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ISA calculator

ISA Calculator UK

See what your Stocks & Shares or Cash ISA could be worth. Enter a starting amount, a monthly contribution and your expected return, and get a year-by-year projection, all of it tax-free. Free, no signup.

Free, no signupTax-free growth modelled£20,000 allowance built in
Your ISA
£
What you'll pay in each month
£
A long-run estimate, returns are not guaranteed
%
yrs
Projected ISA value
£32,703
after 10 years+£7,703 tax-free growth
2y4y6y8y10y
You put inTax-free growth
10y
You put in
£25,000
Growth (tax-free)
+£7,703
Everything inside an ISA is free of UK income tax and capital gains tax, so the full £32,703 is yours to keep. Want to model a plain savings account too? Try the compound interest calculator.
Year-by-year projection
YearPaid inGrowthValue
1£3,400£107£3,507
2£5,800£342£6,142
3£8,200£712£8,912
4£10,600£1,224£11,824
5£13,000£1,885£14,885
6£15,400£2,702£18,102
7£17,800£3,684£21,484
8£20,200£4,839£25,039
9£22,600£6,175£28,775
10£25,000£7,703£32,703
A projection, not a guarantee. Investment returns can fall as well as rise.
Simon Chadwick
Simon Chadwick
Founder, Orbit Money
Method: monthly compounding projectionUpdated: 16 July 2026Sources: gov.uk

How an ISA grows your money tax-free

An ISA, or Individual Savings Account, is a wrapper around your savings or investments that shields them from UK tax. Interest in a Cash ISA, and dividends and gains in a Stocks & Shares ISA, are all free of income tax and capital gains tax. You keep every pound of growth, and there’s nothing to report to HMRC. This calculator projects that growth by compounding your balance month by month and adding your regular contributions on top.

The maths is the same compound growth you’d see in any savings projection: each month your balance earns a slice of the annual return, that growth is added to the pot, and next month’s return is worked out on the slightly larger figure. Over years, that compounding does most of the heavy lifting, which is why starting early tends to matter more than the exact rate.

The £20,000 annual ISA allowance

You can pay up to £20,000 into ISAs in the 2025/26 tax year. That figure is shared across every ISA you hold, so if you put £15,000 into a Stocks & Shares ISA you have £5,000 left for a Cash ISA. The Lifetime ISA has its own £4,000 sub-limit inside that total. The allowance resets on 6 April each year and you can’t roll unused allowance into the next year, so many people top up before the deadline. The calculator flags it if your monthly contributions would push you past £20,000 a year.

Cash ISA versus Stocks & Shares ISA

A Cash ISA works like a tax-free savings account. It pays a set interest rate, your balance can’t fall, and it suits money you might need in the next few years. A Stocks & Shares ISA invests in funds, shares or bonds. Its value moves up and down day to day, but over long periods it has tended to outgrow cash. The trade-off is risk: you could get back less than you put in, so it generally suits money you can leave alone for five years or more. Switch the ISA type at the top of the calculator to compare the two side by side.

How to use this calculator

  1. Choose your ISA type: Stocks & Shares for investments, or Cash ISA for a set interest rate.
  2. Enter your starting amount and how much you'll add each month.
  3. Set your expected annual return. For a Cash ISA, use the account's AER; for Stocks & Shares, a long-run estimate.
  4. Enter how many years you'll keep the ISA running.
  5. Read your projected value, the tax-free growth, and the year-by-year table.

Frequently asked questions

How much can I put in an ISA?
You can pay in up to £20,000 across all your ISAs in the 2025/26 tax year. That total is shared between Cash ISAs, Stocks & Shares ISAs, Lifetime ISAs (capped at £4,000) and Innovative Finance ISAs, so it's £20,000 in total, not per account. The allowance resets each 6 April and you can't carry unused allowance forward.
Are ISA returns tax-free?
Yes. Interest, dividends and capital gains earned inside an ISA are free of UK income tax and capital gains tax, and you don't declare them on a tax return. That's the main draw over a standard savings or investment account, where interest above your Personal Savings Allowance and gains above your CGT allowance can be taxed.
How much will a £20,000 ISA make in a year?
It depends on the type. A Cash ISA at 4% would earn about £800 in a year on a £20,000 balance. A Stocks & Shares ISA has no fixed rate: at an assumed 5% it might grow by roughly £1,000, but investment values rise and fall, so the real figure could be higher or lower, including a loss. All of it is tax-free inside the ISA.
Cash ISA or Stocks & Shares ISA, which is better?
It comes down to your time horizon and appetite for risk. A Cash ISA pays a set interest rate and your balance can't fall, which suits short-term goals and money you might need soon. A Stocks & Shares ISA can grow more over the long run but the value moves up and down, so it tends to suit money you can leave invested for five years or more. Many people hold both.
How do you calculate ISA growth?
This calculator compounds your balance month by month. It applies your expected annual return (divided across twelve months), adds your monthly contribution, and repeats for the number of years you set. The projected value is your total contributions plus the tax-free growth on top. For a Cash ISA, use the account's AER as the rate.
Can I have more than one ISA?
Yes. From April 2024 you can pay into more than one ISA of the same type in a single tax year, as long as your total contributions across all of them stay within the £20,000 allowance. You can also transfer existing ISAs between providers without it counting towards this year's allowance.

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Simon Chadwick
About the author
Simon Chadwick
Founder of Orbit Money

Simon is the founder of Orbit Money, a tool that helps people track subscriptions and recurring spend. He builds Orbit's free money calculators and writes about personal finance for UK and Australian readers.

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This tool is a guide, not financial advice. Investment returns are not guaranteed and the value of a Stocks & Shares ISA can fall as well as rise.