How to Use an ISA Calculator: Inputs, Scenarios and Limits

To use an ISA calculator, a person enters a starting balance, a regular payment, an assumed rate and a number of years, then reads the result as an illustration for the 2026-27 tax year, not a forecast. Running a lower, a middle and a higher rate side by side shows how far the answer depends on the assumption. Results leave out charges, inflation and provider terms unless those are entered.

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.

What does an ISA calculator do?

An ISA calculator estimates how an Individual Savings Account could grow over time from the inputs entered: an initial deposit, monthly or yearly payments, an assumed interest rate or investment return, and the number of years. It makes compound growth visible, because the difference between saving £200 a month and £400 a month over ten or twenty years is hard to see without working it out.

A calculator built for UK ISAs can also reflect UK rules. gov.uk says that in the 2026-27 tax year the maximum you can save in ISAs is £20,000, and that the tax year runs from 6 April to 5 April. It also says you do not need to declare any ISA interest, income or capital gains on a tax return, so the growth in an ISA calculation is not reduced by tax.

Which ISA calculator fits which account?

Cash ISA calculator. For a deposit-based account. Inputs are the balance, regular payments and an annual rate, often given as AER (annual equivalent rate). It can compare, for example, two accounts side by side. At two assumed AERs of 4.5% and 3.1% on £10,000 left for five years with annual compounding, the balances come to about £12,460 and about £11,650, a gap of about £800. The rates are assumptions, not offers. The Cash ISA calculator does this.
Stocks and Shares ISA calculator. The same method with an assumed annual return rather than a set interest rate. Actual returns vary and are never guaranteed, and values can fall as well as rise. A difference of a couple of percentage points in the assumed rate, over 25 years, can change the result by a large amount. The Stocks and Shares ISA calculator takes any rate entered.
Lifetime ISA calculator. gov.uk says the government will add a 25% bonus up to a maximum of £1,000 per year on savings, and that you can put in up to £4,000 each year (GOV.UK Lifetime ISA guide). The Lifetime ISA calculator includes the bonus, so the result can be read next to a calculation without one.

What information is needed before starting?

Gathering the figures first avoids guessing. A calculation needs:

the current ISA balance, if any;
the amount that can be paid in each month or year;
the current interest rate, or the return being assumed;
the number of years of saving or investing;
whether payments will use all of the £20,000 allowance or only part of it.

A guessed input produces a less reliable output, whether it is a payment that cannot be kept up or a return rate that is set too high.

How are scenarios compared?

One result is a single answer to a single set of assumptions. Three versions of the same calculation show a range.

Lower. A lower rate and lower payments.
Middle. The inputs the person expects to use.
Higher. A higher rate and larger payments.

This produces a range rather than one figure. The rates are assumptions, not forecasts, and actual returns can be lower than the lowest scenario. Circumstances change, so a calculator that allows payments to vary over time can show, for example, a rise in three years or a pause during a break from work. The Advanced ISA Planner, a one-off purchase of £14.99, is built for several years and several ISA types at once.

A worked example

Sarah, a hypothetical saver, opens a Stocks and Shares ISA with £2,500 and pays in £300 at the end of each month. The assumed return is 6% a year, worked out monthly at one twelfth of 6%, over 20 years, with no charges. The rate is an assumption, not a forecast.

	£300 a month	£400 a month
Total paid in (including the start)	£74,500	£98,500
Projected balance after 20 years	about £147,000	about £193,000
Of which assumed growth	about £72,000	about £95,000

The comparison shows how much a higher monthly payment changes the projected result on the same assumptions. It does not say which amount suits any person. If Sarah considered a Lifetime ISA, the calculator would add the government bonus described above, within the £4,000 yearly limit on payments into one. The Lifetime ISA also has conditions. gov.uk says you must be 18 or over but under 40 to open one, and that a 25% charge applies to withdrawals for any reason other than those it lists, which include buying a first home (GOV.UK).

What do calculator results often leave out?

A rate set too high. Entering a very high return gives an attractive figure that rarely holds up across long periods.
Inflation. Projections are usually in nominal terms. £150,000 in 20 years buys less than £150,000 today.
Charges. For Stocks and Shares ISAs, platform fees and fund charges reduce the return. Some calculators allow a net rate, after charges, to be entered.
No guarantees. Projections are estimates, not promises, especially for investments.
The allowance. If planned payments exceed £20,000 a year, a UK-specific calculator can show it, while a generic savings calculator may not.
Rule changes. The allowance and Lifetime ISA bonus are set by the government and can change. gov.uk is the place to check the current rules.
Provider terms. Early access penalties on fixed-rate accounts, platform charges and eligibility are outside a standard calculator. gov.uk says you can take your money out of an ISA at any time without losing any tax benefits, but its page also says to check the terms of your ISA for rules or charges.

How can allowance use and ISA types be modelled together?

The ISA Allowance Checker shows how much of the £20,000 allowance has been used and how much remains in the current tax year. gov.uk says you can split the allowance across multiple accounts, so the allowance can be divided between ISA types in any combination, for example an amount in a Cash ISA and the remainder in a Stocks and Shares ISA. One way to see the combined picture is to run a separate calculation for each and add the results. The ISA Contribution Split Planner compares splits between account types.

A longer horizon, such as 30 years, can also be run with lower assumed rates to see the result across a long period. The ISA income calculator shows the arithmetic of taking money out afterwards.

What to check when using an ISA calculator

Before calculating:

the current ISA balance or balances;
the monthly payment that can be kept up;
the current interest rate, or the return assumed;
the goal, such as a home purchase, retirement or an emergency fund;
which ISA type or types the calculation covers.

When calculating:

whether a lower, a middle and a higher scenario have been run;
whether the return rate is before or after platform and fund charges;
whether payments stay within the £20,000 yearly allowance;
for a Lifetime ISA, whether the 25% government bonus is included.

Questions to consider afterwards:

Is the payment one that could be kept up?
Does the projection allow for likely changes, such as a career break, a home purchase or dependants?
Does the ISA type match the goal, and what are its rules and restrictions?
Do the calculator's assumptions match the current rules on gov.uk?

A calculator result is an illustration based on the inputs entered, not a forecast, and it changes as circumstances change. A regular review, for example at the start of each tax year, which begins on 6 April, keeps the inputs current.

Frequently asked questions

What do I need before using an ISA calculator?

A starting balance, the amount to pay in each month or year, an interest rate or assumed return, and the number of years. A calculation also needs to know which ISA type or types it covers.

Why run more than one scenario in an ISA calculator?

One result answers one set of assumptions. A lower, a middle and a higher version show a range, and actual returns can be lower than the lowest scenario.

When is it worth running an ISA calculation again?

When the interest rate, the monthly payment, the goal or the person's circumstances change, and at the start of each tax year. The results change with the inputs.

Can two ISA types go into one calculation?

The allowance can be split across ISA types, so a combined picture can come from running a separate calculation for each type and adding the results. The Advanced ISA Planner models several types and years together.

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.