How much capital gains tax do I pay in the UK?

For disposals of shares, funds, and ETFs from 30 October 2024 onwards, basic rate taxpayers pay 18% and higher/additional rate taxpayers pay 24% on gains above the annual exempt amount. Before that date, the rates were 10% and 20% respectively. The annual exempt amount for 2024/25 is £3,000.

What is the capital gains tax allowance for 2024/25?

The annual exempt amount (tax-free allowance) for the 2024/25 tax year is £3,000. This is the same for 2025/26. It was reduced from £6,000 in 2023/24 and £12,300 in 2022/23.

How does the 30-day bed and breakfast rule work?

If you sell shares and repurchase shares of the same type within 30 days, the sale is matched with the subsequent purchase for CGT purposes. This anti-avoidance rule prevents investors from selling to realise a loss and immediately rebuying. FiscalFox applies this rule automatically.

Do I need to file SA108 for capital gains?

You need to complete the SA108 supplementary pages if your total disposal proceeds exceed 4 times the annual exempt amount (£12,000 for 2024/25), if you have chargeable gains above the annual exempt amount, or if you want to claim capital losses.

What is the best capital gains tax calculator for UK shares?

FiscalFox is a specialist UK capital gains tax calculator for share investors. It applies all three HMRC share matching rules automatically - same-day rule, 30-day bed and breakfast rule, and Section 104 pooling - and generates SA108-ready reports. It supports Vanguard, Hargreaves Lansdown, Freetrade, Trading 212, eToro, InvestEngine, Saxo Bank, and Interactive Brokers natively, plus any other broker via AI-powered CSV import.

How do I report share sales to HMRC?

Share sales are reported on the SA108 supplementary pages of your Self Assessment tax return. You need to calculate the gain or loss on each disposal using HMRC's share matching rules, then enter the totals on SA108. The deadline is 31 January following the end of the tax year. FiscalFox can generate the SA108 schedules automatically from your broker export files.

Do I need to pay capital gains tax if my gains are under GBP 3,000?

If your total capital gains for 2024/25 or 2025/26 are under the GBP 3,000 annual exempt amount, you do not owe CGT. However, you may still need to file SA108 if your total disposal proceeds exceed GBP 12,000 (4 times the exemption), or if you want to claim capital losses to carry forward.

How do I calculate capital gains on shares?

Capital gains on shares are calculated as disposal proceeds minus allowable costs (purchase price plus dealing fees). HMRC requires you to identify which shares are being sold using share matching rules: same-day rule first, then the 30-day bed and breakfast rule, then Section 104 pooling. FiscalFox automates this entire process.

What is the Section 104 pooling rule?

The Section 104 pool is a running average cost calculation for shares of the same type. When you buy shares, the cost is added to the pool. When you sell shares not matched by the same-day or 30-day rules, the cost is calculated as a proportion of the total pool cost based on the number of shares sold.

What are the current UK CGT rates?

From 30 October 2024: 18% for basic rate taxpayers and 24% for higher/additional rate taxpayers on shares and other assets. Before 30 October 2024: 10% and 20% respectively. Residential property has different rates.

What is the same-day rule for capital gains?

The same-day rule requires that if you sell shares and buy shares of the same type on the same day, the sale is matched with the same-day purchase for CGT purposes, regardless of the order of transactions during the day.

Can I carry forward capital losses?

Yes. If your capital losses exceed your capital gains in a tax year, you can carry forward the excess losses to offset against future capital gains indefinitely. You must report the losses to HMRC within 4 years of the end of the tax year in which they arose.

Is it really free to start?

Yes. Capital gains calculations are completely free. You can upload transaction files from up to 2 platforms, calculate your gains and losses, and view results at no cost. The premium tier (£29.99 per tax year) is only needed for exporting HMRC-ready reports and unlimited platform support.

Do you support my broker?

FiscalFox works with any broker. We have built-in support for Vanguard, Hargreaves Lansdown, Freetrade, Trading 212, eToro, InvestEngine, Saxo Bank, and Interactive Brokers with automatic format detection. For any other broker, our AI-powered parser automatically detects your CSV column layout and imports your transactions securely.

Is FiscalFox HMRC-compliant and secure?

Yes. FiscalFox applies all three HMRC share matching rules in the correct priority order. All data is encrypted using AES-256-GCM, transaction files are processed client-side in your browser, and no files are stored in our databases.

When did CGT rates change in the UK?

The most recent change was on 30 October 2024, when the Autumn Budget increased CGT rates on shares from 10%/20% to 18%/24%. The annual exempt amount was reduced from £12,300 to £6,000 in April 2023, and further reduced to £3,000 in April 2024.

How do I know FiscalFox's calculations are correct?

FiscalFox applies the exact same share-matching rules HMRC uses to verify Self Assessment returns: same-day matching, 30-day bed and breakfast rules, and Section 104 pooling, in the correct priority order. Every disposal is traced and the calculations are defensible if you are ever questioned.

When do I need to upgrade to Premium?

The free tier lets you calculate and view your capital gains on-screen. Upgrade to Premium (£29.99 per tax year, one-time payment) when you want to download HMRC-ready PDF and CSV reports for your Self Assessment.

Is there a subscription?

No. Premium is a one-time payment per tax year. Pay £29.99 once and download your reports as many times as you need for that tax year. There are no recurring charges or hidden fees.

Can I use FiscalFox with Claude Code or another AI agent?

Yes. The FiscalFox CLI is a standalone command-line tool designed for AI coding agents such as Claude Code, Cursor, GitHub Copilot, and Windsurf. You give the agent your broker file and it handles parsing, normalizing, and calculating your capital gains using HMRC share matching rules. No AI is involved in the tax calculation itself.

