Self Assessment for investors: SA108, SA106 & SA100

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Most of the work in a tax return is figuring out which boxes your numbers go in. If you bought and sold shares, held some crypto, traded a few options, or got a dividend from a US or European company, three forms cover almost all of it. Here's what each one is for, and how this calculator fills them in for you.

The three forms

FormWhat it coversMain boxes
SA108Capital gains — shares, crypto, options, currency23–27, 13.1–13.8, 14–19, 45, 47
SA106Foreign income — overseas dividends and interest2, 3, 4, 5, 6 + per-country grid
SA100UK dividends and UK interest (the main return)Page TR 3

You only file the pages you need. No disposals? Skip SA108. No foreign income? Skip SA106. The calculator works out which ones apply from your broker file and shows you the figure for every box.

SA108 — Capital gains

This is the Capital Gains Summary. Every time you sell an asset for more (or less) than you paid, the gain or loss goes here, grouped by the type of asset:

SA108 sectionProceedsCostGainsLosses
Listed shares & securitiesBox 24Box 25Box 26Box 27
CryptoassetsBox 13.2Box 13.3Box 13.4Box 13.5
Other (bonds, options, FX)Box 15Box 16Box 17Box 19
HMRC SA108 Capital Gains summary form, the Listed shares and securities section, showing boxes 23 to 30
The SA108 “Listed shares and securities” section (2025–26 form): proceeds in box 24, allowable cost in box 25, the gain in box 26, losses in box 27.

Each section wants three numbers: total proceeds, total allowable cost, and the gain (or loss). The calculator applies HMRC's share-matching rules — same day, then the 30-day “bed and breakfast” rule, then the Section 104 pool — converts every foreign trade to sterling at HMRC's monthly rate, and totals each section for you. Losses you can't use this year land in box 47 to carry forward.

Crypto got its own section (boxes 13.1–13.8) from 2024–25 onwards. Before that it went in “Other property”.

HMRC SA108 form, the Cryptoassets section, showing boxes 13.1 to 13.8
Cryptoassets now have their own SA108 boxes 13.1–13.8 (2025–26 form). Disposal proceeds in 13.2, cost in 13.3, gains in 13.4, losses in 13.5.

SA106 — Foreign income

If a foreign company paid you a dividend — or an overseas account paid interest — it usually goes on the Foreign pages, not the main return. SA106 is a per-row grid: one line per country, with the gross amount, the foreign tax taken off, and the taxable amount.

HMRC SA106 Foreign form, the Dividends from foreign companies grid, with columns A country code, B amount before tax and C foreign tax taken off
The SA106 “Dividends from foreign companies” grid (2025–26 form): column A is the country, B the gross dividend, C the foreign tax taken off — one row per country.
US DIVIDEND
$100 gross
WHT 15%: −$15
You get: $85
Treaty
TAX RETURN
Declare: £79
Tax credit: £11.85
FX rate: 0.79
15% TREATY RELIEF

$100 gross dividend, US withholds $15 (15%). At FX rate 0.79: declare £79 income, claim £11.85 tax credit. You pay UK tax on £79 minus the £11.85 credit. The calculator matches withholding tax to dividends automatically.

The country matters, and this is where a lot of tools get it wrong. The country is where the company is registered, not the currency it trades in. A German company paying in euros is German income — not Irish — even though plenty of euro-denominated funds are Irish. The calculator reads the country from each holding's ISIN (the code that starts with two letters for the country of registration), so a DE… stock is Germany and an IE… fund is Ireland, even when both pay in euros.

Foreign Tax Credit Relief, worked through

Say you received £1,000 of dividends from a German company and Germany withheld tax at 26.375% — that's £263.75. You don't lose that money twice. You claim Foreign Tax Credit Relief: a credit against your UK tax for the foreign tax already paid.

The relief is the smaller of two figures: the foreign tax you paid, or the UK tax due on the same income. On top of that, the UK–Germany tax treaty caps the credit on portfolio dividends at 15%. So on £1,000 you can claim back up to £150, even though £263.75 was withheld. On SA106 you'd enter Germany, the £1,000 gross, the £263.75 foreign tax, and tick the box to claim the relief.

There's a shortcut for small amounts: if your foreign dividends are £500 or less and you're not claiming Foreign Tax Credit Relief, you can report them in box 6 on the SA100 main return instead of filling in SA106 at all. The calculator flags this for you.

SA100 — the main return

UK dividends and UK interest go straight on the main return. The only trick is keeping UK and foreign income apart — a foreign dividend with no tax withheld can look like a UK one if you only go by the totals. The calculator splits them by the holding's currency so each figure ends up on the right form.

HMRC SA100 tax return, page TR 3, the Dividends and interest from UK banks and building societies section, boxes 1 to 7
SA100 page TR 3 (2025–26 form): UK dividends and interest. Box 6 even lets you report up to £500 of foreign dividends without filing SA106 at all.

How the calculator fills them in

Upload your broker CSV and you get a report with a Self Assessment guide built in: every section shows which box and the figure to type into it. SA108 disposals are grouped by asset class, the SA106 foreign grid is laid out country by country, and UK income is separated out for SA100. You copy the numbers across — no spreadsheet, no manual share-matching, no exchange-rate lookups.

See it on a real report: the foreign dividends example shows the SA106 grid with US, Irish and Cayman holdings, and the Trading 212 example shows a full SA108 mapping. Or upload your own file and check your figures in seconds.

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