ISA calculator
See how much your ISA could be worth in 5, 10, 20 or 30 years - and how much tax you save by keeping your savings inside an ISA wrapper. Updated for the 2025/26 ISA allowance.
The ISA allowance is use it or lose it - it resets every April 5th and cannot be carried forward. Even if you cannot fill the full £20,000, putting in whatever you can before the tax year ends means that money grows tax-free forever. Peter would always fill his ISA before putting money anywhere else.
The £20,000 annual limit
You can put up to £20,000 per tax year into ISAs. This limit applies across all your ISAs combined. Unused allowance cannot be carried to the next year.
Cash vs Stocks and Shares
A Cash ISA gives a fixed interest rate - safe and predictable. A Stocks and Shares ISA invests in markets - higher potential returns over the long term but values can go down as well as up.
Lifetime ISA
Under 40? A Lifetime ISA lets you save £4,000 per year toward a first home or retirement. The government adds 25% on top - up to £1,000 free every year. It counts within the £20,000 limit.
Withdrawals are flexible
Unlike a pension, you can withdraw from most ISAs whenever you want with no tax to pay. Some fixed-rate cash ISAs have withdrawal restrictions - always check the terms before opening.
Please note: ISA allowances and tax rules are correct for the 2025/26 tax year and are subject to change by HMRC. Stocks and Shares ISA projections are illustrative only - investment returns are not guaranteed and values can fall as well as rise. Cash ISA rates will vary between providers and over time. This is not financial advice.
Calculator results are estimates based on the information you enter and the assumptions built into the tool. They are illustrative only and should not be relied upon as accurate projections. Rates, thresholds and rules change regularly; always verify figures with an appropriate professional or official source before acting on them.
What is an ISA calculator?
An ISA calculator projects the growth of regular contributions in an Individual Savings Account over a chosen timeframe, taking into account the compounding effect of returns and the tax-free nature of the wrapper.
The key difference between an ISA and a standard savings or investment account is that all returns – interest, dividends and capital gains — accumulate free of UK tax inside the ISA. Over time, this tax-free compounding produces a meaningfully larger outcome than the equivalent amount in a taxable account, particularly for higher-rate taxpayers and for longer timeframes.
How to use this calculator
Monthly contribution: The amount you plan to contribute each month. This can be anything from £25 upward, up to the annual limit of £20,000 (approximately £1,666 per month). Be realistic – use the amount you will actually maintain by standing order, not the aspirational maximum.
Starting balance: Any existing ISA balance you already hold. The calculator includes this in the projection.
Annual return rate: For a cash ISA, enter the current savings rate. For a stocks and shares ISA, typical long-run projections use 4 to 6% for a cautious allocation and 6 to 8% for a globally diversified equity allocation – though investment returns are not guaranteed and past performance does not predict future returns.
Timeframe: The number of years over which you want to project.
Reading the result: The calculator shows the projected final balance, the total contributions made, and the total growth (interest or investment return) earned. The difference between contributions and final balance represents the tax-free return for a stocks and shares ISA over long periods, which typically exceeds the contributions themselves.
Why starting early matters more than the amount
The most important variable in any compound growth calculation is time. A £100 monthly contribution starting at 25 produces a significantly larger outcome by 65 than £300 per month starting at 45, even though the total contributions in the second scenario may be higher. The calculator makes this visible: run it at your current age and contribution, then run it again with a ten-year earlier start to see the difference.
This is not an argument for guilt about past decisions — it is an argument for starting now rather than waiting until the amount is larger or the timing feels better.
Frequently asked questions
What is the annual ISA allowance? The annual ISA allowance is £20,000 per tax year, covering all ISA types combined – cash ISA, stocks and shares ISA, Lifetime ISA (maximum £4,000 per year) and Innovative Finance ISA. The allowance resets on 6 April each year, and unused allowance cannot be carried forward.
What is the difference between a cash ISA and a stocks and shares ISA? A cash ISA holds cash and earns interest — similar to a savings account but tax-free. A stocks and shares ISA holds investments (funds, shares, bonds) and the returns — dividends, interest, capital gains — are tax-free. Cash ISAs are lower-risk and have lower expected returns; stocks and shares ISAs have variable returns but have historically produced higher returns over long periods.
Can I have more than one ISA? You can hold multiple ISAs, but can only pay into one of each type per tax year. You can transfer previous years’ ISA savings between providers without using your annual allowance.
What happens to my ISA if I need the money early? Most ISAs are flexible – you can withdraw money at any time without penalty (though a Lifetime ISA has a 25% withdrawal penalty if withdrawn before age 60 for non-qualifying purposes). Withdrawing from a flexible ISA and redepositing in the same tax year does not use additional allowance.
Is the ISA allowance likely to change? The £20,000 allowance has been stable for several years. Future changes are possible – contributions already made retain their tax-free status permanently, regardless of future rule changes.
The information on this site is for educational and informational purposes only. Nothing on Be More Peter constitutes financial advice, and nothing should be treated as such. The calculators, articles and tools on this site are designed to help you understand financial concepts and run indicative numbers - they are not a substitute for professional advice tailored to your specific circumstances. Before making any financial decision - including decisions about mortgages, pensions, investments, savings or insurance — you should seek independent financial advice from a qualified adviser who is authorised and regulated by the Financial Conduct Authority (FCA). Calculator results are estimates based on the information you enter and the assumptions built into the tool. They are illustrative only and should not be relied upon as accurate projections. Rates, thresholds and rules change regularly - always verify figures with an appropriate professional or official source before acting on them. Be More Peter is not authorised or regulated by the FCA. The content on this site does not constitute a financial promotion.
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- Compound interest calculator — compare taxable and tax-free growth
- Mortgage overpayment calculator — compare ISA investing versus overpaying
- What is an ISA and why have I been ignoring mine?
- ISA vs savings account — which is better?
- ISA deadline — why April 5th matters