Loss harvesting before 5 April, the version that survives the rules.
Positions standing at a loss are a tax asset waiting to be used. Sell before the tax year ends on 5 April and the loss offsets this year's gains. Done carelessly it achieves nothing, three rules decide the difference.
Work out what your harvest is actually worth
Four figures and you get the tax saved, the amount worth harvesting, and a plain warning if part of the loss would buy nothing. It runs on this page in your browser, nothing you type is transmitted or stored.
Nothing is computed until the figures are real. Still needed:
- Gains realised this tax year
- The loss you are considering realising
- Your other income
Computed in your browser at 2026 to 2027 rates, nothing transmitted. Copy link puts your figures after the # in the URL, the part browsers never send to a server.
Rule one, the 30-day trap
Sell to realise a loss and rebuy within 30 days, and the bed and breakfast rule matches your sale against the rebuy, not the pool, and the loss largely evaporates. The boundary is exact, sell on 13 August and the first clear rebuy day is 13 September, day 31. Anything you harvest, be ready to stay out of for a full month, or to accept the different asset you swap into.
Give the sale date and the window is counted from today.
- A sale date
Day 31 is the first clear day, counted in your browser from your own clock, so it is right whatever day you read this. Matching is per taxpayer and per asset.
Rule two, losses only work when claimed
A loss offsets gains in its own year automatically once reported, and carries forward, but only if claimed, on your return or within four years of the end of the tax year you disposed. The app's losses view tracks used and carried amounts, and the SA108 helper outputs both figures labelled for the return.
Rule three, never harvest into the allowance
The first £3,000 of net gains is exempt this year anyway. Losses spent bringing gains below that line buy nothing, the allowance would have covered them, and the allowance dies each 5 April while claimed losses carry forward. Harvest down to the allowance, not through it. The ladder in the calculator above makes the wasted zone visible, the bars stop growing while the harvest keeps growing.
See your own harvestable losses, counted and dated
The planner lists your unrealised losses with the offset quantified against this year's taxable gains, warns when a harvest would be wasted below the allowance, and counts down open 30-day windows per asset. Your data, your machine, free up to 1,000 transactions.
Open the harvesting viewCommon questions
What is loss harvesting?
Selling positions that stand at a loss so the loss becomes real and offsets your gains in the same tax year, cutting the tax bill. Unused losses carry forward once claimed.
Can I sell and immediately buy back to bank the loss?
Not usefully. A rebuy within 30 days matches the sale against the rebuy under the bed and breakfast rule, and the loss you wanted largely disappears. The first clear day is day 31.
Do I need to claim losses for them to count?
Yes. Losses work only when claimed, on your return or within four years of the end of the tax year of the disposal. Unclaimed losses are money left on the table.
Is harvesting worth it under the £3,000 allowance?
Usually not. Losses used to bring gains below the allowance are wasted, the allowance would have covered those gains anyway. The calculator on this page warns you before you harvest into the allowance.
How much should I harvest?
Down to the allowance line and no further. Harvest the amount that takes your net gains to £3,000, keep the rest of the loss unrealised, and it stays available for a future year when it can meet taxed gains.
Does the calculator send my figures anywhere?
No. It runs on this page in your browser, nothing is transmitted or stored. The copy link button puts your inputs after the # in the URL, and browsers never send that part to any server.
Loss rules, gov.uk, capital gains tax losses, allowance, gov.uk, allowances, both accessed 22 August 2026. The 30-day rule sits in section 106A TCGA 1992. The calculator is a computation on the figures you type, not personal tax advice.