ISA Calculator — Tax-Free Compound Growth
See how your ISA could grow tax-free over 5, 10, or 30 years. Adjust your annual contribution, expected return, and starting balance to project your wealth.
ISA allowance for 2025/26: £20,000 per person. All growth inside an ISA is free from income tax, capital gains tax, and dividend tax.
Growth over time
Contributions vs growth
Year-by-year projection
| Year | Contributed | Growth | Balance |
|---|---|---|---|
| 1 | £20,000 | £1,400 | £21,400 |
| 2 | £40,000 | £4,298 | £44,298 |
| 3 | £60,000 | £8,799 | £68,799 |
| 4 | £80,000 | £15,015 | £95,015 |
| 5 | £100,000 | £23,066 | £123,066 |
| 10 | £200,000 | £95,672 | £295,672 |
| 15 | £300,000 | £237,761 | £537,761 |
| 20 | £400,000 | £477,304 | £877,304 |
Why compound growth matters for ISAs
Compound interest is the single most powerful force in long-term saving. When your returns generate their own returns, growth accelerates — £10,000 growing at 7% annually becomes £19,672 after 10 years and £76,123 after 30 years, without adding another penny. Inside an ISA, every penny of that growth is tax-free. No income tax on interest, no capital gains tax on investment profits, no dividend tax. Outside an ISA, a higher-rate taxpayer would owe 40% on interest above their £500 Personal Savings Allowance, and 20% on capital gains above the £3,000 annual exempt amount.
Cash ISA vs stocks & shares ISA
Cash ISAs currently pay 3–5% depending on the provider and whether you lock your money away. Stocks & shares ISAs have historically returned 7–10% annually over long periods (FTSE All-Share total return since 1986 averages around 8.5%), but with significant volatility year-to-year. Over 20+ years, equities have beaten cash in every rolling period in modern UK market history — but over 1–3 years, you could easily lose money. Use this calculator to model both scenarios and see when the equity premium overtakes the volatility risk.
Making the most of your 2025/26 allowance
The ISA allowance for 2025/26 is £20,000per person. You can split this across cash, stocks & shares, innovative finance, and Lifetime ISAs (up to £4,000 into a LISA, which adds a 25% government bonus for first-time buyers or retirement). Unused allowance does not carry forward — if you don't use it before 5 April, it's gone. Couples can shelter £40,000 between them, and children under 18 get a separate Junior ISA allowance on top.
This calculator is for illustrative purposes only and does not constitute financial advice. Projected returns are hypothetical and not guaranteed — investments can go down as well as up, and you may get back less than you invest. Past performance is not a reliable indicator of future results. Tax treatment depends on individual circumstances and may change. GiltEdge is not regulated by the FCA.