ISA Growth Calculator
An ISA does not change how your money grows — it changes what happens to that growth at tax time, since everything earned inside an ISA, whether interest, dividends or capital gains, is completely free of UK tax. This calculator projects how your ISA balance grows over time based on your contributions and expected rate of return, so you can see the tax-free total you are working toward.
ISA Growth Calculator
Project how much your ISA could grow over time — all interest and returns are tax-free.
Why £20,000 Really Is Use-It-Or-Lose-It
The annual ISA allowance, currently £20,000, resets every tax year on 6 April and does not carry forward — unused allowance from one year is simply gone once the new tax year starts, with no way to claim it back later. This is genuinely different from some other allowances that can be carried forward, and it is a common source of regret for people who intended to use their full allowance but let the tax year end without doing so. If you have a lump sum you intend to shelter from tax over several years, spreading it across multiple tax years, rather than trying to fit it all into a single year’s allowance, is often the only way to get it all inside an ISA at all.
This is worth planning around deliberately rather than leaving to the last minute — many people find it easier to use the allowance steadily through the year via regular monthly contributions rather than trying to find a large lump sum in the final weeks before the tax year ends, when the deadline can otherwise be easy to miss entirely.
Four Types, One Shared Limit
The £20,000 allowance is a single, shared limit across all your ISAs in a tax year, not £20,000 per type — a Cash ISA, a Stocks and Shares ISA, a Lifetime ISA and an Innovative Finance ISA all draw from the same total allowance, split however you choose between them. You can pay into more than one type in the same tax year, provided the combined total across all of them does not exceed £20,000, which gives genuine flexibility to combine, for example, some cash for shorter-term security with some invested for longer-term growth, all within the same overall limit.
This shared-limit structure is worth understanding clearly before assuming you have far more total tax-free room than you actually do — a common misconception is treating each ISA type as having its own separate £20,000, when in reality every pound paid into any of them counts against the same single annual total.
The 25% Bonus With Strings Attached
A Lifetime ISA is the one type genuinely different from the others: contributions are capped at £4,000 a year, counting toward your overall £20,000 limit, but the government adds a 25% bonus on top — effectively free money, provided the funds are eventually used either to buy a first home or withdrawn from age 60 onward. Withdrawing for any other reason before age 60 triggers a 25% government withdrawal charge, which is a genuinely significant penalty — it does not just claw back the bonus, it can leave you with less than you actually paid in. This makes a Lifetime ISA a strong option specifically for its intended purposes, and a genuinely risky place to keep money you might need access to for something else.
Seeing the Numbers Add Up
£5,000 invested initially with a further £300 a month, growing at 4.5% a year for 10 years, projects to roughly £53,200 — built from £41,000 of actual contributions and around £12,200 of tax-free growth on top. Paying the maximum £4,000 a year into a Lifetime ISA instead over the same 10 years contributes £40,000 in total, and the 25% government bonus alone adds £10,000 on top of that, before any investment growth is even factored in — a guaranteed uplift no ordinary ISA type offers.
Cash or Stocks and Shares — The Real Trade-Off
A Cash ISA behaves like a savings account, with a known interest rate and protected capital up to the FSCS limit, while a Stocks and Shares ISA invests the money in the market, offering higher long-term expected growth alongside genuine risk of the balance falling in value, particularly over shorter periods. Neither is universally correct — the right choice depends on your time horizon and comfort with risk, and this calculator lets you enter your own expected rate to model either scenario. Our Investment Calculator goes further into the risk and real-value considerations specific to market-based investing if a Stocks and Shares ISA is the direction you are leaning toward. And if you already hold an ISA and are wondering whether a different provider or type would serve you better, our ISA Switch Calculator covers that comparison directly.
Frequently Asked Questions
Can I have more than one ISA of the same type?
You can hold multiple ISAs of the same type from previous tax years, but you can generally only pay new money into one of each type within the current tax year, splitting your allowance across different types rather than multiple accounts of the same type simultaneously.
What happens to my ISA if I do not use the full allowance?
Nothing negative happens to money already inside the ISA — it stays tax-free and continues growing. It is only the unused portion of that year’s £20,000 allowance that disappears; existing ISA balances are never affected.
Is a Lifetime ISA only for buying a first home?
No — it can also be accessed penalty-free from age 60 for any purpose, functioning as a retirement savings vehicle with a government bonus, not solely a house-deposit product, though the first-home route is the more commonly used one for younger savers.
Can I transfer between ISA types without losing tax-free status?
Yes, provided the transfer is done correctly through your provider’s official ISA transfer process, rather than withdrawing and reopening a new one yourself, which would count as new contributions against your current year’s allowance instead of a like-for-like transfer.
Do children have their own ISA allowance?
Junior ISAs have a separate annual allowance, currently £9,000, entirely independent of an adult’s own £20,000 limit, and the funds become the child’s own money, accessible once they turn 18.
Is there an age limit for opening a Lifetime ISA?
Yes — you must open a Lifetime ISA between the ages of 18 and 39, though once opened, contributions can continue until age 50, and the account itself can be held and continue growing well beyond that.
Important Information
This calculator provides an estimate based on the assumptions you enter and does not constitute financial advice. ISA allowances and rules may change, and investment-based ISAs can fall as well as rise in value. For advice specific to your circumstances, consult a qualified financial adviser. See our Disclaimer for further information.