If you sold shares this tax year and want a quick estimate of your Capital Gains Tax (CGT), use this calculator. It is designed for a straightforward share disposal and gives you a practical, easy-to-understand estimate in seconds.
CGT Calculator (Shares)
Assumption: this is a simplified estimate for one purchase lot and one sale. Complex HMRC matching rules (same-day, 30-day, Section 104 pooling across multiple buys) are discussed below and may change the final tax figure.
How this CGT calculator on shares works
This tool estimates your taxable gain in five steps:
- Allowable cost = share purchase cost + dealing fees (apportioned if you only sold part of your holding).
- Net proceeds = sale value minus selling fees.
- Gain before reliefs = net proceeds minus allowable cost.
- Adjusted gain = gain after deducting carried-forward losses.
- Taxable gain = adjusted gain minus annual CGT allowance; then apply your CGT rate.
Example: quick share CGT estimate
Suppose you bought 1,000 shares at £2.50 and paid £10 in fees. Later, you sold 600 shares at £4.20 and paid £10 to sell.
- Total acquisition cost of 1,000 shares = £2,500 + £10 = £2,510
- Allowable cost for 600 sold shares = 600/1000 × £2,510 = £1,506
- Net sale proceeds = (600 × £4.20) - £10 = £2,510
- Gain before reliefs = £2,510 - £1,506 = £1,004
If your annual exemption is £3,000 and no other gains are considered, the taxable amount could be zero. If total yearly gains are above the allowance, tax is then charged at your applicable CGT rate.
Important UK share CGT rules to know
1) Same-day rule
If you buy and sell the same shares on the same day, HMRC generally matches these first for cost basis purposes.
2) 30-day “bed and breakfast” rule
If you sell shares and repurchase the same shares within 30 days, those later purchases may be matched against the disposal before your pooled holding.
3) Section 104 pooling
Most longer-term holdings are grouped in a pooled average-cost calculation. If you have multiple buy transactions at different prices, your real gain may differ from a single-lot estimate.
4) ISA and SIPP wrappers
Investments held inside ISAs and SIPPs are generally outside CGT. This calculator is for taxable (non-wrapper) share disposals.
How to legally reduce CGT on shares
- Use your annual CGT allowance each year rather than realizing very large gains at once.
- Harvest losses where suitable to offset gains.
- Consider spouse/civil partner transfers (usually no gain/no loss) to use two allowances and tax bands.
- Shelter future growth by moving assets into ISA/SIPP where possible.
- Keep accurate records of fees and transaction dates to claim all allowable costs.
Records you should keep
Good records make your tax return easier and reduce risk of mistakes. Keep:
- Contract notes for buys and sells
- Broker statements
- Dates, quantities, and prices
- Transaction fees and stamp duty
- Any prior-year capital losses used in your calculation
Frequently asked questions
Is this calculator tax advice?
No. It is an educational estimate. If your affairs are complex, get guidance from a qualified tax adviser.
What if I sold shares from multiple purchase dates?
Then HMRC matching rules and Section 104 pooling become important. Use this tool as a first pass, then complete a full tax computation before filing.
Do broker fees really matter?
Yes. Purchase and selling costs usually affect your gain and can reduce tax due, so include them.
Bottom line
This CGT calculator on shares is a practical starting point for planning and estimating your tax bill. Use it to understand the mechanics, then confirm final figures with complete transaction history and current HMRC rules for your tax year.