Self assessment, the SA108 helper box by box

Checked against app build 2026-08-26-01-41 on 2026-08-26.

Where do crypto disposals go on a self assessment return?

In the Other property, assets and gains section of the SA108, the Capital Gains Tax summary pages of the self assessment return.

The helper report outputs one sheet per tax year with the figures ready to copy. If you are working out whether you have run out of time to file, the crypto tax deadline countdown carries the dates.

Which SA108 boxes does the helper fill?

Boxes 23 to 27, verified against the actual SA108 2025 form.

Box 23, number of disposals. Box 24, disposal proceeds. Box 25, allowable costs including the purchase price. Box 26, gains in the year before losses. Box 27, losses in the year. Whether you owe anything at all depends on the annual exempt amount, see the CGT allowance figures.

How are the figures rounded?

To whole pounds, and never against you.

Proceeds and gains round down, costs and losses round up, the direction HMRC accepts and the helper applies consistently.

Why are the loss figures labelled rather than numbered?

Because the Losses and adjustments section’s box numbering varies by year.

Losses brought forward and used this year, and losses available to carry forward, belong in that section. The helper gives them as labels on purpose, match the label to the form in front of you. Filing for a year other than 2025, check that year’s numbering before copying, the helper says the same on its face. Where the two loss figures come from is covered in the report family.

What should I check before I file?

Your reconciliation, first and last.

Only file from figures whose reconciliation you trust. The working paper carries the audit trail behind every number, each disposal with its matching rule cited, see reading citations, and the helper’s output is a computation, not a filed return and not personal tax advice.

The rest of the filing kit sits alongside it. The capital gains report lists every disposal with proceeds, cost, gain and rule. The negligible value claim letter, when your data holds a case for one, is a section 24(2) TCGA 1992 template with the cost figures highlighted for you to confirm. Keep the session file alongside the return, record keeping covers the habit.