UK ISA Calculator

An ISA (Individual Savings Account) is one of the simplest ways to grow your money in the UK without handing a share of the gains to HMRC. Every interest payment, dividend and capital gain earned inside an ISA is completely tax-free, which makes a real difference over many years of saving or investing. This calculator projects how a Cash, Stocks and Shares, Lifetime or Junior ISA could grow over time, and shows roughly how much more you would keep compared to a taxed savings account.

How to use the ISA calculator

Choose the type of ISA you are using or considering, then enter your initial deposit, how much you plan to add each month, the annual return you expect, and how many years you plan to keep saving. The calculator projects your final balance, splits it between your own contributions and the tax-free gains on top, and compares the result to a similar taxed savings account.

If you are opening a Lifetime ISA, the calculator also shows the 25% government bonus on your contributions, up to the £4,000 annual Lifetime ISA limit. This bonus is one of the most valuable features available to first-time buyers and long-term retirement savers under 40, and it is worth factoring into any comparison with a standard Stocks and Shares ISA.

How the calculation works

The calculator compounds your balance monthly: your monthly contribution is added, then growth is applied to the new, larger total. This repeats every month for the number of years you enter, so later months benefit from growth on a bigger balance than earlier ones. The maths is the same principle behind our compound interest calculator, with the key difference being that everything inside an ISA grows completely free of tax.

For the taxed savings comparison, the calculator assumes the same growth rate outside an ISA, with any interest earned above your Personal Savings Allowance taxed at a simplified 20% each year. This is a rough illustration rather than a precise tax calculation, since your real tax position depends on your total income and whether the growth comes from interest, dividends, or capital gains, each of which is taxed differently outside an ISA.

Worked example

Say you open a Stocks and Shares ISA with £1,000, add £200 a month, and expect an average annual return of 5% over 10 years.

Over that time you would contribute £25,000 of your own money (the £1,000 start plus £200 a month for 120 months). The calculator projects a final balance of around £32,833, meaning roughly £7,833 of tax-free growth on top of what you paid in. Because the whole amount sits inside an ISA wrapper, none of that growth is subject to Income Tax or Capital Gains Tax, no matter how large your other income or investments become.

Try switching the ISA type to Lifetime ISA in the calculator above, using the same £200 a month. You will see a 25% government bonus added on top of your contributions, which is a guaranteed uplift that no investment return can match, though remember the annual Lifetime ISA limit is lower, at £4,000 a year rather than the full £20,000 ISA allowance.

Common mistakes to avoid

A common mistake is treating the ISA allowance as if it resets or rolls over. It does not: any unused allowance is lost at the end of each tax year on 5 April, so contributing little and often across the year is usually more reliable than waiting until the last few weeks and finding you cannot fit in a large lump sum. It is also easy to forget that the Lifetime ISA's £4,000 limit counts towards, not on top of, your overall £20,000 allowance, so a large Lifetime ISA contribution leaves less room for other ISAs in the same tax year.

Another mistake is choosing an ISA type based purely on the interest rate or projected return without thinking about your timeframe. A Stocks and Shares ISA earning a higher long-term average return can still lose value in any given year, which matters a great deal if you need the money within the next year or two. For short-term goals, such as a house deposit you plan to use soon, a Cash ISA or Lifetime ISA cash option usually makes more sense than chasing a higher headline return.

Finally, do not assume every provider's ISA is flexible. A flexible ISA lets you withdraw money and pay it back in later in the same tax year without it counting against your allowance twice, but not every provider offers this feature. Check before you rely on being able to move money in and out freely.

Related calculators

If you want to see how your savings would grow outside an ISA wrapper, our compound interest calculator and savings goal calculator use the same underlying growth maths. If you are saving specifically for retirement, try our pension calculator to project your pot and possible retirement income, and check your take-home pay to see how much you can realistically afford to put aside each month. Our guide to compound interest explains the underlying formula in more depth, and our income tax guide covers how tax works on income outside an ISA.