{"id":104,"date":"2026-03-23T08:39:17","date_gmt":"2026-03-23T08:39:17","guid":{"rendered":"https:\/\/taxbull.co.uk\/blog\/how-to-fill-in-sa108-capital-gains\/"},"modified":"2026-03-23T08:39:17","modified_gmt":"2026-03-23T08:39:17","slug":"how-to-fill-in-sa108-capital-gains","status":"publish","type":"post","link":"https:\/\/taxbull.co.uk\/blog\/how-to-fill-in-sa108-capital-gains\/","title":{"rendered":"How to Fill in SA108 \u2014 Capital Gains Summary for Self-Assessment (Box-by-Box Guide)"},"content":{"rendered":"

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.<\/p>\n

Do you actually need to fill in SA108?<\/h2>\n

You must complete SA108 if any of these apply:<\/p>\n

Your total disposal proceeds (the amount you received from all sales) exceed \u00a312,000<\/strong> \u2014 which is four times the annual exempt amount.<\/p>\n

Your total chargeable gains exceed the \u00a33,000<\/strong> annual exempt amount.<\/p>\n

You want to claim a loss<\/strong> to carry forward to future years.<\/p>\n

You’ve previously claimed losses<\/strong> and want to use them this year.<\/p>\n

Even if you owe no tax after using your annual exemption, if your proceeds exceed \u00a312,000, HMRC wants to see the form. Missing this triggers penalties.<\/p>\n

The structure of SA108<\/h2>\n

SA108 has several sections. For share and option traders, the two that matter are:<\/p>\n\n\n\n\n\n\n
Section<\/th>\nBoxes<\/th>\nWhat goes here<\/th>\n<\/tr>\n<\/thead>\n
Listed shares & securities (Page CG3)<\/td>\n23 \u2013 30<\/td>\nShares, ETFs, funds traded on a stock exchange<\/td>\n<\/tr>\n
Other property, assets & gains (Page CG2)<\/td>\n14 \u2013 22<\/td>\nOptions, unlisted shares, other chargeable assets<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

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.<\/p>\n

Listed shares \u2014 Boxes 23 to 30<\/h2>\n

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.<\/p>\n\n\n\n\n\n\n\n\n\n
Box<\/th>\nName<\/th>\nWhat to enter<\/th>\nExample<\/th>\n<\/tr>\n<\/thead>\n
23<\/td>\nNumber of disposals<\/td>\nCount of how many sell transactions you made<\/td>\n12<\/td>\n<\/tr>\n
24<\/td>\nDisposal proceeds<\/td>\nTotal amount received from all sales (in GBP)<\/td>\n\u00a345,200.00<\/td>\n<\/tr>\n
25<\/td>\nAllowable costs<\/td>\nTotal cost basis of what you sold, including purchase cost and fees<\/td>\n\u00a338,750.00<\/td>\n<\/tr>\n
26<\/td>\nGains in the year, before losses<\/td>\nSum of all individual gains (only the positive ones)<\/td>\n\u00a38,200.00<\/td>\n<\/tr>\n
27<\/td>\nLosses in the year<\/td>\nSum of all individual losses (positive number)<\/td>\n\u00a31,750.00<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Boxes 28-30 exist but aren’t commonly used by retail investors \u2014 they cover special situations like negligible value claims.<\/p>\n

Important:<\/strong> 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 \u2014 because gains and losses are reported separately.<\/p>\n

Other assets (options) \u2014 Boxes 14 to 22<\/h2>\n

If you traded options (calls, puts), those go here. The boxes mirror the shares section:<\/p>\n\n\n\n\n\n\n\n\n\n
Box<\/th>\nName<\/th>\nWhat to enter<\/th>\n<\/tr>\n<\/thead>\n
14<\/td>\nNumber of disposals<\/td>\nCount of option disposals (closes, expiries \u2014 but NOT assignments if rolled into stock)<\/td>\n<\/tr>\n
15<\/td>\nDisposal proceeds<\/td>\nTotal proceeds from closing\/expiring options<\/td>\n<\/tr>\n
16<\/td>\nAllowable costs<\/td>\nTotal cost of closing\/expiring options<\/td>\n<\/tr>\n
17<\/td>\nGains in the year, before losses<\/td>\nSum of all option gains<\/td>\n<\/tr>\n
18<\/td>\nLosses in the year<\/td>\nSum of all option losses<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Remember: assigned options where the premium is merged into a stock trade (under TCGA s.144) show as \u00a30 gain here, with the premium folded into the stock section instead.<\/p>\n

