How does the ISA calculator work?

It treats the rate you enter as a yearly rate (an AER for cash, or a yearly growth rate for investments), converts it to a monthly rate, and adds your monthly payment at the end of each month. Because it's an ISA, there's no tax to take off. It assumes the rate stays the same, which a variable cash rate or investment returns won't.

How long does it take to become an ISA millionaire?

Paying in the full £20,000 allowance every year, it would take 25 years and 5 months to reach £1 million at a steady 5% a year, starting from nothing. Investment returns aren't steady or guaranteed, so treat that as an illustration: at lower returns it takes longer.

What rate should I use for a stocks and shares ISA?

There's no reliable figure: investment returns vary from year to year and can be negative. Try a few rates to see the range, and remember that fund and platform charges come off the return.

ISA Calculator: What Your ISA Could Grow To, and How to Become an ISA Millionaire

Work out what a cash or stocks and shares ISA could grow to with regular payments, or how long it would take to reach a target such as £1 million. Enter any rate; the starting rate is the Bank of England's average 1-year fixed cash ISA rate.

By James Whitfield · 15 June 2025 · Last reviewed: 30 September 2026 · 2 min read

Use the calculator to see what regular ISA payments could grow to, or switch it to work out how long it would take to reach a target. Enter the rate as a yearly figure: an AER for a cash ISA, or a yearly growth rate for a stocks and shares ISA.

The starting rate is the Bank of England’s average 1-year fixed rate cash ISA for August 2026 (4.40%). Results assume the rate never changes, which variable cash rates and investment returns won’t, and don’t take off any fund or platform charges. There’s no tax on ISA interest or growth, so no tax is deducted.

How ISA growth works

An ISA grows the same way as any savings or investment: the interest or growth is added to the balance and then grows itself. The difference is that none of it is taxed, so the whole return stays in the account. The allowance limits what you can pay in each tax year (£20,000), not what the ISA grows to.

For how compounding works, and the formula, see the compound interest calculator.

Becoming an ISA millionaire

Paying in the full £20,000 allowance every year (£1,666.67 a month) and growing at a steady 5% a year, an ISA starting from nothing would pass £1 million after 25 years and 5 months. Try it: set the calculator to “How long to reach a target”, with a monthly payment of £1,666.67, a rate of 5% and a target of £1,000,000.

Three things to keep in mind:

The rate is an illustration. Stocks and shares returns vary from year to year and can be negative. A cash ISA paying the August 2026 average fixed rate of 4.40% would take longer to get there.
Charges reduce the return. A 5% return before charges is less after them.
The allowance may change. From 6 April 2027, people under 65 can put at most £12,000 a year into cash ISAs, so a mainly-cash millionaire plan would need the rest in another type of ISA (the cash ISA limit).

ISAs guide: the ISA types, the allowance and the main choices in one place
Cash or stocks and shares ISA: which suits your money
Lifetime ISA calculator: the government bonus by age
Pension or ISA: where long-term money grows best

ISA Calculator: What Your ISA Could Grow To, and How to Become an ISA Millionaire

Contents

How ISA growth works

Becoming an ISA millionaire

Sources

Contents

How ISA growth works

Becoming an ISA millionaire

Related guides

Sources