Stocks and Shares ISA Calculator United Kingdom

Money in a Stocks and Shares ISA can be invested in funds, shares and bonds, and everything it earns, capital gains, dividends and interest, is sheltered from tax while it stays there. This calculator takes a starting amount and a monthly contribution, compounds them at the rate of return you choose, and projects what the pot could be worth at the end of your chosen term.

How the projection works

The calculator compounds monthly, matching how many people invest: a lump sum now plus a fixed amount every month. Each month it applies a twelfth of your expected annual return to the balance and adds your contribution, which gives the standard future value formula for a starting sum plus a regular series of payments. The longer the term and the higher the rate, the more the result comes from growth rather than the cash you put in.

You get three figures: the projected ISA value, the total you contributed and the tax-free growth, the difference between the two. That growth would be exposed to Capital Gains Tax and dividend tax in an ordinary account.

The ISA allowance

The ISA allowance is £20,000 a tax year, across every ISA you hold, not per account. You can put the full amount into a Stocks and Shares ISA or split it with a Cash ISA, but combined subscriptions cannot exceed the limit. The allowance resets on 6 April and unused allowance does not carry over. The calculator flags when your monthly contributions add up to more than the limit over a year; remember that a lump sum paid in the same tax year also counts towards it.

Why the tax shelter matters over time

In a taxable account, dividends can be taxed each year and each sale can bring a Capital Gains Tax bill. Inside an ISA neither applies, so the investments compound on their full return. Over a decade or two the money that would have gone in tax stays invested and earns its own return, which is why it usually makes sense to use the ISA allowance before investing in an unwrapped account, particularly for assets expected to grow substantially.

A worked example

Suppose you start with £5,000 and add £300 a month for 20 years at an assumed 7% annual return. You contribute £77,000 in total, the initial £5,000 plus £72,000 over 240 monthly payments. With monthly compounding the pot grows to about £176,472, so about £99,472 of the final balance is investment growth, all of it tax-free inside the ISA. Change any input to see how a higher contribution, a longer term or a more cautious return changes the outcome.

Making the most of the wrapper

Paying in regularly by direct debit keeps you investing through market rises and falls. Reinvesting dividends keeps the compounding going. Checking the allowance each spring makes sure you use it before it resets. Use the calculator to test scenarios before you commit money.

Assumptions and limitations

This is a projection, not a promise. It assumes one constant rate of return every year, while real returns are volatile and can be negative. It shows gross figures before platform fees, fund charges and adviser costs, all of which reduce the final pot; even a 1% annual charge makes a visible difference over 20 years. It does not adjust for inflation, so the real value of the projected balance will be lower than the headline number. It assumes your contributions stay within the ISA allowance, which is stored as a parameter and reviewed each tax year. Investments can fall as well as rise, so treat the result as an illustration and take regulated advice for decisions that matter.

About This Stocks and Shares ISA Calculator

Use this Stocks and Shares ISA calculator to get an instant, accurate estimate. The figures follow official GOV.UK guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.