Stocks & Shares ISA Calculator
The ISA is Britain's best tax deal: up to £20,000 a year into investments, and HMRC never touches a penny of the growth. Type in a monthly amount and watch what the wrapper can build.
Your inputs
Pot growth over time
Year-by-year breakdown
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What is a Stocks & Shares ISA?
An ISA is a wrapper, not an investment. You put money in, choose what to invest in — index funds, shares, bonds — and everything inside grows free of UK tax: no income tax on dividends, no capital gains tax when you sell, however big the pot gets. For the 2026/27 tax year (6 April 2026 to 5 April 2027) you can contribute up to £20,000 across all your ISAs combined. The allowance resets every 6 April and can't be carried forward — use it or lose it.
The math behind this calculator
Each month your balance compounds at one-twelfth of the annual return, then your monthly contribution is added. Contributions are capped so you never exceed £20,000 in a year (£1,666.67 a month). If you enter more, the calculator trims it and tells you.
Capped so yearly contributions never exceed £20,000
Worked example
Invest £500 a month for 20 years at 7%. You contribute £120,000 — and the pot reaches roughly £260,000, all of it tax-free. Growth (£140,000) does more than half the work. Outside an ISA, a higher-rate taxpayer would lose 40% of dividends above the allowance and 24% of gains above the annual exempt amount; inside, the whole £260,000 is yours.
Caveats worth knowing
- Investments can fall. The ISA shields you from tax, not from markets. You could get back less than you put in — time horizon and diversification manage that risk, not the wrapper.
- US withholding tax still applies. The 15% US withholding tax on American dividends can't be reclaimed inside an ISA. On a global tracker it's a small drag, but it's real.
- Cash ISA rules change in April 2027. From 6 April 2027, under-65s will be limited to £12,000 a year in Cash ISAs (the overall £20,000 allowance is unchanged, so Stocks & Shares ISAs become relatively more attractive).
- You must be 18+ and UK resident to open a Stocks & Shares ISA. Junior ISAs (£9,000 a year) are the equivalent for children.
Sources: AJ Bell — ISA allowance 2026/27; MoneyWise UK — 2026/27 allowance & 2027 changes; GOV.UK — Individual Savings Accounts.