How much interest will £10,000 earn in a cash ISA?

At 4% AER, about £400 in the first year. Left for 5 years with the interest added, it grows to about £12,167. All of it is tax-free.

After 5 years: about £11,593 at 3%, £12,167 at 4% and £12,763 at 5%.

Possibly. Outside an ISA, interest above your Personal Savings Allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate) is taxed. £10,000 at 4% earns about £400 a year, so it matters most if you have other savings or pay higher-rate tax.

Fixed-rate cash ISAs usually pay more but lock your money away for a set time. Easy-access ISAs let you withdraw any time, but their rate can change. Enter the rate you're offered to compare.

At 4%, about £800 in the first year, and about £24,333 after 5 years with the interest left in.

AER (annual equivalent rate) shows what you'd earn in a year if interest is added and left in the account, so you can compare accounts that pay interest monthly or yearly on the same basis. Enter the AER in the calculator.