How Do ISA Calculators Work? Reading the Returns They Show
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An ISA calculator works out a projected balance from four inputs, a starting balance, a regular payment, an assumed rate of return and a number of years, and applies compound growth to them for the 2026-27 tax year. The return it shows is the assumed rate carried forward, not a measured result. Reading it well means checking whether the rate is before or after charges and whether the answer is in today's money.
The site's rules data was last checked against gov.uk on 26 September 2026. Figures typed in this guide may not yet reflect a later change, and the descriptions of gov.uk pages are summaries that may also lag. The current figures are on the ISA rules at a glance page. General information only, not financial advice.
How does an ISA calculator work?
Most ISA calculators use the same core inputs to project growth:
- Starting balance. Any lump sum already held.
- Regular payment. The amount added each month or year.
- Time horizon. The number of years of saving or investing.
- Assumed rate of return. An interest rate for a Cash ISA, or an estimated annual growth rate for a Stocks and Shares ISA.
The calculator applies compound growth, so returns themselves start earning returns, and projects a final balance. Some also show a chart comparing the projected balance with a scenario that has slightly higher payments, so the effect of a small change is visible. Payments across all ISAs in a tax year are limited: gov.uk says that in the 2026-27 tax year the maximum you can save in ISAs is £20,000.
How is the return worked out?
The return is the rate multiplied by the balance, repeated for each period. How often the growth is added matters a little. At an assumed 4% a year on £10,000, adding interest once a year gives £400 in the first year. Adding one twelfth of 4% every month gives about £407, because later months earn on earlier months' interest. The two figures are the same account described two ways: 4% is the gross rate, and the monthly version is equivalent to an annual rate of about 4.07%, usually called the AER.
This is why the label on a rate matters. A figure given as AER already includes the effect of compounding, while a gross rate does not. The site's Cash ISA calculator shows which method it uses.
How do Cash ISA and Stocks and Shares ISA calculations differ?
The sum is the same for both ISA types. The assumptions behind it differ:
- Cash ISA. The rate is set by the provider. On a fixed-rate account it is known for the term, and on a variable-rate account it can change, so the projection is only as good as the rate entered.
- Stocks and Shares ISA. The rate is an assumed average annual return. Actual returns vary from year to year and are never guaranteed, so the projection is an estimate, not a promise.
The Stocks and Shares ISA calculator takes any assumed return. gov.uk says you do not need to declare any ISA interest, income or capital gains on a tax return, so neither calculation takes tax off the growth.
A worked example: reading one result three ways
Take an assumed starting balance of £8,000 and £250 paid in at the end of every month for 10 years, with growth worked out monthly. The total paid in is £38,000, the £8,000 start plus 120 payments of £250. The rates are assumptions, not forecasts.
| Assumed 3% a year | Assumed 5% a year | |
|---|---|---|
| Projected balance after 10 years | about £45,700 | about £52,000 |
| Of which assumed growth | about £7,700 | about £14,000 |
| The same balance in today's money, at an assumed 2% a year of inflation | about £37,500 | about £42,700 |
The first row is the nominal figure, the amount the calculator displays. The third row is a real figure, the nominal balance divided by the assumed price rise over 10 years. In the 3% column the nominal balance is above the money paid in, but in today's money it is about the same as the £38,000 paid in, which shows why nominal and real figures can tell different stories.
Charges also change the reading. If the 5% column is a gross rate and an annual charge of 0.5% comes off, using 4.5% instead gives a balance of about £50,300, about £1,700 lower. A calculator that has no charges field gives the gross figure unless a net rate is entered.
What affects how accurate the result is?
- The assumed rate. The result changes with it, as the table above shows. Historical averages do not guarantee future performance, so a lower rate is a way to see a less favourable case.
- Charges. Platform and fund charges reduce the real-world return. Entering a rate net of charges is one way to allow for them.
- Inflation. A projected balance is worth less in real terms by the time it is reached.
- Changes to the plan. A calculator assumes steady payments. Missed payments or early withdrawals change the outcome.
- Rule changes. gov.uk sets the allowance for each tax year. A calculator uses the rules as entered.
Can calculators help with the allowance as well as growth?
Beyond growth projections, some calculators address the allowance itself. The ISA Allowance Checker shows the monthly amount needed to use the full allowance before the tax year ends. The ISA Contribution Split Planner shows how contributions could be divided between Cash and Stocks and Shares ISAs. gov.uk says you can split the allowance across multiple accounts.
To compare different approaches side by side, for example all cash against a mix, or different time horizons, the Advanced ISA Planner models several scenarios at once rather than one calculation at a time. The ISA Calculator is free and runs a single set of numbers.
What to check when reading a returns figure
- Is the rate a gross rate, an AER or a figure after charges?
- Is the result in nominal terms or adjusted for inflation?
- Is the rate an offered interest rate or an assumed investment return?
- How many scenarios have been run, and how far apart are the rates?
- Do the payments stay within the yearly allowance?
An ISA calculator is a planning tool, not a guarantee. It shows what is possible under a given set of assumptions, so that changes to payments or time can be compared.
Frequently asked questions
How accurate are ISA calculator projections?
A projection is only as accurate as its assumed rate. A fixed Cash ISA rate is known for its term, but a variable rate can change, and a Stocks and Shares ISA projection is an estimate because investment returns vary and are never guaranteed.
Do ISA calculators account for fees?
Many basic calculators do not unless they have a charges field. Entering a rate after charges is one way to allow for them. In this guide's example, using 4.5% instead of 5% lowers a 10-year result by about £1,700.
Can an ISA calculator help with the allowance as well as growth?
Yes. The site's ISA Allowance Checker and Contribution Split Planner are built for allowance questions, separate from growth projections.
What is the difference between a nominal and a real return?
A nominal figure is the amount the calculator displays. A real figure adjusts it for price rises. In this guide's example, a nominal balance of about £45,700 is about £37,500 in today's money at an assumed 2% a year of inflation.
This guide does not constitute financial advice. Your personal situation, tax position, and savings goals may mean different strategies are appropriate. Consider speaking to a regulated financial adviser if you are uncertain.
