ISA growth calculator
ISA growth calculator: what could your ISA be worth?
Put in what you have now, what you can add each month and how long you will leave it, and see how much of the final figure is your own money and how much is compound returns. Every result is an illustration, not a forecast.
What could your ISA be worth?
Put in what you have now, what you can add each month and how long you'll leave it. See how much of the final figure is your own money and how much is growth.
Your numbers
Illustrative value after 25 years
£0
How to read this
Your monthly amount is added at the end of each month, any extra yearly top-up at the end of each year, and the whole pot grows at the same steady rate every year. Real markets never behave that neatly, but the shape is honest: early on most of the pot is your own money, and the longer you leave it, the more of it is growth on growth. I explain why in the maths that changes everything, and why what you put in matters so much in why savings rate matters more than returns. Figures are before inflation.
How it works
How an ISA grows
An ISA grows in two ways at once: the money you add, and the returns on everything already in it. Inside a Stocks and Shares ISA none of that growth is taxed, so every pound of return stays invested.
Growth on growth
In the early years almost all of the pot is your own money. Leave it long enough and the returns start earning returns of their own, until that growth on growth is most of the final figure.
That’s what people mean by compound interest, although with shares it’s really compound returns.
Time and the amount you add
These matter more than the growth rate you pick. Move the years from 20 to 30 and watch the growth share of the pot, then try an extra £50 a month.
Small changes held for decades make large differences, which is the case for starting early and staying invested through the falls.
The growth rate
It’s an assumption, not a promise. Over long periods UK shares have returned roughly 5% a year after inflation, but real returns arrive unevenly, with some years sharply down.
I set out the long-run figures in what a realistic average return is.
Questions
Common questions
Quick answers about the figures. For your own circumstances, gov.uk and your provider have the details.
Is this a compound interest calculator?
The maths is the same. Shares pay returns rather than interest, so I call it compound returns, but a compound interest calculator and this one work the same way.
Does it allow for inflation?
No. The figures are before inflation, so £100,000 in 25 years will buy less than £100,000 today. Using a lower growth rate, such as 4% or 5%, gives a rough picture in today’s money.
How much can I put in?
Up to £20,000 a year across all your ISAs at the time of writing. The calculator flags it if your monthly and yearly amounts go over.
Rates used
All rates are at the time of writing and were checked at source on 3 November 2026. They’re updated each year on 6 April, and after any Budget that changes them.
| What | 2026/27 | Source |
|---|---|---|
| ISA allowance | £20,000 a year, shared across all your ISAs | gov.uk: Individual Savings Accounts |
Other calculators
New to ISAs?
Read the sixteen articles in order.
Getting Started takes you from the basics to opening your ISA, then the principles that keep you invested and how wealth builds over time.
Go to Getting StartedAll results are illustrations, not forecasts, and not financial advice. They assume steady rates that real markets never deliver. The value of investments can go down as well as up, and you can get back less than you put in. Allowances, tax rates and ISA rules can change, so always check gov.uk for the current figures.
One email a month
The latest articles, what’s changed in the ISA rules, and ideas inspired by the book and my own experiences. Join, and The ISA Starter is yours straight away.
JOIN THE NEWSLETTER