UK options CGT calculator 2026/27

What the calculator does, and what it assumes

List a tax year’s option events and the calculator applies the UK capital gains rules to each: a premium received, a bought option sold or left to lapse, exercise, assignment, a buy-back under section 148 and cash settlement. It then works out the year, from gains and losses to the 18% and 24% split and the SA108 boxes.

It is built for a UK-resident individual who holds options as an investment, HMRC’s starting view for individuals dealing in shares and derivatives (BIM56850). It covers listed options: ICE stock options on UK shares (1,000-share standard and 100-share mini contracts), ICE FTSE 100 options, and US equity and index options converted into pounds. Each result is an illustration under the assumptions stated with it: the calculator does not know your circumstances and does not replace your records or a tax adviser. Nothing you type is uploaded; Remember on this device keeps your figures and events in this browser only, and Link to these events carries the events but never your income figures.

The calculator

It opens with the worked example below. Change any figure, add events or clear everything; The working for each event shows every figure and the rule behind it.

Eleven outcomes and the year each lands in

For a writer the date is what matters: a premium is taxed in the year the option is written, and what happens later mostly adjusts that grant. Longer examples are in the written options and bought options sections of the tax worked examples.

On exercise or assignment the share trade is dated when the option is exercised, not at settlement two business days later (TCGA 1992 s28(2)). When a grant from one tax year is assigned or cash-settled in the next, the grant stops being chargeable and any tax already returned on it is set off or repaid (CG12317). The calculator notes both.

Worked example: one reader’s 2026/27

The reader has taxable income of £36,000 after the Personal Allowance and sold other shares for £12,000 at a gain of £5,000. Commission follows the model sheet: £1.40 a contract on ICE stock options (charged on assignment too), £1.70 on FTSE 100 options and the $1.00 US minimum. Premiums were priced with the site’s model on Monday 17 August 2026 and rounded to the tick; later prices are the assumptions shown. Model inputs: r 3.75% (US 3.625%); FTSE 100 10,750, dividend yield 3.05%, IV from the site’s FTSE surface (15.71% at 10,300, 13.08% at 11,000, kept per strike); BP 530p, IV 26%; HSBC 1,530p, IV 25%, American; the US share IV 30%; the UK share IV 26%; no ex-dividend date in any share option’s life. BP and HSBC are model underlyings, not a view on either company.

Gains before losses are £7,372.26 (£2,372.26 from these events plus the £5,000) and losses £951.80, so net gains are £6,420.46 and, after the £3,000 annual exempt amount, taxable gains £3,420.46. The unused basic rate band is £37,700 less £36,000, or £1,700, taxed at 18% (£306.00); the other £1,720.46 is taxed at 24% (£412.91). Capital gains tax: £718.91 on these assumptions. Proceeds of £21,679.68 are under £50,000, but gains exceed £3,000 and a loss is claimed, so the SA108 is needed.

The 30-day rule on an option series

HMRC pools options of one series in a section 104 holding and applies the share identification rules (CG55535): a disposal is matched first with the same series bought that day, then with any bought in the next 30 days, then with the holding at average cost (TCGA 1992 s105, s106A, s104). So the calls sold on 18 September for £1,150 are matched with those bought back seven days later for £1,200 plus commission: a loss of £53.40, not £1,118.40. The first purchase, £2,266.70 with commission, stays in the holding for the November sale, turning a £166.60 gain into a £898.40 loss.

Within one year the net is the same, −£951.80, though the boxes differ. It bites when the repurchase falls after 5 April or the second position is still open at the year end. The calculator matches rows with the same underlying and series, and bought options only; leaving grants out is an inference, as HMRC’s text is silent. More in the 30-day rule section.

A US option: two dates, two rates

Dollar costs and proceeds are each converted at the rate on their own date, never as one net figure (CG78310). In event 5 the $540 of proceeds at $1.3344 give £404.68; the $311 cost at $1.3559 gives £229.37, and the $1 closing commission £0.75: a gain of £174.56. The net $228 converted in one step would give £170.86 or £168.15, depending on the rate. HMRC prescribes no rate source, so the calculator uses the rate you enter, such as your broker’s; these are European Central Bank reference-rate crosses. See the FX rule and US options from the UK.

A buy-back after 5 April

Event 7 was written in March 2027 and bought back on 6 April, the first day of 2027/28. Section 148 adds the buy-back to the costs of the grant, which is the disposal, so the £12.20 belongs to 2026/27. That follows from s148 and s144(1); HMRC gives no worked cross-year example, so a large position across 5 April may justify a tax adviser’s view. The trap runs the other way: a bought option that lapses or is sold at a loss after 5 April makes a 2027/28 loss, which cannot be carried back (across 5 April).

Assignment: a share sale, or a share cost

When the BP call is assigned, its premium joins the share sale proceeds in the listed shares section. When the HSBC put is assigned there is no disposal: the reader buys 1,000 shares for £15,000, less the £440 premium, plus £2.80 commission and £75.00 SDRT, which the buyer of the shares pays (who pays SDRT). The £14,637.80 goes into the reader’s share records. If the same shares were sold at a loss in the 30 days before, the calculator warns that the sale is matched with these shares (CG51560); it does not re-work the share records.

How the year is worked out

1. Only events dated 6 April 2026 to 5 April 2027 enter the totals; the rest are listed separately.
2. Same-year losses are set off in full, even if that wastes part of the annual exempt amount; losses brought forward reduce what is left only down to £3,000 (GOV.UK: losses). A loss is claimed within four years of the end of its tax year.
3. After the £3,000 exempt amount, gains are taxed at 18% within the unused part of the £37,700 basic rate band and 24% above it (GOV.UK: rates).
4. Gross Gift Aid extends the band (ITA 2007 s414(2)), and so do gross relief-at-source pension contributions where higher-rate income tax is otherwise due (FA 2004 s192(4)); the link to capital gains tax is TCGA 1992 s1J(8). That chain is our reading of the statutes, with no HMRC options example. Scottish and Welsh taxpayers use the UK bands here (s1J(6)).

Where the figures go on the SA108

The SA108 is needed when proceeds exceed £50,000, gains before losses exceed £3,000, or a loss is claimed. Its notes do not name options, so the calculator follows HMRC’s box definitions: option results in Other property, assets and gains (boxes 14 to 22 on the 2026 form) and share disposals on exercise or assignment in Listed shares and securities (boxes 23 to 30). In the example: five option disposals with gains of £1,155.06 and losses of £951.80, and one share disposal. HMRC’s working sheet cannot be used for an asset acquired by exercising an option, so computations go with the return; the SA108 guide covers the form, whose 2027 edition may renumber the boxes.

Checks against HMRC’s own examples

The calculator is also tested on these HMRC and GOV.UK figures.

What the calculator does not cover

• Trading. It assumes investment; above 250 events it points to HMRC’s badges of trade (BIM20205) and draws no conclusion.
• Share records. You enter the cost of shares delivered; share matching after exercise or assignment is flagged, not worked out.
• Spread bets and CFD options. Spread bets give no gains or losses for tax (CG56105) and s148 does not apply to CFD-style options (the three routes compared).
• ISAs. Options cannot be held in one (wrappers).
• Employee share options (see the RSU and share option calculator), adjusted contracts, non-residents and trusts.

Nor can it judge whether a loss is artificial (the TCGA 1992 s16A anti-avoidance rule) or whether a locked-in combination is taxed as disguised interest; both turn on facts only you know.