Calculator
ISA savings calculator
Capital at risk. The value of investments can fall as well as rise, and you may get back less than you put in. Past performance is not a guide to future returns.
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See how an ISA could grow with compound interest, how much of each year’s allowance your deposits would use, and how much growth would be tax-free. New deposits are capped at the annual ISA allowance of £20,000 (2026/27).
FSCS protects eligible deposits up to £120,000 per person, per authorised institution.
Illustrative only, not financial advice. Results depend entirely on the figures you enter. Figures filled in below are labelled “Example figure” — they aren’t live rates, offers or recommendations.
Estimate your ISA growth
Your illustrative results
- Total ISA value
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- New money paid in
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- Interest earned
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- Tax-free growth
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- Allowance used in year 1
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- Illustrative tax avoided
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Balance at the end, including interest.
Lump sum plus monthly deposits, after any allowance cap.
Growth on your existing balance and new deposits.
Interest inside an ISA isn’t taxed, so all of it is tax-free.
How much of the £20,000 allowance your first-year deposits use.
Interest × the tax rate you entered, ignoring allowances that might cover it outside an ISA.
| Year | Paid in | Allowance used | Interest | Balance |
|---|
How this ISA calculator works
It assumes one rate for the whole term, deposits at the end of each month and a lump sum at the start. Each 12-month block is treated as a tax year, which is a simplification — real tax years run from 6 April to 5 April. Stocks and shares ISAs don’t pay a fixed rate, and their value can fall as well as rise.
The tax illustration ignores the Personal Savings Allowance and other allowances, which could mean less or no tax on the same interest outside an ISA.
Things to check
- Whether the rate is fixed or variable, and any withdrawal limits — see ISAs for beginners.
- The current allowance rules — see ISA allowance rules.
- Transfer rules if you’re moving an existing ISA — see how to transfer an ISA.
The formulas we use
Monthly rate from AER: r = (1 + AER)^(1/12) − 1.
Compound growth (no cap reached): A = P(1 + r)^n + C((1 + r)^n − 1) ÷ r, where P is the existing balance plus the lump sum, C the monthly deposit and n the number of months.
Allowance cap: in each year, lump sum + monthly deposits ≤ £20,000 (2026/27). Deposits over the cap aren’t added, so the calculator works month by month.
Interest earned = tax-free growth: A − existing balance − new money paid in. Illustrative tax avoided: interest × your tax rate.
The ISA allowance
For the 2026/27 tax year the maximum you can pay into ISAs is £20,000 (GOV.UK, checked 4 October 2026), shared across cash, stocks and shares, innovative finance and Lifetime ISAs (GOV.UK). Money already in an ISA from earlier years doesn’t use the current allowance. Rules can change, so check GOV.UK before relying on a figure.
To keep a running total of what you’ve paid in this tax year, use our ISA allowance tracker.
FAQ
What happens if I try to pay in more than £20,000?
The calculator stops adding deposits once a year’s allowance is used and tells you how much was left out.
Does my existing ISA balance count towards the allowance?
No. Only new money paid in during the tax year counts. Transfers of previous years’ ISA money don’t use the allowance.
Is all ISA interest tax-free?
Interest and growth inside an ISA aren’t subject to UK income tax or capital gains tax.
Does this calculator store my figures?
No. Everything is worked out in your browser and nothing is stored or sent anywhere.
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