The SA108 is the supplementary page to the Self Assessment tax return (SA100) that covers capital gains and losses. HMRC publishes the SA108 form and notes, which explain how to complete each section. If you have sold shares, funds, or other chargeable assets during the tax year, you may need to complete the SA108. This guide explains when you need to file it, what information goes in each section, and how to avoid common mistakes.
When do you need to complete the SA108?
You must include the SA108 supplementary pages with your Self Assessment tax return if any of the following apply for the tax year:
- Your total disposal proceeds (the amount you received from selling assets) exceed £50,000 for the 2023/24 tax year onwards, if you are registered for Self Assessment.
- Your chargeable gains (before deducting losses and the annual exempt amount) exceed the annual exempt amount of £3,000.
- You want to claim an allowable capital loss or make any other capital gains claim or election.
- You have received a chargeable gain through a trust, or you are the personal representative of a deceased person's estate.
Lower threshold warning
The annual exempt amount is now £3,000, so modest investment gains can create a CGT bill. GOV.UK also gives a separate Self Assessment reporting threshold based on total sale proceeds: £50,000 for the 2023/24 tax year onwards.
Structure of the SA108
The SA108 is divided into several sections. For share investors, the most relevant sections are:
- Section 1 - Listed shares and securities
- Section 2 - Unlisted shares and securities
- Section 4 - Other property, assets, and gains
- Section 5 - Summary and additional information (losses, annual exempt amount)
Most individual investors selling shares through a broker will only need to complete Section 1 and the summary in Section 5.
Key boxes for listed shares (Section 1)
The following boxes need to be completed in the listed shares section. HMRC updates box numbers periodically, so always check you are using the correct version of the SA108 form for your tax year. You can find the latest form and notes on GOV.UK under "Self Assessment: Capital Gains Summary (SA108)". The key data points are:
| Field | What to enter |
|---|---|
| Number of disposals | The total count of individual disposals (sales) you made during the tax year. |
| Disposal proceeds | The total amount you received from all disposals, before deducting any costs. |
| Allowable costs (including purchase price) | The total allowable cost of the shares disposed of, calculated using the HMRC share matching rules. This includes the acquisition cost, dealing fees, and stamp duty. |
| Gains in the year, before losses | The total of all individual gains (where proceeds exceeded costs). Do not net off losses here. |
| Losses in the year | The total of all individual losses (where costs exceeded proceeds). Enter as a positive number. |
The summary section (Section 5)
Section 5 brings together your total gains and losses across all asset types and applies the annual exempt amount. The key boxes include:
- Total gains - the sum of gains from all sections.
- Total losses in year - the sum of losses from all sections.
- Losses brought forward and used in the year - any losses from previous tax years that you are applying against this year's gains. Remember, you should only apply enough to reduce your net gains to the annual exempt amount.
- Net chargeable gains - your gains after deducting losses and the annual exempt amount. This is the figure on which CGT is calculated.
- Annual exempt amount used - normally £3,000 for 2024/25 (or the amount of your net gains if they are below £3,000).
- Losses available to carry forward - any unused losses that can be carried to future years.
Common mistakes to avoid
Watch out for these common errors
- Mixing up gains and proceeds. Disposal proceeds is the total sale amount, not the gain.
- Forgetting to apply share matching rules. You must apply the same-day rule (TCGA 1992 s.105), the 30-day rule (s.106A), and Section 104 pooling (s.104) to calculate the correct allowable cost. See HMRC Helpsheet HS284 for details.
- Netting gains and losses incorrectly. In the individual sections, report gross gains and gross losses separately. Only in the summary section are they brought together.
- Over-using brought-forward losses. Only use enough to reduce net gains to the level of the annual exempt amount.
- Missing the filing deadline. The online filing deadline is 31 January following the end of the tax year. Late filing results in an automatic £100 penalty.
How FiscalFox reports map to the SA108
When you upload your broker data to FiscalFox, the platform generates a capital gains report designed to map directly to the SA108. The report provides:
- Number of disposals - counted automatically from your transaction history.
- Total disposal proceeds - summed from all sell transactions.
- Total allowable costs - calculated using the HMRC share matching rules, including dealing fees.
- Total gains and total losses - separated so you can enter them in the correct SA108 boxes.
- Detailed computation schedule - a breakdown of each disposal showing which matching rule was applied, the cost basis, and the resulting gain or loss. This is not submitted to HMRC but serves as your supporting evidence if HMRC queries your return.
You can download the report and use it to populate your SA108 directly, or keep it as a record alongside your tax return.
Filing online vs. paper
If you file your Self Assessment online (which most people do), you enter the SA108 figures directly into HMRC's online portal or compatible third-party software that supports the SA100 and SA108 forms. The online system calculates the tax due automatically based on your entries. If you file on paper, you need to request the SA108 supplementary pages from HMRC and submit them with your SA100 by the earlier paper deadline of 31 October.
Tips for a smooth filing
- Gather all your broker statements and transaction histories before you start.
- Use a tool like FiscalFox to apply the share matching rules correctly - manual calculations are error-prone.
- Keep your detailed computation schedule for at least five years in case HMRC opens an enquiry.
- If you have disposals across multiple brokers, combine them all into a single SA108 filing.
- Double-check that your total proceeds figure is correct - this determines whether you need to file.
- If in doubt about any aspect of your return, consult a qualified tax adviser.