Claiming UK crypto losses on Self Assessment (2026 guide)

Crypto losses are valuable. Used correctly, they offset gains in the same year, carry forward indefinitely, and protect future profits from CGT. Used incorrectly — or not claimed at all — they evaporate. UK rules are precise about what counts as a loss, when you can claim, and what the deadline is for getting the claim on file. This guide walks through the mechanics with a worked example.

Two rules to remember

In-year offset

Losses crystallised between 6 April and 5 April are netted against gains crystallised in the same window before the £3,000 annual exempt amount is applied. The mechanics on SA108 are:

1. Box 17 — gains in the year, before losses (gross).
2. Box 18 — losses in the year (gross).
3. HMRC subtracts box 18 from box 17, then deducts the £3,000 AEA.
4. What remains is the chargeable gain, taxed at 18% / 24%.

You do not get to choose which gains the losses offset — the netting is automatic. Practically this means a loss harvested late in the tax year is fully usable against a gain earlier in the same year.

Carry-forward indefinitely

If your losses exceed your gains, the excess does not vanish. It carries forward — but only if you claim it. Carry-forward is automatic in software systems but not in HMRC's. You must:

• Report the loss on the SA108 of the year it arose, even if you have no gains and otherwise would not file.
• Tick the relevant box to indicate carry-forward.
• Keep records — HMRC may ask for proof of basis years later.

Once on file, the loss sits in your CGT history indefinitely. In a future year you draw from it via SA108 box 19 (losses brought forward used). The annual exempt amount is applied first, so you only ever use brought-forward losses against gains exceeding £3,000.

The 4-year claim window

This is where filers lose money. The deadline to claim a loss is four years from the end of the tax year in which the loss arose. After that, the loss is gone — irrespective of how carefully you recorded it.

Example timeline:

• Loss crystallised: 15 January 2026 (in 2025/26 tax year).
• Tax year ends: 5 April 2026.
• Four-year deadline: 5 April 2030.
• Last opportunity to claim by amending or writing to HMRC: 5 April 2030.

The simplest way to never miss this deadline is to declare every realised loss on the SA108 of the year it happens — even if you don't have any gains.

Negligible value claims

For tokens that have collapsed to effectively zero, you do not need to actually sell them to crystallise the loss. UK CGT allows a negligible value claim under TCGA 1992 s.24. You declare the asset has become of negligible value, and HMRC treats the claim as a deemed disposal at zero, producing the loss.

For HMRC to accept the claim, two things must be true:

1. The asset must be genuinely of negligible value at the time of the claim (not just down 80%).
2. You must still own the asset.

Backdating is allowed: you can claim that the asset became of negligible value up to two tax years before the year of the claim, provided it was actually negligible then and you owned it then.

Rugged tokens, lost wallets and stolen crypto

Three common patterns:

• Rugged tokens. If the project collapsed, the token typically qualifies for a negligible value claim — book the loss in the tax year of the collapse.
• Lost wallet (forgotten seed phrase, dead drive).HMRC's position is that you still own the asset; loss of access is not loss of ownership. Generally not deductible unless you can prove permanent and complete loss with extensive evidence.
• Stolen crypto (hack, exploit, social engineering). Often deductible as a capital loss, but the case must be documented with police reports and exchange correspondence. HMRC reviews theft claims case-by-case.

See HMRC's position at gov.uk Cryptoassets Manual.

Worked example

Suppose during the 2025/26 tax year:

• You realised £15,000 of gains on BTC and ETH disposals.
• You realised £30,000 of losses when you sold a basket of underperforming alts.

The treatment on SA108:

1. Box 17 (gains): £15,000.
2. Box 18 (losses): £30,000.
3. Net position: −£15,000.
4. £3,000 AEA cannot reduce a loss further; CGT due is £0.
5. £15,000 of unused loss is claimed for carry-forward.

Result: zero CGT for 2025/26, plus a £15,000 loss bank available to offset gains in 2026/27 and beyond until used. If you don't claim it on the 2025/26 return, the four-year clock starts ticking and the loss is at risk of expiry.

How to file the loss

On the HMRC online return:

1. Tailor your return — tick yes to chargeable assets disposed.
2. SA108 — enter total proceeds, allowable costs and gains as normal.
3. Box 18 — enter total losses for the year.
4. Indicate carry-forward in the relevant prompt.
5. Attach the computation PDF showing each disposal and matched cost.

For the full filing walkthrough, see how to report crypto on taxes. For a refresher on share-pooling and the 30-day rule, see how to calculate crypto taxes. To put losses in the wider tax-planning context, see getting the most from your tax return.