How much interest does £10,000 or £20,000 earn in a Cash ISA?

What do £10,000 and £20,000 earn in a year?

Over one year, a Cash ISA pays the amount saved multiplied by its AER, as long as nothing is taken out. So £10,000 earns £100 for every 1% of AER and £20,000 earns £200, and the rate decides the rest.

At the three rates below, £10,000 earns between £325 and £500 in a year and £20,000 earns between £650 and £1,000.

A year’s interest at three rates, checked 6 October 2026

£10,000 at the best easy access rate, 4.86% AER

£486

Variable, so it can change

£20,000 at the best easy access rate, 4.86% AER

£972

Variable, so it can change

£10,000 at the best 1-year fixed rate, 5.00% AER

£500

Fixed for 12 months

£20,000 at the best 1-year fixed rate, 5.00% AER

£1,000

Fixed for 12 months

£10,000 at the median easy access rate, 3.25% AER

£325

The middle of 64 easy access rates

£20,000 at the median easy access rate, 3.25% AER

£650

The middle of 64 easy access rates

An illustration. These are not forecasts, and they are not the rate on any one account. They use three rates from our Cash ISA tables, last checked on 6 October 2026: the best easy access rate, the best 1-year fixed rate and the median easy access rate, which is the middle one when the 64 easy access rates are lined up from lowest to highest. They assume the money stays in for a full year at that rate with nothing taken out, and they name no account.

The best easy access rate in our table comes with a note about a bonus or a condition on the rate, so the account’s own terms say how long it lasts and what it pays after that. 21 of the 64 easy access rates in our table carry a note like that.

What does AER mean?

AER stands for annual equivalent rate. It shows what the rate works out as if the interest were paid into the account once a year, so accounts that pay interest monthly, annually or at maturity can be compared on a level footing. If nothing is taken out, it is the interest the account pays over a year as a percentage of the balance.

That is why a year’s interest is simply the amount multiplied by the AER. At an example rate of 4% AER, £10,000 earns £400 and £20,000 earns £800. Over more than a year, each year’s interest is added to the balance and earns interest of its own from then on, which is how the five-year figures further down are worked out.

How much does each amount earn in a year?

The table works out a year’s interest on five amounts at each of the three rates, in whole pounds. It is the same sum as the box above, for more amounts.

The interest in the table is before any tax. Inside a Cash ISA none of it is taxed, as the section on tax below explains.

How much is that a month?

Divide a year’s interest by 12 for an average month. The table does that for each figure above, to the nearest penny.

These are averages, not what an account pays each month. When interest is paid depends on the account: some pay it monthly and some once a year, and the terms say which. Accounts with the same AER earn the same over a year if the interest stays in the ISA. Interest that is paid out of the ISA stops earning interest of its own, so the year’s total can come out a little lower. See whether ISA interest is paid monthly or annually.

What is the balance after five years?

If the best easy access rate stayed at 4.86% AER for five years and every year’s interest stayed in the ISA, the balances would be as in the table. It will not stay the same: easy access rates are variable, so the provider can change them at any time.

The table is arithmetic, not a forecast. A rate that rises or falls during the five years gives a different answer, and a bonus or condition on the rate can change what an account pays after the first year. The interest over five years is more than five times a single year’s because each year’s interest is added to the balance and earns interest of its own.

Is the rate fixed or variable?

A variable rate can go up or down at any time. A fixed rate stays the same for its term, such as the 1 year in the tables. In our tables every easy access rate is variable and every 1-year fixed rate is fixed.

So the 1-year fixed figures show what a year’s interest would be if the rate holds for the full 12 months, which a fix does. The easy access figures assume the rate stays put, which a variable rate does not promise.

Fixed rate ISAs usually charge for early access, so the money is tied up for the term. The fixed rate ISA penalty calculator works out what an early withdrawal charge could cost.

Can you pay in £50,000 or more?

Not as new money in one tax year. The ISA allowance is £20,000 a year across all your ISAs, and the Cash ISA limit stays £20,000 after the Budget on 28 October 2026 cancelled a planned cut. At £20,000 a year, £50,000 takes at least 3 tax years to pay in. Money already in another Cash ISA can be moved by an ISA transfer, which does not use the allowance. See the Cash ISA limit.

The £50,000 rows in the tables show what that balance would earn once it is all inside Cash ISAs. Until then, money still outside an ISA earns interest outside one, where the Personal Savings Allowance applies.

Is the interest tax-free?

Inside a Cash ISA, yes. gov.uk says you do not pay tax on interest on cash in an ISA, and that if you complete a tax return you do not need to declare it.

Outside an ISA, the Personal Savings Allowance covers £1,000 of interest a year for a basic rate taxpayer, £500 for a higher rate taxpayer and £0 for an additional rate taxpayer. Interest above the allowance can be taxed.

The table compares a year’s interest at the best 1-year fixed rate, 5.00% AER, with each allowance, for money held outside an ISA.

The table assumes no other savings interest, and no Personal Allowance or starting rate for savings left to cover it. Those can cover more interest for some savers, and tax treatment depends on your circumstances and may change. The savings tax calculator works out the tax on your own interest, and do you pay tax on ISA interest covers the rules.

How do you work out your own figure?

The Cash ISA interest calculator takes any amount, AER and term and shows the interest and a year-by-year balance. Our Cash ISA rate tables rank accounts by rate, with easy access and 1-year fixed rate Cash ISAs on their own pages.