Summary section \u2014 putting it together<\/h2>\n

The summary boxes combine both sections:<\/p>\n\n\n\n\n\n\n\n\n
Box<\/th>\nWhat it is<\/th>\nHow to calculate<\/th>\n<\/tr>\n<\/thead>\n
7<\/td>\nTotal gains<\/td>\nBox 26 + Box 17 (gains from shares + gains from options)<\/td>\n<\/tr>\n
8<\/td>\nTotal losses<\/td>\nBox 27 + Box 18 (losses from shares + losses from options)<\/td>\n<\/tr>\n
9<\/td>\nNet gain (gains minus losses)<\/td>\nBox 7 \u2212 Box 8<\/td>\n<\/tr>\n
47<\/td>\nLosses carried forward<\/td>\nIf Box 8 > Box 7, the excess carries forward<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The annual exempt amount (\u00a33,000) is applied to your net gain. You only pay tax on the excess.<\/p>\n

A real filing example<\/h2>\n

Let’s say David traded shares on Trading 212 and options on Robinhood UK in 2025\/26:<\/p>\n

Shares:<\/strong> 8 disposals, total proceeds \u00a332,000, total costs \u00a326,500. Individual gains: \u00a37,200 across 6 profitable sales. Individual losses: \u00a31,700 across 2 loss-making sales.<\/p>\n

Options:<\/strong> 5 disposals (3 closed, 2 expired). Total proceeds \u00a32,800, total costs \u00a31,600. Individual gains: \u00a31,400. Individual losses: \u00a3200.<\/p>\n

David’s SA108:<\/strong><\/p>\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Box<\/th>\nValue<\/th>\n<\/tr>\n<\/thead>\n
14 (option disposals)<\/td>\n5<\/td>\n<\/tr>\n
15 (option proceeds)<\/td>\n\u00a32,800<\/td>\n<\/tr>\n
16 (option costs)<\/td>\n\u00a31,600<\/td>\n<\/tr>\n
17 (option gains)<\/td>\n\u00a31,400<\/td>\n<\/tr>\n
18 (option losses)<\/td>\n\u00a3200<\/td>\n<\/tr>\n
23 (share disposals)<\/td>\n8<\/td>\n<\/tr>\n
24 (share proceeds)<\/td>\n\u00a332,000<\/td>\n<\/tr>\n
25 (share costs)<\/td>\n\u00a326,500<\/td>\n<\/tr>\n
26 (share gains)<\/td>\n\u00a37,200<\/td>\n<\/tr>\n
27 (share losses)<\/td>\n\u00a31,700<\/td>\n<\/tr>\n
7 (total gains)<\/td>\n\u00a38,600<\/td>\n<\/tr>\n
8 (total losses)<\/td>\n\u00a31,900<\/td>\n<\/tr>\n
Net gain (7 \u2212 8)<\/td>\n\u00a36,700<\/td>\n<\/tr>\n
After \u00a33,000 exemption<\/td>\n\u00a33,700 taxable<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

If David is a basic rate taxpayer: \u00a33,700 \u00d7 18% = \u00a3666 CGT due<\/strong>.<\/p>\n

Box 51 \u2014 the October 2024 warning<\/h2>\n

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.<\/p>\n

Filing deadline<\/h2>\n

The deadline for online self-assessment is 31 January<\/strong> following the end of the tax year. For 2025\/26 (ending 5 April 2026), the deadline is 31 January 2027.<\/p>\n

Paper returns have an earlier deadline: 31 October 2026.<\/p>\n

Late filing attracts an automatic \u00a3100 penalty, with further penalties if you’re more than 3 months late.<\/p>\n

Let TaxBull do the SA108 mapping<\/h2>\n

TaxBull<\/a> 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.<\/p>\n

You still need to enter the numbers into HMRC’s online system yourself \u2014 but having them pre-calculated and checked saves hours and reduces the risk of errors.<\/p>\n

This guide is for informational purposes only. Always verify figures with a qualified accountant before filing your return.<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

Step-by-step guide to filling in the SA108 Capital Gains Summary supplementary page for your UK self-assessment tax return.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[30,28,29,10,11,27],"class_list":["post-104","post","type-post","status-publish","format-standard","hentry","category-cgt-guides","tag-boxes-23-30","tag-capital-gains-summary","tag-filing-guide","tag-hmrc","tag-sa108","tag-self-assessment"],"_links":{"self":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/104","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/comments?post=104"}],"version-history":[{"count":1,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/104\/revisions"}],"predecessor-version":[{"id":328,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/posts\/104\/revisions\/328"}],"wp:attachment":[{"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/media?parent=104"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/categories?post=104"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/taxbull.co.uk\/blog\/wp-json\/wp\/v2\/tags?post=104"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}