SA108 is the Capital Gains Tax summary in your Self Assessment return. If you sold shares, funds or ETFs listed on a stock exchange, most of the work is five totals for the year in boxes 23 to 27: how many disposals you made, what you sold for, what those holdings cost, your total gains and your total losses. The figures go in before any losses brought forward and before the £3,000 annual exempt amount.
For the 2025/26 tax year (6 April 2025 to 5 April 2026), the matching paperwork is HMRC's SA108 (2026) form and notes. This guide shows how to get from a broker's list of sales to those box figures, with a worked example.
Start with your broker records
Find the transaction history for your taxable account: Hargreaves Lansdown, Freetrade, Fidelity UK or AJ Bell. For employee shares, see the Morgan Stanley guide.
Already have your files? Calculate your gains free to work towards the figures below.
How this guide differs from the official SA108 download
The GOV.UK page gives you the paper form and HMRC's notes, which define each box. If you file online you do not need to download the form: the online return asks the same questions once you say you had capital gains. The notes assume you have already worked out your gains. They do not show how to apply the share matching rules to a broker export, how to treat dealing fees, or how to count part-filled orders. That is the gap this guide covers. Where HMRC's notes are the authority, we say so and link to them.
Do you need to fill in SA108?
For 2025/26, HMRC's notes say to fill in the Capital Gains Tax summary pages if any of these apply:
- you are registered for Self Assessment and sold or disposed of chargeable assets worth more than £50,000 in total
- your chargeable gains, before taking off any losses, were more than £3,000 (the annual exempt amount)
- you want to claim an allowable capital loss, or make another capital gains claim or election
- you have gains from an earlier year that are taxable in this year, or other special cases listed in the notes, such as claims under the foreign income and gains regime
You do not need the pages if you only sold investments held in an ISA, or UK government gilts. Sales in a general investment account still count, even if you also have an ISA with the same broker.
Small gains can still need reporting
The two tests are separate. If you are registered for Self Assessment, you can have gains well under £3,000 and still need SA108 because your total sale proceeds were over £50,000. HMRC's reporting guidance explains this condition. Equally, proceeds of only £15,000 can need SA108 if the gains on them were more than £3,000. Not sure which applies? Use the free Do I need to report capital gains? check.
Which section to use
On SA108 (2026), listed shares and securities are on page CG 2 (boxes 23 to 30). That covers shares and ETFs listed on a recognised stock exchange. Shares in companies on the Alternative Investment Market (AIM) are treated as unlisted and go in boxes 31 to 44 on page CG 3, along with shares where certain reliefs apply. Losses and adjustments are on CG 3 and CG 4. If you also traded options, read how options are taxed before filling in the boxes.
Choose the form for the tax year of the sale, not the year you file. The 2026 form covers sales from 6 April 2025 to 5 April 2026. Sales since then fall in 2026/27; see the2026/27 capital gains tax guide.
Listed-share boxes for 2025/26
Report gains and losses separately, before the annual exempt amount. Box titles below are from the SA108 (2026) notes; the explanations are ours.
| Box | What to enter |
|---|---|
| 23: Number of disposals | How many disposals you made. HMRC counts all sales of the same class of share in the same company on the same day as one disposal, so an order filled in three parts is one. |
| 24: Disposal proceeds | Total sale proceeds for the year, before any reliefs, claims or elections. |
| 25: Allowable costs | The cost of what you sold, using the same-day rule, the 30-day rule and then the Section 104 pool. Include purchase dealing fees and stamp duty. Sale fees can be counted here if box 24 shows gross proceeds; count them once, not twice. |
| 26: Gains in the year, before losses | The total of every disposal that made a gain. Do not net off losses or take off the allowance. |
| 27: Losses in the year | The total of every disposal that made a loss, entered as a positive figure. |
| 28 to 30 | Box 28 is for a claim or election code. Boxes 29 and 30 only apply if you already reported gains through HMRC’s real-time Capital Gains Tax service. |
Box 26.1 is for amounts claimed under the foreign income and gains regime. Most UK-resident investors leave it blank.
Worked example: from broker sales to boxes 23 to 27
Priya holds shares and an ETF in a general investment account. During 2025/26 she makes these sales. The pool costs come from her Section 104 pools, and she made no purchases of these holdings on the same day or in the following 30 days.
| Sale | Proceeds | Pool cost | Fees | Gain or loss |
|---|---|---|---|---|
| 14 May 2025: 300 ABC plc, two fills | £1,500 | £1,200 | £6 | £294 gain |
| 3 Sep 2025: 50 ETF units | £4,000 | £4,600 | £5 | £605 loss |
| 20 Jan 2026: 1,000 DEF plc | £9,000 | £5,500 | £10 | £3,490 gain |
Her listed-share boxes are:
| Box | Figure | Working |
|---|---|---|
| 23 | 3 | The two ABC fills on 14 May count as one disposal. |
| 24 | £14,500 | £1,500 + £4,000 + £9,000 |
| 25 | £11,321 | (£1,200 + £6) + (£4,600 + £5) + (£5,500 + £10) |
| 26 | £3,784 | £294 + £3,490 |
| 27 | £605 | The ETF loss |
As a check, box 24 minus box 25 (£3,179) equals box 26 minus box 27. Priya must fill in SA108 because her gains before losses (£3,784) are more than £3,000, even though her proceeds are well under £50,000. Her net gain is £3,179. After the £3,000 annual exempt amount, £179 is taxable at the rate for her income band.
