Cash ISA Calculator United Kingdom

This calculator projects what your cash ISA will be worth after a set number of years and shows, in pounds, what the ISA wrapper saves you compared with an ordinary savings account. Enter an opening balance, a monthly deposit, the AER your provider quotes and how long you plan to save. The tool compounds the account month by month and keeps within the annual ISA subscription allowance.

What a cash ISA is and why the wrapper matters

A cash ISA is a deposit account with a tax shelter around it. It earns interest like any savings account, but that interest never enters your taxable income: it is free of Income Tax, uses none of your Personal Savings Allowance and does not need to be declared. Withdrawals are tax free too, unlike a pension, where you get tax relief going in and pay tax coming out.

For some savers the wrapper makes little difference. A basic rate taxpayer with £1,000 of Personal Savings Allowance and a modest balance may earn all their interest tax free in an ordinary account. That changes as balances and rates rise and as your income moves you into a higher band. A higher rate taxpayer gets only £500 of allowance and an additional rate taxpayer none, so interest outside an ISA is taxed at 40% or 45%. The comparison in the results shows what that means for your figures.

How the cash ISA calculator works

The calculator converts the quoted AER into an equivalent monthly rate, so a 4.5% AER becomes about 0.3675% a month. Each month it credits interest to the balance, then adds your deposit at the end of the month, which is the cautious ordering. It repeats this for every month of the term.

In each tax year the tool caps what you can pay in at the remaining ISA allowance, so a large monthly deposit stops once the allowance is used. If you have already paid into an ISA this year, enter that in the advanced options and the first year’s headroom is reduced. The results show how much allowance is left, which is useful before 5 April, since unused allowance is lost rather than carried forward.

Alongside the ISA, the tool runs a shadow account holding the same money. At the end of each year it adds up that account’s interest plus any other savings interest you entered, subtracts your Personal Savings Allowance and any unused 0% starting rate for savings, and charges the rest at your marginal rate. The tax comes out of the shadow balance, so the cost compounds year after year. The difference between the two closing balances is the ISA advantage.

Reading the results

The main figure is the projected cash ISA balance. Below it, the tax-free interest earned separates growth from the money you paid in, and the total paid in is your cost base. The taxed comparison, the tax paid outside an ISA and the ISA advantage together tell you whether the ISA was worth using.

For a basic rate taxpayer with a small balance, the advantage is often a few pounds a year, because the Personal Savings Allowance covers most of the interest. On a six-figure balance held by a higher rate taxpayer, the advantage over a decade runs into thousands, because tax is taken every year on interest that would otherwise have compounded. The ISA advantage is larger than the tax paid for the same reason: money lost to tax in year one would have earned interest in every later year.

Flexible, fixed and easy-access accounts

Easy-access cash ISAs allow withdrawals at any time but usually pay a variable rate that can be cut. Fixed-rate cash ISAs lock the money away for one to five years for a higher guaranteed rate and usually charge an interest penalty for early access. Notice accounts sit between the two. Some providers offer a flexible ISA, which lets you withdraw money and put it back in the same tax year without the replacement counting against your allowance. Providers do not have to offer this, so check before you rely on it.

The projection uses one rate for the whole term, so for a variable-rate account run it twice: at the current rate and at a lower rate you see as a realistic floor. The gap between the two balances shows how much of the projected growth depends on the rate holding.

Transfers

To move ISA money from one provider to another, always use the transfer process rather than withdrawing the cash and paying it in again. Money withdrawn from an ISA loses its tax-free status immediately, and paying it back in uses fresh allowance. A transfer keeps the wrapper and does not touch your annual allowance. Money from previous tax years can be transferred in full or in part; money paid in during the current year must generally be transferred in full.

Getting the most from the allowance

Paying in early in the tax year gives the money twelve months of tax-free interest instead of one, and using the allowance each year matters because it does not roll forward. Each adult has their own allowance, and a Junior ISA gives each child a separate one. Where your savings are well above what the Personal Savings Allowance covers, moving them into an ISA is usually the cheapest improvement a cash saver can make. The government has announced that from April 2027 savers under 65 will be able to put at most £12,000 a year into cash ISAs, within the £20,000 overall allowance.

The figures are an estimate based on the rate, deposits and term you enter, and on ISA and tax rules that can change at any Budget. They are not personal financial advice. Check the terms of your account and speak to a regulated adviser before making a decision that matters.

About This Cash ISA Calculator

Use this cash ISA calculator to get an instant, accurate estimate. The figures follow official GOV.UK guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.