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, Moneybox, 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, Moneybox, 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.

FiscalFox CLI Guide - Use AI Agents to Calculate UK CGT

The FiscalFox CLI calculates UK Capital Gains Tax using the same HMRC-compliant engine as the FiscalFox web app. It is designed to be used by AI coding agents. The process is simple: paste the npx command into your agent, point it at your broker files, and the agent handles the rest.

No AI is involved in the tax calculation itself - the maths follows HMRC's share matching rules (TCGA 1992) exactly. The agent only handles parsing your broker file. The calculation is deterministic, auditable, and reproducible.

Three steps. That's it.

1. Paste this into your AI agent

Open Claude Code, Cursor, Windsurf, GitHub Copilot - any agent that can run terminal commands. Then paste:

npx fiscalfox --help

No install needed. npx downloads and runs it instantly. The agent now has access to the FiscalFox CLI and knows what commands are available.

2. Point the agent to your broker files

Tell the agent where your transaction file is and what you want. For example:

Example prompt

"Use all the files in ./tax-files to calculate my capital gains for 2025/26"

The agent reads your file, figures out the columns, normalizes the data, and runs the calculation. It handles every broker format - Trading 212, Vanguard, Hargreaves Lansdown, Freetrade, eToro, InvestEngine, Moneybox, or anything else with a CSV or XLSX export.

3. Get your results

The agent extracts everything you need for your tax return:

Every disposal with the matching rule applied (same-day, 30-day, Section 104)
Total gains and losses
Taxable gain after the annual exempt amount
Estimated CGT liability
SA108 Self Assessment box values - ready to copy into your return

Example session

$ claude
> I have a Trading 212 CSV at ./t212-2024.csv.
  Calculate my capital gains for 2024/25.
  My income is £45,000. Generate SA108 values.

# The agent reads your CSV, parses it, and runs:

$ npx fiscalfox run trades.json \
    --tax-year 2024/25 \
    --income 45000 \
    --sa108 \
    --out ./results

# Total disposals: 12
# Total gains: £4,820.50
# Total losses: £310.00
# Net gain: £4,510.50
# Annual exempt amount: £3,000.00
# Taxable gain: £1,510.50
# Estimated CGT: £271.89 (18% basic rate)
# SA108 values written to ./results/sa108.json

Deterministic and auditable

The CLI applies the same HMRC share matching rules as the FiscalFox web app: the same-day rule (TCGA 1992 s.105), the 30-day bed and breakfast rule (s.106A), and Section 104 pooling (s.104). The same input always produces the same output - no AI randomness in the calculation. Every disposal records which matching rule was applied.

Compatible agents

Any AI agent that can read files and run terminal commands works with FiscalFox CLI:

Claude Code - Run claude in your terminal
Cursor - Open your folder, use the agent chat panel
GitHub Copilot - Copilot Chat in VS Code with terminal access
Windsurf - Open your folder, use Cascade

Tips

Include your income - so the CLI can determine your basic or higher rate.
Ask for SA108 values - the exact box values you need for Self Assessment.
Multiple brokers? - Give the agent all your files. The CLI merges and calculates across all holdings.
Keep the output files - the detailed computation in disposals.json is your supporting evidence if HMRC queries your return.

How to Use the FiscalFox CLI with AI Agents