ISA Calculator (UK)

What is an ISA?

An ISA (Individual Savings Account) is a UK account for saving or investing in which you pay no UK income tax or capital gains tax on the interest or returns. Adults can pay in up to £20,000 each tax year across Cash ISAs, Stocks and Shares ISAs, Innovative Finance ISAs and Lifetime ISAs.

Use this ISA calculator to estimate how your savings or investments could grow tax-free over time inside a UK Individual Savings Account (ISA). Whether you're contributing monthly, making a lump sum investment, or planning long-term growth, this tool helps you model potential outcomes clearly and quickly. Because you set the rate yourself, it works as an ISA savings calculator for cash (enter the interest rate) and as an investment ISA calculator for a Stocks and Shares ISA (enter an assumed annual return).

This calculator is ideal for UK savers who want to understand how compound growth works within the £20,000 annual ISA allowance. You can adjust contributions, time horizon and assumed return to see how your ISA value may develop.

For more advanced modelling, including risk scenarios and allocation comparisons, explore the Advanced ISA Planner. If you're new to ISAs, visit our ISA Guides to understand ISA rules, allowance limits and tax treatment.

How to use the ISA Calculator

1. Enter your monthly contribution (and any lump sum)
2. Choose an assumed annual return and time horizon
3. Compare scenarios by adjusting contributions or years

How the ISA calculator works

The calculator adds your lump sum at the start and your monthly contribution at the end of each month. It divides the annual return you enter by 12 and compounds it monthly, so 6% works out at about 6.17% over a full year. It shows the final value, the total you contributed and the estimated growth, and the chart adds a dotted line for £50 a month more.

The "ISA allowance used this tax year" box shows how much of the £20,000 allowance you have used and how much is left, based only on the amount you type there. Goal mode works out the monthly amount needed to reach a target at your return and at 2 percentage points lower and higher, within each tax year's allowance; open the calculator in goal mode to start there. Figures are before charges and inflation and assume the same return every year.

ISA growth, return or interest calculator: which figure to enter

This ISA growth calculator applies one annual rate for the whole period. For a Cash ISA that rate is the interest rate, so it doubles as an ISA interest calculator (the Cash ISA Calculator also applies the cash ISA limit for your age band). For a Stocks and Shares ISA it is your assumed return, so the "estimated growth" figure is what an ISA return calculator would show. Interest rates and investment returns vary, so run a lower and a higher figure as well as your central estimate.

Worked example

With the example inputs of a £1,000 lump sum, £200 a month, a 6% return and 10 years, the calculator estimates £34,595.27: £25,000 contributed and £9,595.27 of growth. This illustrates the maths; it is not a forecast.

ISA allowance rules for 2026-27

• You can pay in up to £20,000 a tax year (6 April to 5 April) across all your ISAs. Unused allowance does not carry over to the next tax year. HMRC has said the £20,000 limit will stay the same until April 2031 (HMRC tax-free savings newsletter).
• You can split the allowance across several ISAs, including more than one of the same type, but you can only pay into one Lifetime ISA a year, up to £4,000 (GOV.UK: how ISAs work).
• Junior ISAs have a separate allowance of £9,000 a year per child, which does not reduce your own.
• From 6 April 2027, savers under 65 will be able to put no more than £12,000 a year of the £20,000 into cash ISAs (GOV.UK).

ISA rules on this page are correct as of 26 September 2026. For every rule with its gov.uk source, see ISA rules at a glance.

Other calculators: Junior ISA Calculator, ISA vs Savings Calculator and Fixed-Rate ISA Switch Calculator.

Where to go next

Embed this calculator

You can add the ISA Calculator to your own website with the snippet below. It loads a chromeless version of this tool from isaplan.co.uk with the same figures and rules, sets no cookies, and links back here. Adjust the height to suit your page.