does not produce the loss you were after.
01 When harvesting does nothing at all
Capital gains within the annual exempt amount are not taxed, and losses of the same tax year are set against gains before that exemption is applied. Put those two facts together and the conclusion is that harvesting below the threshold achieves nothing: the exemption was going to cover the gain anyway, and the loss has been spent to no effect.
The exempt amount cannot be carried forward — unused, it is gone at the end of the tax year. Losses can be carried forward indefinitely. So in a year where your gains are small, the better move is usually to leave the loss unrealised and let the exemption do its work.
This is the reverse of the advice that circulates each March. Harvesting is a tool for people whose realised gains exceed the exempt amount, which since the amount was reduced is a larger group than it once was, but still not everyone.
02 What the loss is actually worth
The saving is the amount of gain the loss removes from charge, multiplied by your CGT rate. Since UK CGT rates differ by the type of asset and by whether you are a basic or higher rate taxpayer, the same loss is worth materially different amounts to different people.
That makes the timing question a rate question. A loss carried forward into a year when you expect to be a higher rate taxpayer, or to realise a large gain, is worth more than the same loss used now against a small gain taxed at a lower rate.
The worked example applies your own figures against the exempt amount so you can see whether this year's loss does anything, and what is left over to carry.
Shows: the CGT a realised loss saves once your annual exempt amount has already been used, and what is left to carry forward. Ignores: dealing costs, the share identification rules on repurchase, and any losses already carried forward from earlier years.
Source: Losses and Capital Gains Tax
03 The identification rules, and why repurchase is constrained
The UK matches disposals against acquisitions in a set order, and one of those matching rules covers shares of the same class in the same company acquired within 30 days after the disposal. If you sell and buy back inside that window, the disposal is matched to the repurchase rather than to your original holding, and the loss you intended to realise does not arise as expected.
There is no equivalent restriction on buying something different. Selling one fund at a loss and buying a different fund tracking a similar market keeps you invested and produces the loss, because the asset acquired is not the asset disposed of.
The other route is the one the rules leave open deliberately. Selling in a taxable account and repurchasing inside an ISA — a Bed and ISA — moves the holding into a shelter, and the mechanics and costs are covered in Bed and ISA.
04 Reporting, which is the part people skip
A loss you do not report is a loss you cannot use. Carrying a capital loss forward requires it to have been notified to HMRC, and there is a time limit for doing so. Losses sitting unreported in an old contract note are worth nothing when the large gain finally arrives.
This is the most common failure in the whole strategy, because the years in which a loss does nothing are exactly the years people do not think about reporting it. The loss arises, no tax is due, nothing is filed, and the asset quietly ceases to exist.
The fix is administrative rather than clever. Report losses in the year they arise even when they save nothing, keep the records, and the carried-forward figure is available when it is worth something.
Source: Individual Savings Accounts
05 What to actually do
Work out your realised gains for the tax year first, and compare them to the annual exempt amount. If you are below it, harvesting is usually the wrong move and the exemption is doing the job for free.
If you are above it, look for holdings you were willing to change anyway. Selling into a different fund keeps you invested and sidesteps the identification rules, and it is the cleanest version of the strategy because the portfolio decision stands on its own.
Then report the loss, whatever it did this year. And consider whether the underlying problem is that the money is in a taxable account at all — using the ISA allowance removes the question entirely for future years.
Source: Tax on savings and investments
The UK version of this strategy is mostly an exercise in finding out you did not need it. The annual exempt amount covers a lot of people's realised gains entirely, and for them harvesting spends a loss to save nothing. Where I have seen it matter is the opposite case — someone with a genuinely large gain and a forgotten loss from years earlier that was never reported, and therefore never usable. The valuable habit is the boring one: report the loss in the year it happens, even when it does nothing.
FAQ
Does realising a loss always reduce my UK tax bill?
No. Losses of the same year are set against gains before the annual exempt amount is applied, so if your gains are already within the exemption you owe nothing and the loss has been used for no benefit. Below the threshold it is usually better to leave the loss unrealised.
Can I sell a share for the loss and buy it back the next day?
Not effectively. The share identification rules match a disposal against an acquisition of the same shares within the following 30 days, so an immediate repurchase means the loss does not arise as intended. Buying a different holding, or repurchasing inside an ISA, avoids the problem.
How long can I carry a UK capital loss forward?
Indefinitely, but only if you have reported it to HMRC within the time limit. An unreported loss cannot be used later, and the years in which a loss saves nothing are exactly the years people forget to report it.
Is a Bed and ISA a way of harvesting a loss?
It can be both at once. Selling in a taxable account realises the loss and repurchasing inside an ISA moves the holding into a shelter, which the identification rules permit. The costs and the allowance it consumes are the things to check first.
Sources
Regulator references
- Capital Gains Tax rates and allowances · GOV.UK · 2025How Australian capital gains and losses are calculated.Last verified: 2026-09-07
- Losses and Capital Gains Tax · GOV.UK · 2025How Australian capital gains and losses are calculated.Last verified: 2026-09-07
- Shares and Capital Gains Tax: identification rules · HM Revenue & Customs · 2025How Australian capital gains and losses are calculated.Last verified: 2026-09-07
- Individual Savings Accounts · GOV.UK · 2025The ISA rules a Bed and ISA repurchase relies on.Last verified: 2026-09-07
- Tax on savings and investments · MoneyHelper · 2025Government-backed guidance on tax on savings and investments.Last verified: 2026-09-07
Calculator unit tests · the assertions this page's worked example is checked against, and their last result
Changelog
- 2026-09-07 — initial publish (new format)
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Jordan Reeves
Founder of Talk Through Wealth. A software engineer for over a decade before turning to retirement planning, Jordan built the projection engine after watching family members get fragmented, country-by-country advice that never reconciled. He writes about retirement the way the engine computes it: month-by-month, lifetime-long, and skeptical of any rule of thumb that hasn't been run through the math.