ISACALCULATOR
See how your ISA could grow tax-free. Compare Cash ISA, Stocks & Shares ISA and Lifetime ISA — with government bonus calculations built in.
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What Is an ISA?
An Individual Savings Account (ISA) is one of the best deals in UK personal finance. It's a tax-free wrapper around your savings or investments — meaning every penny of interest, dividends, or capital gains you earn inside it is yours to keep. No income tax, no capital gains tax, nothing. The government gives every UK adult a £20,000 annual ISA allowance, and honestly, if you're not using at least some of it, you're leaving free money on the table. There are several types: Cash ISAs work like savings accounts, Stocks & Shares ISAs let you invest in funds and shares tax-free, and Lifetime ISAs give you a 25% government bonus on top. Whether you're saving for a rainy day or building long-term wealth, ISAs should be part of your plan.
How This ISA Calculator Works
Plug in your numbers and see exactly how your ISA could grow over time. The calculator handles compound interest for Cash ISAs and projected growth for Stocks & Shares ISAs — and if you pick the Lifetime ISA option, it automatically works out your government bonus.
Choose Your ISA Type
Pick from Cash ISA, Stocks & Shares ISA, or Lifetime ISA. Each has different rules and expected returns. The calculator adjusts automatically — for example, selecting LISA shows the bonus calculation.
Enter Your Deposit & Contributions
Set your initial lump sum and how much you plan to add each month. Remember the total annual ISA allowance is £20,000 across all ISA types, and LISA contributions are capped at £4,000 per year.
Set Your Rate & Timeframe
For Cash ISAs, enter the AER your provider offers (typically 3-5%). For Stocks & Shares ISAs, 5-7% is a reasonable long-term assumption, though returns are not guaranteed.
Review Your Projection
See your final ISA value, total growth, government bonus (for LISA), and how much tax you've saved compared to holding the same investments outside an ISA.
ISA Growth Formula
Where FV is the final ISA value, P is the initial deposit, r is the annual interest/growth rate as a decimal, t is time in years, and PMT is the monthly contribution. For Lifetime ISAs, the government bonus is calculated as 25% of annual contributions up to £4,000 — giving a maximum bonus of £1,000 per year. The bonus is added to the ISA and compounds alongside your own contributions. Tax saved is estimated at 20% (basic rate) of total growth, representing the income tax you would have owed on interest or gains outside an ISA.
Tips & Best Practices
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Use your £20,000 ISA allowance every tax year if you can — you lose it on 6 April and it never rolls over
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If you're a first-time buyer under 40, open a Lifetime ISA immediately — even with £1 — to start the clock on the 12-month qualifying period
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For Stocks & Shares ISAs, low-cost global index funds (like a FTSE Global All Cap tracker) are hard to beat for long-term growth
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Couples can shelter £40,000 per year between them — that's serious tax-free compounding over a decade
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Don't leave large sums in a current account earning nothing when Cash ISAs are paying 4-5% tax-free
Frequently Asked Questions
Mandeep Singh · 25+ Years UK Financial Services
Important Information
This calculator is for informational purposes only and does not constitute financial advice, a personal recommendation, or an offer to buy or sell any investment or asset class.
Projected figures are illustrative estimates based solely on the inputs you provide. Returns are not guaranteed and actual outcomes will differ.
Past performance is not a guide to future performance, nor a reliable indicator of future results.
If you are unsure about the suitability of any investment or savings strategy for your circumstances, you should seek independent advice from a qualified financial adviser.