Figures are illustrative only, ignore charges and tax, and are not a forecast or personal recommendation.
This calculator projects the potential future value of a Stocks & Shares ISA, based on how much you contribute each year, how many years you plan to contribute for, and an assumed annual rate of return. It uses annual contributions compounded yearly, reflecting how many ISA investors actually fund their account — often as a single contribution near the start of the tax year, or built up gradually and treated as one annual figure for planning purposes.
The ISA allowance
UK residents can contribute up to £20,000 per tax year (2025/26) across all their ISAs combined, and all growth and income within a Stocks & Shares ISA is free of UK income tax and Capital Gains Tax — a significant advantage over a General Investment Account once you factor in tax. There's no lifetime limit on how much can be held in an ISA, only an annual limit on new contributions, so a disciplined long-term contributor can build a substantial tax-free portfolio over enough years. Our ultimate guide to Stocks & Shares ISAs covers the rules, allowance mechanics, and how to get started in full.
How the projection works
The calculator assumes your annual contribution is invested at the start of each tax year and compounds annually at the rate you enter, for the number of years specified. It reports the projected ISA value, your total contributions, and the growth on top. If you'd rather model monthly contributions specifically, our Regular Investment Calculator or combined Investment Growth Calculator may better match how you actually invest.
Worked example
Contributing the full £10,000 a year for 15 years at an assumed 5% annual return produces a projected ISA value well above the £150,000 paid in — with the compounding effect becoming particularly noticeable in the final third of the period. Because none of this growth is taxed, the full projected figure (minus any platform and fund charges) is what you'd actually have, unlike a taxable account where gains above your annual exempt amount could be subject to Capital Gains Tax.
Should you max out your ISA before investing elsewhere?
For most UK investors, using the ISA allowance in full before investing via a taxable General Investment Account makes sense, given the tax advantages. Once both your pension (for the employer contribution and tax relief) and ISA allowances are being used, a GIA becomes the natural next step for additional saving — see our guide on using a GIA once you've maxed your ISA and pension for how to think about that stage.
Frequently asked questions
Can I contribute more than £20,000 a year?
Not within an ISA in a single tax year — £20,000 (2025/26) is the combined limit across all your ISAs. Additional saving beyond that goes into a taxable account, or a pension if you have allowance remaining there.
What if I don't contribute the same amount every year?
The calculator assumes a flat annual contribution for simplicity. For a more realistic picture with varying contributions, project each differing period separately and combine the results, or use a lower "average" annual figure across the whole term.
Can I withdraw money from an ISA without losing my allowance?
With a Flexible ISA, you can withdraw and replace money within the same tax year without it counting against your annual allowance twice — see our guide on Flexible ISAs for how this works.
What return assumption should I use?
This depends entirely on what you're invested in within the ISA — a global equity tracker, a multi-asset fund, and a cash-heavy allocation have very different realistic long-run return expectations.
Should children have their own ISA-style account?
Yes, in the form of a Junior ISA — see our guide to Junior ISAs (JISAs) for the separate rules and allowance that apply to saving or investing for a child.
Common mistakes when projecting an ISA
A frequent mistake is assuming the full £20,000 annual allowance will always be used every year without checking this is actually realistic given other financial priorities and allowances (such as a workplace pension or a Lifetime ISA for a first home, which count separately or, in the LISA's case, toward the same overall ISA allowance). It's worth using a contribution figure that reflects a genuinely sustainable amount rather than the maximum allowance, since a lower but consistent contribution, kept up for the full term, usually beats an ambitious figure abandoned partway through.
A second mistake is forgetting that the £20,000 limit is a use-it-or-lose-it annual allowance — unused allowance doesn't carry forward to future tax years, unlike the pension annual allowance's carry-forward rules. If your contribution amount varies significantly year to year, this calculator's flat annual figure is best treated as a rough average across the whole period rather than a precise year-by-year plan.
What happens to my ISA allowance if I don't use it all?
It's lost at the end of the tax year (5 April) — there's no carry-forward, unlike some pension allowances, so any unused allowance simply resets to the new £20,000 limit (or whatever the current limit is) at the start of the next tax year.
Can I hold multiple types of ISA at once?
Yes — you can hold a Cash ISA, Stocks & Shares ISA, Innovative Finance ISA, and Lifetime ISA simultaneously, but your total contributions across all of them combined cannot exceed the overall annual allowance.
Can I use both a Cash ISA and a Stocks & Shares ISA in the same tax year?
Yes — you can split your annual allowance across different ISA types in the same tax year (for example, part in cash and part in stocks and shares), as long as the combined total across all of them doesn't exceed the overall annual limit.
What happens to my ISA if I move abroad?
You can generally keep an existing ISA and it retains its tax-free status, but you typically cannot make new contributions to it while not a UK resident for tax purposes — rules here can be nuanced, so check current HMRC guidance if this applies to you.