Cash ISA interest calculator
Enter a deposit, an AER and a term to see the interest it would actually earn, with a year-by-year balance chart. Interest inside a Cash ISA is always tax-free, whatever your income.
Optional. Assumed to be paid in at the start of each month.
Your numbers
Interest earned
£1,249
Final balance
£11,249
Balance by year
How this works
- We turn your AER into a monthly compounding rate using (1 + AER) to the power of one twelfth, minus 1. Compounding that monthly rate for 12 months grows your balance by exactly the AER you entered, matching how providers quote the rate.
- If you add a monthly top-up, we assume it is paid in at the start of each month, so it earns that month's interest too.
- The chart shows your balance at the end of each complete year, plus the end of the term itself if that is not a whole number of years.
- Interest inside a Cash ISA is entirely tax-free, so there is no tax calculation on this page.
- All figures, including the chart, are rounded to the nearest pound.
Tax treatment depends on your circumstances and may change. This is not financial advice.
Questions people ask
Does compounding make much difference over a short term?
Not hugely over a few months, but it grows faster the longer the term runs. Over several years, even a modest AER earns more than simple, uncompounded interest would suggest, because each year you earn interest on your previous interest too.
Is AER the same as the interest rate I will actually get?
AER stands for annual equivalent rate. It shows what a deposit would earn over a full year once interest is compounded within the account, and it is the standard way UK providers quote Cash ISA rates so you can compare them directly.
Do I pay tax on Cash ISA interest?
No. All interest earned inside a Cash ISA is entirely tax-free, however much you have saved and whatever tax band you are in. It does not use up your Personal Savings Allowance either, because that allowance is only for interest earned outside ISAs.
What if my rate changes part way through the term?
This calculator assumes a single fixed rate for the whole term you enter. If you hold a variable-rate Cash ISA, your real return will differ whenever the provider changes the rate, so treat the result as an estimate at today's rate.