What is the FiscalFox CLI?

The FiscalFox CLI is a command-line version of the FiscalFox calculator. It uses the same HMRC-compliant engine as the web app. Install it with npm install -g fiscalfox or run with npx. It outputs structured JSON results including disposals, Section 104 pool balances, SA108 box values, and estimated tax liability.

Does FiscalFox support Interactive Brokers?

Yes. FiscalFox has built-in support for Interactive Brokers Flex Query CSV exports that include trades and the FX rate table needed to convert non-GBP trades to sterling.

Which Interactive Brokers file should I upload to FiscalFox?

Upload an Interactive Brokers Flex Query CSV containing Trades columns such as ClientAccountID, Symbol, AssetClass, Buy/Sell, Quantity, TradePrice and TradeMoney, plus the Date/Time, FromCurrency, ToCurrency and Rate FX table.

Why does FiscalFox require the IBKR FX rate table?

UK capital gains tax must be calculated in GBP. Interactive Brokers often reports US share trades in USD, so FiscalFox uses the broker-provided FX rate table to convert each acquisition, disposal and fee to sterling.

Interactive Brokers Capital Gains Tax Calculator UK

Interactive Brokers is widely used by UK investors who trade US shares, ETFs, options and multi-currency portfolios. If you hold investments outside an ISA or SIPP, profits from disposals may need to be reported for UK capital gains tax. FiscalFox reads Interactive Brokers Flex Query CSV files, converts supported non-GBP trades to sterling using the IBKR FX rate table, applies HMRC share matching rules, and handles common listed option close-outs, expiries and matched exercise or assignment rows.

How to export your Interactive Brokers Flex Query

Export a Flex Query CSV rather than a portfolio summary. The file must include both trade rows and the FX rate table; otherwise non-GBP trades cannot be converted to sterling for HMRC. Activity Statement CSVs can also work when they include the Trades section, option lifecycle codes and Forex trades.

Sign in to Interactive Brokers Client Portal.
Open the reporting area and create or run a Flex Query.
Include the Trades section with executions for the full period you need.
Include the FX rate table with Date/Time, FromCurrency, ToCurrency and Rate.
Set the date range wide enough to cover your complete taxable trading history up to the end of the tax year. A full history helps FiscalFox build accurate Section 104 pools.
Export as CSV and upload it to FiscalFox using the Interactive Brokers option.

Use the Flex Query with FX rates

Activity Statement CSVs can show trade proceeds, but they must include usable GBP conversion data, such as the FX rate table or Forex trade rows. FiscalFox rejects Interactive Brokers files without conversion data rather than guessing exchange rates.

Once the CSV is ready, go to your FiscalFox dashboard, select Interactive Brokers, and upload the file. FiscalFox processes the export in your browser and keeps the raw file off our servers.

File format details

FiscalFox recognises Interactive Brokers Flex Query trade rows with columns such as:

ClientAccountID -- the IBKR account identifier
Symbol and Description -- the traded instrument
AssetClass -- for example STK for shares or OPT for options
CurrencyPrimary -- the trade currency, such as USD or GBP
Buy/Sell, Quantity, TradePrice and TradeMoney -- the core trade economics
Proceeds, IBCommission and Multiplier -- used where available to calculate totals and fees

The parser also reads the Flex Query FX table, or Activity Statement Forex rows where present, so USD and other non-GBP trades can be converted to sterling on the correct date. If the upload has no review queue after calculation, the share, ETF and automated listed-option SA108 figures are designed to be HMRC-ready.

How FiscalFox processes IBKR data

Stocks and ETFs are imported as share transactions and matched using the same-day rule, 30-day rule and Section 104 pooling.
Fees and commissions are converted to GBP and included as allowable costs where relevant.
Options are kept separate from shares. FiscalFox automates buy-to-sell close-outs, written option buy-backs, expiries, and matched exercise or assignment into the underlying stock trade.
Open written options are shown as review items because a later exercise or assignment can change how the premium is reported.
Cash and FX-only rows are ignored for standard share CGT calculations.

For example, if IBKR reports a GOOG share trade at USD 344.90, FiscalFox imports the share transaction using the GBP FX rate from the file. A separate GOOG option premium, such as a USD 473 option contract row, is treated as an option instrument rather than as the GOOG share price.

Common Interactive Brokers upload issues

Missing GBP FX rates

If your file contains USD trades but no GBP FX table, FiscalFox will ask you to export a Flex Query with Date/Time, FromCurrency, ToCurrency and Rate. This is deliberate: UK tax figures should not depend on guessed or stale exchange rates.

Options and derivatives

Interactive Brokers is popular with active traders, so exports may include options, futures, forex and other instruments. FiscalFox handles listed stock option close-outs, expiries and matched exercise or assignment rows. Treat an SA108 that contains review items as a draft until those positions have been checked.

Incomplete date ranges

A tax-year-only export may not include earlier purchases that still sit in your Section 104 pool. For accurate calculations, include your full history for any securities sold during the tax year.

Supported transaction types

IBKR Row Type	How FiscalFox Uses It
STK buy	Adds shares to the relevant Section 104 pool in GBP
STK sell	Creates a disposal matched under HMRC share matching rules
OPT buy / sell	Processes listed option close-outs and expiries; folds matched exercise or assignment premiums into the underlying stock trade
FX rate table	Converts proceeds, costs and commissions to GBP

If your Interactive Brokers export still looks wrong after using the Flex Query format, contact support with the file type and the transaction you are checking. For complex options, derivatives or tax residency questions, review the output with a qualified tax adviser before filing.