ISA calculator
See how much a tax-free ISA could be worth by the time you need it. The allowance sounds like something for other people. Over a decade it's the difference between paying tax on your gains and keeping them.
Updated for the 2026/27 tax year · reviewed
Your ISA
The 2026/27 ISA allowance is £20,000 a year, so the monthly slider is capped at £1,667 - HMRC sets this allowance each tax year and the rules can change.
Your ISA could be worth
£34,885
tax-free, after 5 years
Total contributed
£30,000
Growth
£4,885
Annual contribution
£6,000
Allowance used
30.0%
per tax year, of £20,000
Contributions vs growth, by year
Each bar is cumulative - what you've paid in versus what tax-free growth has added on top.
What if that monthly amount went into your pension instead?
An ISA is tax-free and reachable; a pension gets relief up front but locks the money away. Quidworth models both across your household so you can see which actually leaves you better off, and when.
Create your free accountHow this calculator works
We grow your monthly contribution using standard monthly compound interest at the growth rate you choose. Because none of the growth inside an ISA is taxed, the whole figure shown is what actually lands in your pocket - there's no separate tax adjustment to make, unlike a general investment account.
The monthly slider is capped at £1,666.67 because that's a twelfth of the current £20,000 annual ISA allowance - pay in more than that across all your ISAs in one tax year and the excess loses its tax-free status. The allowance resets every 6 April and any unused amount doesn't carry over.
This assumes a steady monthly contribution and a constant growth rate for the whole period. A cash ISA pays a rate closer to typical savings rates; a stocks and shares ISA can grow faster on average but moves up and down along the way - the number above is an illustration either way, not a promise.
Questions people ask
How much can I put into an ISA each year?
The annual ISA allowance for 2026/27 is £20,000, which can be split across cash, stocks and shares, innovative finance and Lifetime ISAs in any combination, subject to the Lifetime ISA's own £4,000 sub-limit. The allowance resets each 6 April and doesn't carry over if unused.
Cash ISA or stocks and shares ISA?
A cash ISA suits money you need within a few years or can't afford to see fall in value, since returns broadly track savings rates. A stocks and shares ISA carries more risk and short-term ups and downs but has historically grown more over longer periods - this calculator lets you compare growth rates for either.
Is ISA growth really tax-free?
Yes - all interest, dividends and capital gains within an ISA are free of UK income tax and capital gains tax, with no need to declare them. That's the main reason ISAs are usually the first place to save or invest once you have an emergency fund.
What happens if I pay in more than the allowance?
Your ISA provider should reject or flag contributions over £20,000 in a tax year, but if HMRC later spots a breach, the excess loses its tax-free status and may need to be withdrawn. Keep track of contributions across all your ISAs, since the £20,000 limit applies in total, not per account.
Last reviewed against 2026/27 UK rules. Projections are estimates for education, not financial advice. Understanding your projections.
A calculator answers one question at a time.
The planner keeps all of them next to each other, so an overpayment you model today still shows up in the pension picture in twenty years.
- Free to start
- No card needed
- Your figures carry over