How to File Crypto Tax on SA108: Step-by-Step for 2025/26

SA108 is the capital gains supplementary page of the UK Self Assessment return. If you have any crypto disposals that crystallise a gain — or losses you want to register for carry-forward — they go here. This guide walks the exact boxes for 2025/26.

Who needs to file SA108

You need SA108 if any of these apply for 2025/26:

Your total capital gains (across all assets, not just crypto) exceed £3,000. Your total proceeds from disposals exceed £50,000 (the reporting threshold, separate from the gains threshold — you can be under the gains allowance but over the proceeds threshold and still owe a return). You have losses you want to register so they're available for carry-forward against future gains. You're already filing Self Assessment for other reasons and have any crypto disposals at all.

Where SA108 sits in the return

Open your Self Assessment online or paper form. The main return (SA100) asks if you have capital gains in box 7. Tick yes. That unlocks SA108 — an additional set of pages titled "Capital Gains Summary".

The form has multiple sections: Listed shares, Unlisted shares, Property and other assets, Losses and adjustments, plus a final summary. Crypto disposals go into "Other property, assets and gains" (box 14 onwards for the totals; boxes 23-34 for the per-disposal detail if completing paper).

The boxes you actually need

Box 14 — Number of disposals: Enter the count of individual disposal events. For an active trader this can be hundreds; HMRC accepts that number and expects the supporting computation to detail them.

Box 15 — Disposal proceeds: Total GBP proceeds across every disposal. Include the £-equivalent value of crypto-to-crypto swaps. This is gross proceeds, not net of cost basis.

Box 16 — Allowable costs (including the purchase price): Total cost basis applied against the disposals. Comes from your Section 104 pool computation.

Box 17 — Gains in the year, before losses: Box 15 minus box 16 if positive. Zero if negative.

Box 18 — Losses in the year: Total losses from disposals that produced a negative gain.

Box 19 — If any disposals are between connected parties, "Yes"/"No": Spouse transfers go here.

Box 25 — Total losses available to bring forward against future gains: If you're carrying any losses forward, register them here.

The required attached computation

HMRC requires a supporting computation alongside SA108 when crypto disposals are present. The computation is a per-disposal breakdown showing:

Date of disposal. Asset (token symbol + chain). Disposal proceeds (GBP). Cost basis (GBP, from Section 104 pool average at the moment of disposal). Gain or loss. Notes on same-day or 30-day matches if applicable.

CryptoLens generates this computation as a PDF that you attach to your online filing (Self Assessment lets you upload supporting documents) or post with a paper return.

Without the computation, HMRC may issue an enquiry. With the computation, the return is self-explanatory — they have everything they need.

The £3,000 allowance and how it shows up

The Annual Exempt Amount for 2025/26 is £3,000. The form calculates net gains as (box 17 minus box 18) and applies the allowance against that net figure. If your net gain is £2,500, the form charges £0 of CGT. If your net gain is £6,000, the form charges CGT on £3,000.

The allowance can't be carried forward or back. If you don't use it in 2025/26, you don't get to apply it to 2026/27.

The £50,000 proceeds threshold

You must file SA108 if total disposal proceeds exceed £50,000 even if your net gain is under the £3,000 allowance. This is a reporting threshold for HMRC's audit visibility, not a tax threshold.

For active crypto users this threshold is easy to cross without realising. £50k of gross proceeds is roughly £200-£500 of memecoin trades, or two ETH disposals at recent prices. CryptoLens flags the threshold in the annual report so you know whether you need to file.

How crypto-to-crypto swaps show up

Each crypto-to-crypto swap is one disposal AND one acquisition. The disposal side goes on SA108 like any other disposal — proceeds = GBP value of the token received, cost basis = your pool's average for the token sold.

The acquisition side doesn't go on SA108; it just adds to your Section 104 pool for the new token, with cost basis equal to the GBP-value-received. The next time you dispose of the new token, that cost basis applies.

For high-frequency traders this means SA108's disposal count can easily reach hundreds in a year. That's fine — HMRC accepts the per-disposal computation, and the totals (boxes 14-17) summarise.

Filing the return

Online: log into your Government Gateway, navigate to Self Assessment, complete SA100 and tick box 7 for capital gains. Complete SA108. Upload the supporting computation PDF in the documents-and-attachments section. Submit by 31 January 2027 for the 2025/26 tax year.

Paper: post the SA108 form alongside SA100 to HMRC's central Self Assessment address by 31 October 2026 (the paper-filing deadline is earlier than the online one).

Paying the CGT

If you owe CGT, payment is due by 31 January 2027 — the same date as the filing deadline. Pay via the Government Gateway (bank transfer, debit card) or set up a Time-to-Pay arrangement if you can't pay in full.

There's no equivalent of the 60-day Capital Gains on Property service for crypto. Crypto CGT is paid annually as part of the Self Assessment cycle, not in 60-day increments.

Common SA108 mistakes

Reporting only the net gain in box 15 instead of gross proceeds. Box 15 is gross proceeds; the net figure comes after subtracting box 16. Conflating crypto income (staking, airdrops, mining) with capital gains. Income goes on a different page entirely (SA100 box 17, "Other UK income"). Not registering losses. If you don't claim losses on SA108 box 18 or box 25, you can't carry them forward. Submitting a return without the supporting computation. Possible, but invites an enquiry.

CryptoLens's annual report formats every number for the boxes above and produces the supporting computation HMRC expects. Drop it into your filing and the boxes copy across in minutes.