The SA108 is the supplementary page you attach to your self-assessment tax return when you have capital gains or losses to report. It’s two pages long and surprisingly dense. This is a plain-English walkthrough of every box that matters for share and option traders.
Do you actually need to fill in SA108?
You must complete SA108 if any of these apply:
Your total disposal proceeds (the amount you received from all sales) exceed £12,000 — which is four times the annual exempt amount.
Your total chargeable gains exceed the £3,000 annual exempt amount.
You want to claim a loss to carry forward to future years.
You’ve previously claimed losses and want to use them this year.
Even if you owe no tax after using your annual exemption, if your proceeds exceed £12,000, HMRC wants to see the form. Missing this triggers penalties.
The structure of SA108
SA108 has several sections. For share and option traders, the two that matter are:
| Section | Boxes | What goes here |
|---|---|---|
| Listed shares & securities (Page CG3) | 23 – 30 | Shares, ETFs, funds traded on a stock exchange |
| Other property, assets & gains (Page CG2) | 14 – 22 | Options, unlisted shares, other chargeable assets |
There’s also a summary section (Boxes 1-13) where everything gets totalled up, and a section for losses (Box 47) and the annual exempt amount.
Listed shares — Boxes 23 to 30
This is where most people’s trades go. If you bought and sold shares on a recognised stock exchange (London Stock Exchange, NYSE, NASDAQ, etc.), this is your section.
| Box | Name | What to enter | Example |
|---|---|---|---|
| 23 | Number of disposals | Count of how many sell transactions you made | 12 |
| 24 | Disposal proceeds | Total amount received from all sales (in GBP) | £45,200.00 |
| 25 | Allowable costs | Total cost basis of what you sold, including purchase cost and fees | £38,750.00 |
| 26 | Gains in the year, before losses | Sum of all individual gains (only the positive ones) | £8,200.00 |
| 27 | Losses in the year | Sum of all individual losses (positive number) | £1,750.00 |
Boxes 28-30 exist but aren’t commonly used by retail investors — they cover special situations like negligible value claims.
Important: Box 24 and 25 are totals across all disposals. Box 26 is the sum of profitable disposals only, and Box 27 is the sum of loss-making disposals only. They do NOT simply equal Box 24 minus Box 25 — because gains and losses are reported separately.
Other assets (options) — Boxes 14 to 22
If you traded options (calls, puts), those go here. The boxes mirror the shares section:
| Box | Name | What to enter |
|---|---|---|
| 14 | Number of disposals | Count of option disposals (closes, expiries — but NOT assignments if rolled into stock) |
| 15 | Disposal proceeds | Total proceeds from closing/expiring options |
| 16 | Allowable costs | Total cost of closing/expiring options |
| 17 | Gains in the year, before losses | Sum of all option gains |
| 18 | Losses in the year | Sum of all option losses |
Remember: assigned options where the premium is merged into a stock trade (under TCGA s.144) show as £0 gain here, with the premium folded into the stock section instead.
Summary section — putting it together
The summary boxes combine both sections:
| Box | What it is | How to calculate |
|---|---|---|
| 7 | Total gains | Box 26 + Box 17 (gains from shares + gains from options) |
| 8 | Total losses | Box 27 + Box 18 (losses from shares + losses from options) |
| 9 | Net gain (gains minus losses) | Box 7 − Box 8 |
| 47 | Losses carried forward | If Box 8 > Box 7, the excess carries forward |
The annual exempt amount (£3,000) is applied to your net gain. You only pay tax on the excess.
A real filing example
Let’s say David traded shares on Trading 212 and options on Robinhood UK in 2025/26:
Shares: 8 disposals, total proceeds £32,000, total costs £26,500. Individual gains: £7,200 across 6 profitable sales. Individual losses: £1,700 across 2 loss-making sales.
Options: 5 disposals (3 closed, 2 expired). Total proceeds £2,800, total costs £1,600. Individual gains: £1,400. Individual losses: £200.
David’s SA108:
| Box | Value |
|---|---|
| 14 (option disposals) | 5 |
| 15 (option proceeds) | £2,800 |
| 16 (option costs) | £1,600 |
| 17 (option gains) | £1,400 |
| 18 (option losses) | £200 |
| 23 (share disposals) | 8 |
| 24 (share proceeds) | £32,000 |
| 25 (share costs) | £26,500 |
| 26 (share gains) | £7,200 |
| 27 (share losses) | £1,700 |
| 7 (total gains) | £8,600 |
| 8 (total losses) | £1,900 |
| Net gain (7 − 8) | £6,700 |
| After £3,000 exemption | £3,700 taxable |
If David is a basic rate taxpayer: £3,700 × 18% = £666 CGT due.
Box 51 — the October 2024 warning
If you made any disposals on or after 30 October 2024 in the 2024/25 tax year, tick Box 51. This alerts HMRC that the new higher rates (18%/24%) may apply to some of your gains. For 2025/26, this box isn’t relevant since the new rates apply to the full year.
Filing deadline
The deadline for online self-assessment is 31 January following the end of the tax year. For 2025/26 (ending 5 April 2026), the deadline is 31 January 2027.
Paper returns have an earlier deadline: 31 October 2026.
Late filing attracts an automatic £100 penalty, with further penalties if you’re more than 3 months late.
Let TaxBull do the SA108 mapping
TaxBull generates all the SA108 box figures automatically. Upload your broker CSV, and it separates listed shares from options, calculates gains and losses per disposal, and produces a text report with every box pre-filled. You also get a PDF report suitable for attaching as supporting documentation.
You still need to enter the numbers into HMRC’s online system yourself — but having them pre-calculated and checked saves hours and reduces the risk of errors.
This guide is for informational purposes only. Always verify figures with a qualified accountant before filing your return.
Free HMRC-compliant calculator with SA108 output. Supports Robinhood UK, Trading 212, Freetrade, and more.
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