Model how your ISA could grow over time. Compare a Cash ISA against a Stocks & Shares ISA, see your total tax-free gain, and discover exactly how much the ISA wrapper saves you versus a standard taxable account.
Enter your details above to see your personalised ISA projection.
How is your ISA projected growth calculated?
The ISA calculator uses the standard compound interest formula, applied to both your starting pot and your ongoing annual contributions. Because all growth inside the ISA wrapper is sheltered from UK tax, the formula runs at the full rate — unlike a taxable account where the effective growth rate is reduced by your marginal tax band each year.
The first part of the formula, P × (1 + r)n, grows any existing balance at compound interest. The second part, C × [(1 + r)n − 1] / r, calculates the future value of a stream of equal annual deposits — the standard annuity formula. Both are added together to give your total projected balance.
Worked example — Emma, Leeds
Emma is a primary school teacher in Leeds earning £32,000 a year. She opens a Stocks and Shares ISA with a £2,000 starting pot and contributes £6,000 per year (£500 a month). Using a 7% annual return over 20 years:
ISA balance: £2,000 × (1.07)20 + £6,000 × [(1.07)20 − 1] / 0.07 ≈ £253,700
Her total contributions over 20 years are £122,000 (£2,000 + £6,000 × 20), so her tax-free growth is approximately £131,700. Because every penny of that growth compounds untaxed inside the ISA wrapper, Emma keeps the full amount — no Capital Gains Tax, no Income Tax on dividends, nothing to declare. The calculator estimates she saves around £36,700 in tax versus a standard taxable account at the basic rate. The compound interest calculator can help you explore how changing the return rate or contribution frequency affects the final figure.
How much will my ISA be worth in 10 years? At Emma's figures (£6,000/year, 7%, starting £2,000), the answer is around £89,000 — £62,000 contributed and £27,000 in tax-free growth. This illustrates why the ISA wrapper becomes dramatically more valuable the longer you stay invested. Rates are correct as of 2026-05-01.
What does your ISA result mean?
The final balance your ISA calculator returns is a projection, not a guarantee — particularly for Stocks and Shares ISAs, where returns vary year to year. But the figure gives you a meaningful planning anchor. A result under £50,000 (typically 5–10 years at modest contributions) is a solid emergency buffer or medium-term goal fund. A result between £100,000 and £300,000 represents serious long-term wealth accumulation — enough to materially supplement retirement income or fund a significant life event. A result above £500,000 over 25–35 years is financial independence territory for many people.
The more important figure is often the "Tax saved by ISA" card. This number is what you keep by using the ISA wrapper rather than a general investment account. With the CGT annual exempt amount now cut to just £3,000 (HMRC guidance ↗, correct as of 2026-05-01), the question of how much does capital gains tax reduce my returns is increasingly sharp for higher-rate investors. Even basic-rate taxpayers can expect to save tens of thousands over a 20-year horizon.
How can I make the most of the ISA allowance?
Investing early in the tax year (from 6 April) maximises time in the market. Even if you cannot afford a lump sum, drip-feeding monthly contributions — so-called pound-cost averaging — reduces the risk of investing a large amount just before a market dip. The savings goal calculator can help you work out how much to set aside each month to fully use your allowance by 5 April.
For Cash ISA savers, compare rates across providers regularly — the market is competitive, and loyalty rarely pays. In 2026, leading fixed-rate Cash ISAs are offering between 4% and 5% AER. For Stocks and Shares ISA investors, keeping fund costs low is as important as the return rate. A global index ETF tracking the MSCI World or FTSE All-World typically charges 0.07–0.22% annually — far less than actively managed funds, and evidence consistently shows that low-cost passive funds outperform the majority of active managers over 10+ years.
What are the most common ISA mistakes?
The most costly mistake is simply not using the allowance. Leaving £20,000 sitting in a current account earning near-zero interest is a permanent loss of tax-free capacity — the allowance cannot be reclaimed in a future year. Even a Cash ISA earning 4% is significantly better than most current accounts.