If Priya also had £2,000 of losses brought forward from earlier years, she would use only £179 of them, the amount needed to bring her net gain down to £3,000. She would enter £179 in box 45 and could carry the remaining £1,821 forward.
Losses, adjustments and the annual exempt amount
SA108 (2026) has no box for the annual exempt amount. Do not subtract it from box 26; it is applied in the overall tax calculation. The loss and adjustment boxes most share investors meet are:
- Box 45: losses brought forward and used this year. HMRC's notes say these can only reduce gains down to the annual exempt amount.
- Box 47: losses available to carry forward to later years.
- Box 51: adjustments to Capital Gains Tax, where the notes require one.
- Box 53: put an X here if your computations include estimates or valuations.
- Box 54: any other information, such as an explanation of an estimate.
Planning ahead for this tax year? Thetax-loss harvesting calculator shows how selling at a loss before 5 April could reduce the gains you report.
Box 52EG is a memo box for gains on excluded indexed securities. Some physically backed gold and silver ETCs fall into this category; whether yours does depends on its legal structure. Those gains still go in boxes 23 to 27 as well, so the memo box does not change the tax.
Common mistakes to avoid
Watch out for these errors
- Counting each fill as a disposal. Same-day sales of the same share in the same company are one disposal in box 23.
- Putting gains in the proceeds box. Box 24 is the total sale amount, not the profit.
- Using your broker's book cost without checking it. The cost must follow the same-day rule (TCGA 1992 s.105), the 30-day rule (s.106A) and then the Section 104 pool (s.104). Brokers often show a different average, especially after a transfer from another platform. HMRC Helpsheet HS284 explains the order.
- Netting gains and losses in the share boxes. Box 26 holds total gains and box 27 total losses. Losses and the allowance are applied later.
- Putting AIM shares in the listed section. HMRC treats AIM shares as unlisted.
- Writing "see attached". HMRC's notes ask you to fill in every box that applies and send your computations as well, not instead.
- Missing the deadline. Filing online after 31 January following the end of the tax year brings an automatic £100 penalty.
Sending your computations
HMRC's notes say you must send your computations with the Capital Gains Tax summary pages, showing each gain or loss. If you file online, attach them as a PDF. A computation should show, for each disposal, the date, the proceeds, the cost and which matching rule set it, plus the Section 104 pool working behind the cost.
How FiscalFox reports map to SA108
When you upload your broker history to the FiscalFox capital gains calculator, the report gives you the figures for each SA108 section:
- Number of disposals, counting same-day sales of the same holding as one.
- Disposal proceeds, shown before sale fees.
- Allowable costs, using the HMRC matching rules, with purchase and sale fees included.
- Gains and losses, kept separate for boxes 26 and 27.
- A computation pack showing each disposal, the matching rule applied and the Section 104 pool working, which you can attach as a PDF.
FiscalFox flags some holdings for a classification check. Check the report's listed and unlisted sections against your holdings, particularly AIM shares: an ISIN identifies a security but does not confirm that it belongs in the listed-share section. If a sale has no purchase history behind it, for example after a transfer from another broker, it stays out of the totals until you add the missing cost.
Filing online or on paper
Most people file online, either through HMRC's service or commercial software that supports SA108. You type in the box figures and the system works out the tax. If you file on paper, download the SA108 pages from GOV.UK and send them with your SA100.
Standard deadlines for the 2025/26 return
Paper returns: 31 October 2026. Online returns and payment: 31 January 2027. If you need to register for Self Assessment, the usual notification deadline is 5 October 2026. Check HMRC's deadlines for exceptions.
Tips for a smooth filing
- Download the full transaction history from every broker you used, including any you have since left.
- Combine sales across all brokers into one SA108. Holdings of the same share at different brokers share one Section 104 pool.
- Keep purchase records for as long as you still own the holding. HMRC's record-keeping guidance covers what to keep after you file.
- If you had to estimate a cost, put an X in box 53 and explain how you estimated it in box 54 or your computation.
- If in doubt about any part of your return, consult a qualified tax adviser.
Official UK tax references
This guide is checked against published GOV.UK and HMRC material. FiscalFox is a calculation tool, not personal tax advice; review unusual transactions with a qualified adviser.