Frequently Asked Questions
Common questions about UK Capital Gains Tax and how this calculator works.
This calculator is for informational purposes only and does not constitute tax advice.
Getting Started
What is Capital Gains Tax?
Capital Gains Tax (CGT) is a UK tax on the profit you make when you sell or dispose of shares and securities that have increased in value. You pay CGT on the gain (sale proceeds minus allowable cost), not the total amount received. The rate depends on your income tax band — for shares, it is 18% for basic rate taxpayers and 24% for higher/additional rate taxpayers (from 6 April 2024 onwards).
Do I need to enter ISA or SIPP transactions?
No. ISAs and SIPPs are tax-sheltered accounts. Any gains realised within them are completely exempt from Capital Gains Tax, so there is no need to include these transactions in the calculator.
Does this calculator work for property?
No. Residential property has different CGT rules including Principal Private Residence Relief (PRR), lettings relief, and different tax rates. This calculator only handles shares and securities.
Does this calculator work for cryptocurrency?
Yes. HMRC currently treats crypto disposals the same as shares for CGT purposes, so the same matching rules (same-day, bed & breakfast, Section 104 pool) apply. You can use this calculator for cryptocurrency disposals.
What if the symbol is not publicly listed?
The calculator still works. The CGT share matching rules apply regardless of whether shares are listed on a stock exchange or unlisted. You simply enter the trade details (date, quantity, price) as you would for any other share.
What tax years are supported?
The calculator supports tax years from 2008/09 through 2026/27. The UK tax year runs from 6 April to 5 April the following year. Trades before 6 April 2008 are not supported.
What if I have transactions before the 2008 tax year?
Not supported. The calculator only handles tax years from 2008/09 onwards. If you have earlier transactions that form part of your Section 104 pool, you would need to calculate that pool balance manually and enter it as a single buy at the appropriate cost basis.
How It Works
How does the calculator work?
The calculator implements HMRC share matching rules in the required priority order: same-day rule, bed & breakfast (30-day) rule, then Section 104 pool. All calculation happens entirely in your browser — no server-side processing. The calculation engine is open-source at github.com/damiantanase/capital-gains-calculator.
Is my data stored anywhere?
No. Everything runs client-side in your browser. No data is sent to any server, no cookies are set, and there is no tracking of any kind. Your trade data is held in memory only — when you close the tab, your data is gone.
What are the HMRC share matching rules?
When you sell shares, HMRC requires you to match the disposal against acquisitions in a specific priority order:
- Same-day rule: Shares bought on the same day as the sale are matched first.
- Bed & breakfast (30-day) rule: Shares bought within the following 30 days are matched next. This prevents selling and immediately rebuying to crystallise a loss.
- Section 104 pool: Any remaining shares are matched against the pool, which is a running weighted-average cost of all your holdings in that security.
What is the Annual Exempt Amount (AEA)?
The Annual Exempt Amount is your tax-free allowance for capital gains each tax year. For 2024/25 onwards, it is £3,000. You only pay CGT on gains above this threshold. The AEA cannot be carried forward — if you don't use it in a given year, you lose it.
Entering Trades
How should I enter stock vests (RSUs)?
Enter a buy at the Fair Market Value (FMV) on the vest date. This establishes your cost basis for CGT purposes. The quantity should be the total number of shares that vested.
How should I enter "sell to cover"?
Enter two transactions: a buy for the full vest quantity at FMV, then a sell for the shares sold to cover tax (same day, same price as FMV). The remaining shares enter the Section 104 pool at the FMV cost basis.
How should I enter "sell all" vests?
You can either skip these entirely (there is no net position and no CGT impact) or enter a buy and sell at the same price for complete record-keeping. Note: there may be a small loss due to fees if you choose to record them.
How do transfers to spouse work?
Transfers between spouses or civil partners are no-gain/no-loss disposals under s58 TCGA 1992. The calculator applies same-day and bed & breakfast matching rules to determine which shares are transferred (and their cost basis). The recipient inherits this cost basis. The gain is always zero for the transferor.
Why are transfer fees and unit price ignored?
By design — transfers are always no-gain/no-loss. The unit price field has no effect on the calculation. Fees are set to zero because HMRC does not allow deduction of transfer costs on inter-spouse disposals.
How do exchange rates work?
The rate should be “units of foreign currency per 1 GBP” (matching HMRC convention). For example, if 1 GBP = 1.27 USD, enter 1.27. You can fetch HMRC's official rate using the “HMRC” button next to the exchange rate field.
Which currencies are supported for automatic rate lookup?
The calculator can automatically fetch HMRC monthly exchange rates for USD (US Dollar) and EUR (Euro) for all supported tax years (2008/09 onwards). For other currencies, you will need to enter the exchange rate manually. The rate should be entered as “units of foreign currency per 1 GBP” — you can find HMRC rates on the HMRC exchange rates page.
Unsupported Features
Is Taper Relief supported?
No. Taper Relief was abolished from 6 April 2008. Since the calculator only supports tax years from 2008/09 onwards, it is not relevant.
Is Indexation Allowance supported?
No. Indexation Allowance for individuals was frozen at December 2017 and abolished for disposals after 1 January 2018. The calculator does not apply indexation.
Is Change of Status (business/non-business asset) supported?
No. This related to Taper Relief which is no longer available for disposals after April 2008.
Does it handle loss carry-forward?
No. The calculator computes gains and losses per tax year independently. It does not automatically carry forward losses from previous years. You would need to manually account for this in your self-assessment.
Does it handle Business Asset Disposal Relief (BADR)?
No. Formerly known as Entrepreneurs' Relief, BADR requires specific qualifying conditions that are outside the scope of this calculator.
Does it handle residential property or other asset classes?
No. This calculator only handles shares and securities (including cryptocurrency).
Bugs & Feature Requests
I found a bug — how do I report it?
Report bugs via GitHub Issues at github.com/damiantanase/capital-gains-calculator-website/issues. Include: what you did, what you expected, what happened instead. Sample data that reproduces the issue is very helpful.
I'd like a new feature — how do I request one?
Open a feature request on GitHub Issues at github.com/damiantanase/capital-gains-calculator-website/issues. Describe the use case and why it would be useful.
Is the calculator open source?
Yes. Both the website and the calculation engine are open source. Contributions are welcome.