What an ISA does for your money
An Individual Savings Account (ISA) doesn't change how your money is invested or held — it's a tax wrapper placed around cash or investments that shields any interest, dividends or capital gains from Income Tax and Capital Gains Tax entirely, provided the money stays within the ISA rules. This calculator projects how a fixed annual contribution, invested each year at an assumed return, could grow over your chosen number of years, compounding annually as each year's fresh contribution is added.
Cash ISA versus stocks and shares ISA
The annual ISA allowance can be split across different types of ISA in the same tax year, most commonly a cash ISA (functioning like a tax-free savings account) and a stocks and shares ISA (holding investments such as funds, shares or bonds, which can rise and fall in value). Which is more appropriate depends heavily on your time horizon and comfort with risk — cash suits money you might need at short notice or simply don't want exposed to market movements, while a longer horizon of five years or more is generally when a stocks and shares ISA becomes worth considering for its higher long-term growth potential. Our cash ISA versus stocks and shares ISA guide covers this choice in full.
The annual allowance and use-it-or-lose-it rules
Each tax year comes with its own fresh ISA allowance, and any unused portion doesn't roll over into the next year — if you don't use it, it's simply gone once the tax year ends. This is why the calculator above treats each year's contribution as a separate, fresh amount rather than something you can top up retrospectively. Our full ISA allowance and annual rules guide explains exactly how the allowance works, including what happens if you have ISAs with more than one provider.
What happens if you need to access the money
Unlike a pension, money in an ISA (other than a Lifetime ISA used for a first home or retirement, which carries a withdrawal penalty outside those purposes) can generally be accessed whenever you need it, which is one of the reasons ISAs are so widely used for medium-term goals as well as long-term investing. Some providers allow flexible withdrawals and replacements within the same tax year without it counting against your allowance twice; others don't, so it's worth checking your specific provider's rules before assuming you can freely move money in and out.
Building a realistic contribution figure
£10,000 a year, used as the default above, is a substantial contribution that not everyone can sustain, and £20,000 is the maximum most people can currently pay in across all their ISAs combined. It's worth running this calculator with a contribution figure that genuinely matches what you can commit to consistently, since a smaller, realistic and sustained contribution compounding over many years will typically beat an ambitious figure you can't actually maintain. If you're contributing monthly rather than in one annual lump sum, our regular investment calculator models that pattern directly instead.
A worked example
Using the calculator's defaults — £10,000 contributed every year, for 10 years, at an assumed 5% annual return — you'd pay in £100,000 in total across the decade. Because each year's contribution starts compounding as soon as it's added, the projected value comes to roughly £132,700, meaning growth alone has added about £32,700 on top of your contributions by the end of the tenth year.
Stretch the same annual contribution out to 20 years instead, and total contributions double to £200,000 — but the projected value rises to approximately £347,000, more than two and a half times the 10-year figure. The extra decade lets earlier years' growth itself keep compounding, which is why consistent annual ISA contributions kept up over a longer working life tend to produce disproportionately larger results than the same contributions squeezed into a shorter period.
Choosing what to hold inside your ISA
The ISA wrapper itself doesn't generate any return — that comes entirely from whatever's held inside it, whether that's cash, a single fund, a spread of several funds, or individual shares and bonds. Many providers offer ready-made, risk-graded fund options aimed at people who'd rather not choose individual investments themselves, which can be a sensible starting point if you're newer to investing. Whatever you choose, it's worth revisiting the underlying holdings periodically, since a fund or platform that suited you five years ago may not still be the most appropriate or lowest-cost option available today.
Frequently asked questions
Can I have more than one ISA?
Yes, you can hold multiple ISAs with different providers, and since recent rule changes you can generally pay into more than one ISA of the same type within a single tax year, provided your total contributions across all of them stay within the overall annual allowance.
What happens to my ISA if I move house or country?
Your ISA itself stays open and keeps its tax-free status if you move within the UK. If you become non-UK resident, you generally can't pay in further contributions, though existing funds can typically remain invested — check with your provider for your specific circumstances.
Does this calculator include the Lifetime ISA bonus?
No. A Lifetime ISA adds a 25% government bonus on top of contributions up to its own separate annual limit, which this general ISA calculator doesn't model — see our Lifetime ISA guide for that specific scheme.
What happens to my ISA when I die?
An ISA can be passed to a surviving spouse or civil partner with an additional permitted subscription broadly equal to its value at death, preserving its tax-free status; for other beneficiaries, the ISA tax wrapper generally ends and the assets form part of the estate.
Can I withdraw and replace money within the same tax year?
Some providers offer flexible ISAs that allow this without it counting twice against your annual allowance — check whether your specific ISA is flexible before assuming a withdrawal and later replacement won't affect your allowance.