Cash ISA Calculator (UK)

What is a Cash ISA?

A Cash ISA is a savings account inside an Individual Savings Account: you earn interest and pay no UK income tax on it. In the 2026-27 tax year you can pay up to £20,000 into cash ISAs, and from 6 April 2027 the limit for savers under 65 will be £12,000 a year.

Use this Cash ISA calculator to estimate how your savings could grow tax-free over time at a fixed or assumed interest rate. A Cash ISA allows UK savers to earn interest without paying income tax on returns, up to the £20,000 annual ISA allowance.

This tool estimates how interest on a cash ISA balance could add up at a rate you choose. Cash ISA rates can change, and no result is guaranteed. Adjust your starting balance, monthly contributions and interest rate to see how your ISA value may grow over your chosen time horizon.

If you are comparing savings against investment-based ISAs, explore our Stocks and Shares ISA Calculator for growth estimates with market exposure. For scenario modelling across different ISA types and contribution strategies, use the Advanced ISA Planner.

New to ISAs? Visit our ISA Guides to understand how the allowance works and how Cash ISAs compare to other options.

How to use the Cash ISA Calculator

Use this Cash ISA calculator to estimate how your savings could grow over time. Enter an initial deposit, monthly contributions and an assumed interest rate to see estimated balances. Results are estimates only and based on UK Cash ISA rules.

1. Add an initial amount and monthly contribution.
2. Choose an interest rate.
3. See estimated savings.

How the Cash ISA calculator works

The calculator adds your opening deposit at the start and a monthly deposit at the end of each month. It divides the annual interest rate you enter by 12 and adds interest every month, so 3.5% works out at about 3.56% over a full year. Rates advertised as AER already include compounding, so treat the result as a close estimate rather than the exact interest your provider will pay.

Choose your age band: the calculator applies the cash ISA limit for each tax year, and if your planned deposits go over it, the extra is left out of the estimate and the result says how much. It assumes the same rate for the whole period and shows figures before inflation.

Monthly interest, and the interest on £20,000

Interest is added every month, so the result already includes interest on the interest credited earlier in the year, and you can read the growth figure as the monthly interest on your balance added up over the period. As an example of the interest on £20,000, the cash ISA limit in 2026-27: deposited at the start and left untouched at 3.5% (the calculator's example rate, not a current market rate), it earns about £711 of tax-free interest in 1 year and about £3,819 over 5 years, assuming the same rate throughout. To use the tool as a 5 year ISA calculator with regular saving instead, keep the 5-year default and enter a monthly deposit: each deposit earns interest only from the month it is made, so the total is lower than for a lump sum of the same size paid in at the start.

Worked example

With an opening deposit of £1,000, £200 a month, a 3.5% rate and 5 years, the calculator estimates £14,284: £13,000 deposited and £1,284 of tax-free interest. This illustrates the maths; it is not a forecast.

Cash ISA rules for 2026-27

• You can pay in up to £20,000 a tax year (6 April to 5 April) across all your ISAs, and you can split it across more than one cash ISA in the same tax year (GOV.UK: how ISAs work). Unused allowance does not carry over.
• From 6 April 2027, savers under 65 will be able to pay no more than £12,000 a year into cash ISAs. The overall ISA allowance stays at £20,000, and savers aged 65 or over keep a £20,000 cash ISA limit (GOV.UK: cash ISA limit reduction).
• Easy-access Cash ISAs let you withdraw when you like but usually have a variable rate; fixed-rate Cash ISAs fix the rate for a set term, usually with a charge for early access. Our guide to how interest rates affect Cash ISAs explains the trade-offs.
• To move a Cash ISA to a better rate, ask the new provider for an ISA transfer rather than withdrawing the money, which could lose that part of your tax-free allowance. GOV.UK sets out the time limits providers must meet (GOV.UK: transferring your ISA).

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.

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