Projects what an ISA could be worth, and checks your contributions against the £20,000 annual allowance — the rule that makes an ISA an ISA rather than an ordinary savings pot.
Initial deposit (£)
Monthly contribution (£)
Years to save
Annual growth rate (%)
Projected value after 10 years
£17,175
You pay in£13,000
Growth, entirely tax free£4,175
Annual subscription£1,200
£1,200 a year leaves £18,800 of your £20,000 allowance unused. The allowance does not roll over — whatever is left on 5 April is gone.
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About the UK ISA Calculator
An ISA is not an investment. It is a wrapper you put investments or savings inside, and everything that happens within it is free of income tax and capital gains tax. That is the whole benefit, and it is a large one over time: on a pot that grows to £17,000 from £13,000 of contributions, the £4,000 of growth is yours in full, with nothing to declare and nothing to pay. This calculator projects what an ISA could be worth given a starting amount, a monthly contribution, a term and a growth rate — and, unlike most ISA calculators, it checks your contributions against the £20,000 annual allowance. That check matters because the allowance is the only rule that makes an ISA different from an ordinary savings account. Entering £2,000 a month produces a tidy projection built on £24,000 of annual contributions, which is £4,000 more than you are permitted to pay in. A projection you cannot legally achieve is worse than no projection. The allowance for the 2026 to 2027 tax year is £20,000 across all your ISAs combined, and it resets on 6 April.
Mathematical Formula & Logic
Two pieces are added together. The lump sum grows by compound interest: multiply the initial deposit by one plus the monthly rate, raised to the power of the number of months. The monthly rate is the annual rate divided by twelve, and the number of months is the term in years multiplied by twelve. The contribution stream is the future value of an ordinary annuity: each monthly payment compounds for however many months remain after it is made, and the standard closed form is the monthly amount multiplied by the growth factor minus one, divided by the monthly rate. Where the rate is zero this collapses to the monthly amount multiplied by the number of months, which the calculator handles separately to avoid dividing by zero. Adding the two gives the projected value. Contributions are treated as being made at the end of each month, which is the conservative convention — paying in at the start of the month would give a slightly higher figure. Separately, the annual subscription is the monthly contribution multiplied by twelve, and this is compared against the £20,000 allowance. Note what the allowance does and does not restrict. It caps what you pay in during a tax year. It does not cap what the ISA is worth, so a pot can grow to any size without breaching anything, and growth does not use up allowance. Money you withdraw from most ISAs does not restore the allowance either, unless the account is specifically a flexible ISA.
Step-by-Step Example
Three worked examples.
Example 1: a modest regular saver. You start with £1,000, add £100 a month, and assume 5% annual growth over ten years. The initial £1,000 compounds to £1,647. The contribution stream of £12,000 grows to £15,528. The total is £17,175, of which £13,000 is money you put in and £4,175 is growth. Because it is inside an ISA, that £4,175 attracts no tax at all. Outside one, a higher-rate taxpayer could lose a substantial share of it to dividend or capital gains tax.
Example 2: no growth, to show the mechanics. £100 a month for one year at 0% gives exactly £1,200. Nothing compounds, so the total equals the contributions. This is worth checking on any calculator you use — if it returns something other than £1,200 for that input, the annuity formula is wrong.
Example 3: exceeding the allowance. You enter £2,000 a month, which is £24,000 a year. The projection appears without complaint on most calculators, but it is not achievable: the maximum you may pay into ISAs in the 2026 to 2027 tax year is £20,000, so you are £4,000 over. This calculator says so. The practical maximum monthly contribution is £1,666, which uses £19,992 of the allowance and leaves £8 unused.
Reference Data & Values
label
value
Annual ISA allowance, 2026/27
£20,000 across all ISAs combined
Lifetime ISA limit
£4,000 a year, counting towards the £20,000
Maximum monthly contribution
£1,666 — twelve of these is £19,992
Tax on interest, dividends and gains
None, and nothing to report
Allowance reset
6 April each year — unused allowance does not carry over
Types of ISA
Cash, stocks and shares, innovative finance, Lifetime
Minimum age
18, or under 40 to open a Lifetime ISA
Effect of withdrawals on allowance
None restored, unless the ISA is a flexible one
Frequently Asked Questions
£20,000 in the 2026 to 2027 tax year, across all your ISAs combined rather than £20,000 each. You can split it however you like — £15,000 in a cash ISA and £5,000 in a stocks and shares ISA, for example — with the single exception that no more than £4,000 can go into a Lifetime ISA, and that £4,000 counts towards the same £20,000.
No, and this is the most commonly misunderstood rule. Whatever is left of your £20,000 on 5 April simply disappears. There is no carry-forward as there is with pension contributions. If you have money to invest and unused allowance in March, using it before the tax year ends is worth doing, because that allowance will not exist again.
No. Withdrawals from an ISA are entirely free of tax, at any time, in any amount, with no reporting to HMRC. This is unlike a pension, where withdrawals above the tax-free lump sum are taxed as income. The trade-off is that ISA contributions get no tax relief going in, whereas pension contributions do — which is why the two suit different purposes rather than one being simply better.
Only if your ISA is a flexible one, and not all providers offer this. With a flexible ISA, money you withdraw and replace in the same tax year does not use up allowance twice. With a standard ISA it does: withdraw £5,000 and pay it back, and you have used £5,000 of allowance for nothing. Check with your provider before moving money in and out.
No, and it should not be read as a forecast. For a cash ISA the rate is whatever your provider pays and can change at any time. For a stocks and shares ISA there is no rate at all — returns vary year to year and can be negative. The figure here is an assumption you supply, compounded arithmetically. It shows what would happen if that rate held every year, which no real investment does.
Yes. You can hold multiple ISAs and, since the rules were relaxed, pay into more than one of the same type in a single tax year. What you cannot do is exceed £20,000 in total across all of them. You can also transfer existing ISAs between providers without it counting against your allowance, provided you use the formal transfer process rather than withdrawing and re-depositing.
They are taxed at opposite ends. A pension gives tax relief on the way in and is taxed on the way out; an ISA gives no relief on the way in and is entirely tax free on the way out. For most people a pension wins on pure arithmetic for retirement money, particularly with employer contributions, while an ISA wins on flexibility because you can access it at any age. Many people sensibly use both, and this is a question worth taking advice on rather than settling from a calculator.
No — only to what you pay in during a tax year. Your ISA can grow to any value without breaching anything, and that growth does not consume allowance. Someone who has contributed for twenty years could hold well over £400,000 entirely within ISAs, all of it still free of tax on income and gains. This is the main reason the wrapper becomes more valuable the longer it is held.