United Kingdom · Salary, Tax & Money
ISA Calculator
Charges and tax
Enter what you have, what you add each month and what you expect it to return. The calculator projects the value and — unusually — shows what the annual charge costs you over the whole term.
How much will my ISA be worth?
Your starting balance compounds for the full term while each monthly contribution compounds for however long it remains invested, with charges reducing the return before compounding. £10,000 plus £300 a month at 6% growth and 0.4% charges reaches roughly £163,000 over twenty years — entirely free of UK tax.
How to use this calculator
- Enter your current ISA balanceAcross all your ISAs of the type you are modelling. Old ISAs from previous years still count and still compound — they do not use up the current year's allowance.
- Set a realistic monthly contributionThe annual ISA allowance caps what you can pay in each tax year across all ISA types. Check the current figure before assuming a monthly amount is permitted.
- Be conservative with the returnFor a stocks and shares ISA, 5 to 7 per cent before inflation is a reasonable long-run planning assumption. For a cash ISA, use the actual interest rate.
- Enter the total annual chargePlatform fee plus fund charges. It comes off the return before compounding, and the breakdown shows what it costs across the whole term.
How the calculation works
Two components. The existing balance compounds for the whole term. Each monthly contribution compounds from the month it is paid, and the second term of the formula sums that series.
Charges are handled the way they actually behave — subtracted from the gross return before compounding, so a 6 per cent return with a 0.4 per cent charge compounds at 5.6 per cent. That is why platform charges matter more than they look. Over twenty years on a growing balance, the difference between a 0.25 per cent platform and a 0.75 per cent one is routinely a five-figure sum.
Tax does not appear anywhere, and that is the point of the wrapper. Growth inside an ISA is free of income tax and capital gains tax, there is nothing to declare on a tax return, and withdrawals are tax-free at any time. There is no equivalent of the eight-year deemed disposal that applies to some fund structures elsewhere, so compounding is never interrupted.
That combination — no tax on growth, no tax on withdrawal, no reporting — is unusually generous, and it is why using the allowance before considering a general investment account is close to a default recommendation.
FV = P(1+r)ⁿ + M((1+r)ⁿ − 1) ÷ rWorked example: twenty years of monthly saving
£10,000 to start, £300 a month, 6 per cent assumed growth, 0.4 per cent annual charges, over twenty years.
The net return is 5.6 per cent. The starting £10,000 grows to about £29,800. The contributions, £72,000 in total, grow to roughly £133,000. The projected value is around £163,000, of which £81,000 is growth on £82,000 of contributions.
All of that is tax-free. In a general investment account, the same growth would face capital gains tax on disposal above the annual exempt amount, and any dividends would be taxable above the dividend allowance.
The charge line is worth reading. Without the 0.4 per cent charge the pot would reach about £171,000 — so the charge cost roughly £8,000 across the term, more than two years of contributions. Moving from a 0.4 per cent platform to a 0.15 per cent one would leave around £3,000 more, for no change in contributions or strategy.
The ISA types
| Type | What it holds | Best suited to |
|---|---|---|
| Cash ISA | Deposits paying interest | Short-term savings and emergency funds |
| Stocks and shares ISA | Funds, shares, bonds, investment trusts | Long-term growth, five years or more |
| Lifetime ISA | Cash or investments, with a government bonus | First home purchase or retirement, with age limits and withdrawal penalties |
| Innovative finance ISA | Peer-to-peer loans | Higher risk, less liquid |
| Junior ISA | Cash or investments for a child | Long-term saving, locked until 18 |
The annual allowance is shared across types rather than applying to each, and the total is set by HMRC. Transfers between ISAs do not use allowance provided they are done as a formal transfer rather than a withdrawal and redeposit — withdrawing and paying back in uses your allowance again unless the ISA is a flexible one.
What a projection cannot tell you
This is arithmetic on an assumption. It shows what happens if a constant return is achieved every year, and no investment behaves that way. Real returns arrive unevenly, and for anyone drawing down, the order matters enormously — two portfolios with the same average return can end very differently depending on whether the bad years came first or last.
Inflation is the other omission. The figures are nominal. At 2 per cent inflation, £163,000 in twenty years buys roughly what £110,000 buys today. The working panel shows that real-terms figure, because a long-term projection quoted without it consistently overstates what the money will actually do.
For a cash ISA the risk profile is inverted: the nominal value is safe and the real value is not. Cash paying below the inflation rate loses purchasing power with certainty, which is a genuine risk rather than a safe option, particularly over the timeframes this calculator models.
Use the tool to compare scenarios rather than predict outcomes. The comparisons — this charge against that one, starting now against starting in three years, £300 a month against £400 — hold up even where the absolute number does not.
Starting earlier beats contributing more over long periods. £200 a month for thirty years substantially outperforms £350 a month for eighteen despite the second paying in more, because the first has twelve extra years of compounding on the early contributions.
Assumptions and limitations
- Assumes a constant annual return, which no real investment delivers
- Does not enforce the annual ISA allowance — check the current limit before planning contributions
- Treats charges as a flat reduction to the return rate rather than as separate fixed fees
- Figures are nominal; the inflation-adjusted equivalent is shown in the working only
- Does not model Lifetime ISA bonuses, age limits or withdrawal penalties
- Not financial advice, and not a projection any provider would stand behind
Questions people ask about this in United Kingdom
How much will my ISA be worth?
It depends on your balance, contributions, return and charges. £10,000 plus £300 a month at 6 per cent growth with 0.4 per cent charges reaches roughly £163,000 over twenty years.
At 2 per cent inflation that is worth about £110,000 in today's money, which is the more useful figure for planning.
Do I pay tax on ISA growth?
No. Growth inside an ISA is free of both income tax and capital gains tax, withdrawals are tax-free at any time, and there is nothing to report on a tax return. That combination is unusually generous and is the main reason to use the allowance before a general investment account.
How much do ISA charges cost over time?
More than the headline percentage suggests, because the charge reduces the amount that compounds every year. On the example above, 0.4 per cent annually costs about £8,000 over twenty years — more than two years of contributions.
Can I have more than one ISA?
You can hold several ISAs across different types and different years. What is capped is how much you can pay in during a single tax year, and that allowance is shared across all types rather than applying to each one separately.
Does transferring an ISA use my allowance?
Not if it is done as a formal transfer between providers. Withdrawing the money and paying it back in does use your allowance again, unless the ISA is a flexible one that allows withdrawal and replacement within the same tax year.
Is a cash ISA safe?
The nominal value is protected, but the real value is not. Cash paying below the inflation rate loses purchasing power with certainty. Over the twenty-year horizons this calculator models, that erosion is a genuine risk rather than a safe default.
Related United Kingdom calculators
Tools people usually open in the same sitting.
Method, source and review date
- How it is worked out
- Compound growth with regular contributions, net of an annual charge
- Formula used
FV = P(1+r)ⁿ + M × ((1+r)ⁿ − 1) ÷ r, where r is the monthly return net of charges- Checked against
- HMRC guidance on Individual Savings Accounts
- Rates last verified
- Confidence
- Medium
Projections rest on an assumed return that nobody can guarantee. The annual ISA allowance and the rules on transfers and flexible ISAs are set by HMRC and change — check the current allowance before planning contributions.
Spotted a figure that has moved, or a formula you can show is wrong? Tell us — send a correction. Fixes to rates and formulas are treated as urgent.
Disclaimer
This is an estimate only and not financial advice. Lenders, providers and product terms vary, and the figures a provider quotes you will differ from a generic model. Speak to a qualified financial adviser before making a decision.