Projecting your ISA growth
An ISA is a tax-free wrapper. Money inside it grows free of UK income tax and capital gains tax, and you can pay in up to £20,000 a year (2025/26).
Worked example. Paying £20,000 a year for 10 years at a 5% annual return grows to about £251,558. You put in £200,000, so roughly £51,558 is tax-free growth.
The £20,000 allowance
The allowance resets each tax year and does not roll over. If your annual contribution exceeds £20,000 the calculator flags it, because you cannot pay more than the allowance across all your ISAs combined.
Cash vs Stocks and Shares
The projection uses whatever return you enter. A Cash ISA return follows its interest rate. A Stocks and Shares ISA return is variable and not guaranteed, so treat higher figures as a scenario, not a certainty. For the general mechanics of compounding, see the compound interest calculator.
Source: gov.uk Individual Savings Accounts. General information, not financial advice.