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ISA Calculator UK
Estimate how your ISA could grow, how much of your annual allowance is left, and whether planned contributions may exceed the current tax-year limit.
2026/27 ISA allowance: £20,000Lifetime ISA bonus estimateCash or Stocks & Shares ISA
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Calculate your ISA growth
Enter your ISA balance, contributions and expected return. The calculator updates automatically as you change the inputs.
Existing ISA value before future contributions.
Used to estimate your remaining 2026/27 allowance.
Planned monthly ISA saving or investment.
Use a cautious estimate. Investments are not guaranteed.
Projection period in whole years.
Used for guidance and Lifetime ISA checks.
Used to flag broad ISA eligibility notes.
Use no monthly contributions for lump-sum only projections.
Caps eligible LISA contributions at £4,000 per tax year.
How to use this calculator
- Enter your current ISA balance and how much you have already paid in this tax year.
- Add your planned monthly contribution and an estimated annual return.
- Choose the ISA type and whether to include a Lifetime ISA bonus estimate.
- Review the projected value, growth, contributions and allowance warning.
What your ISA result means
The projected ISA value shows what your balance could be worth after your chosen number of years, assuming the annual growth rate stays constant and contributions are made monthly.
The estimated growth figure is the part of the result that comes from interest or investment return, not from your own contributions.
Projection warning: this calculator is an estimate only. Cash rates can change and investments can fall as well as rise.
Ready to act on your result?
Compare Cash ISA and Stocks and Shares ISA options before deciding which route fits your goal.
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How ISAs work
An Individual Savings Account lets you save or invest without normally paying UK tax on interest, dividends or capital gains inside the account. The main adult ISA limit for the 2026/27 tax year is £20,000.
You can use the allowance across different ISA types, such as Cash ISAs, Stocks and Shares ISAs, Innovative Finance ISAs and Lifetime ISAs. A Lifetime ISA has its own contribution cap of £4,000 per tax year, and those contributions still count towards the overall ISA allowance.
A Cash ISA is usually more suitable for short-term savings where keeping the value stable matters. A Stocks and Shares ISA may suit longer-term investing, but the value can rise and fall.
The main advantage of an ISA is the tax wrapper. Interest earned inside an ISA does not normally use your Personal Savings Allowance, and investment gains inside an ISA are not normally subject to capital gains tax.
What affects your ISA projection
- Starting balance: a higher current ISA balance gives compounding more money to work with from the start.
- Monthly contribution: regular contributions can make a large difference over long periods.
- Annual growth rate: higher assumed returns increase the projection, but returns are not guaranteed.
- Time period: longer periods allow compound interest or compound investment growth to build more strongly.
- Allowance used: your current-year contributions must stay within the annual ISA allowance.
- Lifetime ISA bonus: eligible Lifetime ISA contributions can receive a 25% government bonus, capped by the LISA rules.
ISA calculator formula
The calculator uses a monthly compounding formula for the current balance and monthly contributions.
monthly_rate = annual_rate / 100 / 12
months = years × 12
future_value = current_balance × (1 + monthly_rate)^months + monthly_contribution × (((1 + monthly_rate)^months - 1) / monthly_rate)
remaining_allowance = £20,000 - contributions already made this tax yearThe Lifetime ISA bonus estimate is calculated separately and capped at eligible contributions of £4,000 per tax year.
ISA allowance quick reference
Current figures are shown for the 2026/27 tax year. Always check the latest rules before making a contribution.
| ISA rule | 2026/27 figure | Why it matters |
|---|---|---|
| Adult ISA allowance | £20,000 | Total amount you can usually pay into adult ISAs during the tax year. |
| Lifetime ISA contribution cap | £4,000 | Counts towards the £20,000 annual ISA allowance. |
| Lifetime ISA bonus | 25%, up to £1,000 per tax year | Government bonus on eligible Lifetime ISA contributions. |
| Junior ISA allowance | £9,000 | Separate annual allowance for eligible children. |
ISA calculator FAQs
How much can I put in an ISA this year?
For the 2026/27 tax year, the adult ISA allowance is £20,000. Lifetime ISA contributions are capped at £4,000 and count towards that overall limit.
How much will my ISA be worth in 10 years?
That depends on your starting balance, contributions, interest or investment return, and how long the money stays invested.
Is a Cash ISA better than a Stocks and Shares ISA?
A Cash ISA may be better for short-term savings or money you cannot risk losing. A Stocks and Shares ISA may suit longer-term investing, but the value can go down as well as up.
Does ISA interest count towards my Personal Savings Allowance?
No. Interest earned inside an ISA is normally tax-free and does not usually count towards your Personal Savings Allowance.
Can I pay into more than one ISA?
ISA rules now allow more flexibility than older rules, but your total adult ISA subscriptions in the tax year must stay within the annual allowance.
Key terms used in this calculator
These glossary pages explain the main terms used when comparing ISA growth, tax sheltering and savings allowances.