A close second is mismatching the ISA type to your goal. A Stocks and Shares ISA holding money you will need in 18 months is exposed to short-term market risk. Conversely, keeping a 25-year pension pot in a Cash ISA means accepting lower real returns when you have the time horizon to ride out equity volatility. Think of how much tax does an ISA wrapper actually save you — in Emma's case above, over £36,000 — and then ask whether a Cash ISA at 4.5% or a globally-diversified S&S ISA at a long-run 7% better serves your specific time horizon and risk tolerance. The maths usually favours equities for anything beyond five years.
Finally, always transfer rather than withdraw when moving ISA money between providers. Withdrawing breaks the tax-free wrapper on those funds; an official ISA transfer preserves it. Most platforms offer in-specie or cash transfers that take 15–30 business days.
Frequently Asked Questions
What is the ISA allowance for 2025/26?
The annual ISA allowance is £20,000 per person for 2025/26 (correct as of 2026-05-01, per HMRC ↗). You can split this across a Cash ISA, Stocks and Shares ISA, Lifetime ISA (up to £4,000), and Innovative Finance ISA, but the combined total cannot exceed £20,000. Unused allowance is lost at the end of the tax year on 5 April — there is no carry-forward.
How much will my ISA be worth in 10 years?
It depends on your contribution and assumed return. Investing £6,000 a year into a Stocks and Shares ISA at 7% annual return gives approximately £83,000 after 10 years — £60,000 contributed plus £23,000 in tax-free growth. Use this ISA calculator to model your own figures with your starting balance, contribution, and expected rate.
Should I choose a Cash ISA or a Stocks and Shares ISA?
Use a Cash ISA for money you may need within 3–5 years or as an emergency fund — it offers capital protection and a guaranteed rate. A Stocks and Shares ISA suits goals with a 5+ year horizon, where equities have historically outperformed cash after inflation by a wide margin. The cash isa vs stocks and shares isa calculator above lets you compare both directly using your own numbers.
What is tax drag on investments?
Tax drag is the cumulative reduction in your returns caused by paying Income Tax, Dividend Tax, or Capital Gains Tax on gains each year outside a tax-free wrapper. Over 20–30 years, paying tax annually on investment returns compounds into a substantial shortfall compared to an ISA where every penny of growth reinvests untouched. The ISA wrapper eliminates this drag entirely.
Is ISA growth really tax-free in the UK?
Yes, completely. Inside an ISA you pay no Income Tax on interest, no Dividend Tax on dividends, and no Capital Gains Tax on investment profits — and nothing to declare on your Self Assessment return. With the CGT annual exempt amount now just £3,000, this makes the ISA wrapper more valuable than ever for investors with growing portfolios.
Can I contribute to an ISA if I'm self-employed in the UK?
Yes — self-employed ISA contribution limits UK are identical to those for employees: £20,000 per tax year. ISA eligibility depends only on being a UK resident aged 18 or over, not your employment status. Self-employed people aged 18–39 can also open a Lifetime ISA and receive the 25% government bonus on up to £4,000 per year, subject to the qualifying withdrawal rules.
How does tax-free compound interest work inside an ISA?
Inside an ISA, all returns — dividends, interest, and capital gains — are reinvested with no tax deducted, so the full compound effect applies every year. Outside an ISA, the effective growth rate is reduced by your tax band annually, which meaningfully reduces compounding over long periods. The formula A = P(1+r)^n + C×[(1+r)^n−1]/r shows how to calculate tax-free compound interest: r is your full gross rate, unreduced by tax.
What happens if I withdraw money from my ISA?
Most ISAs allow penalty-free withdrawals at any time (fixed-rate ISA bonds may apply an early-access charge). Withdrawing from a standard ISA permanently uses that portion of your annual allowance. A flexible ISA lets you withdraw and re-deposit in the same tax year without affecting your £20,000 limit. Always use an official ISA transfer — never withdraw and re-open with a new provider, as this breaks the tax-free wrapper on those funds.
This calculator is for illustrative purposes only and does not constitute financial advice. Past performance is not a guarantee of future returns. Projected figures assume a constant annual return rate, which will not reflect the volatility of actual investment returns. Consult a qualified financial adviser before making investment decisions. ISA allowances and tax rules are correct as of 2026-05-01 and are subject to change by HMRC.