See what a monthly amount grows into at your account's rate: total paid in, interest earned, and the year-by-year path. The rate is always your input.
Saving £11,500 over 5 years — £2,500 to start, then £150 a month — becomes £12,968.49 at 4% AER: £1,468.49 of it is interest. The calculator below runs the same arithmetic on your own figures, and the 4% is an example to overwrite with your account's real rate, because no rate printed on a page stays right for long.
Enter what you start with, what you pay in each month, and the rate your account actually pays. The rate shown is an example, not a recommendation.
Start £2,500.00, Year 5 £12,968.49 — rising by £10,468.49 across 6 points.
Paying in £150.00 a month from a £2,500.00 start reaches £12,968.49 after 5 years at 4% — £11,500.00 paid in and £1,468.49 from interest.
Deposits are added at the end of each month, after that month's interest. The rate here is yours to set, not a market rate: promotional and bonus rates that expire, withdrawal limits and provider changes are not modelled. A planning estimate, not financial advice.
| Tax/source year | 2026/27 |
|---|---|
| Region | UK-wide: savings interest is taxed on the UK bands and thresholds in every UK nation, including for Scottish taxpayers |
| Account rates | Never assumed. Any interest rate in a savings calculation is an input you set, stated beside the result. |
| Your other income | Wages and pension income decide how much Personal Allowance and starting rate are left for interest, and which Personal Savings Allowance band applies. |
|---|---|
| The interest itself | Interest counts towards the income that sets your band, so a large interest year can shrink the allowance that was meant to cover it. |
| The account's wrapper | The same balance at the same rate is taxed outside an ISA and untouched inside one; the wrapper, not the rate, decides whether the allowances are needed. |
| What is not decided here | PayBreakdown does not decide payroll correctness, lender approval, benefit entitlement or employer compliance, and does not give regulated financial, mortgage, debt, tax, payroll or legal advice. |
|---|
Three assumptions, all visible. Deposits are added at the end of each month, after that month's interest — so a deposit's first interest arrives the month after it does. Interest compounds monthly, at the monthly rate the AER implies: the twelfth root of one plus the AER, minus one, which is what AER means — the annual figure after monthly compounding has done its work. And the rate is treated as constant for the whole term, which no real account promises — that is exactly why the rate stays an input rather than a figure this page asserts.
An account paying interest monthly quotes two numbers that describe one account. The AER is the annual result after each month's interest starts earning its own; the gross rate is the smaller figure the monthly payments are actually worked out from. At 4% AER, the monthly rate is the twelfth root of one plus the AER, minus one — compound that twelve times and you land back on 4% exactly — and twelve of those monthly rates add up to the gross rate of about 3.93%. So when a statement shows a rate that looks short of the advertised figure, nothing is missing: compounding closes the gap by the year's end. This calculator takes the AER, because it is the number accounts advertise and the one that makes two accounts comparable.
Promotional and bonus rates that expire part-way through a term are not modelled — an account paying a boosted rate for twelve months then dropping is two projections, not one, and pretending otherwise would flatter the result. Withdrawal limits, notice periods and provider rate changes are not modelled either. Nor is tax: interest outside an ISA can be taxable once it outgrows your allowances, and the savings hub explains how those allowances stack before any tax is due.
Change any figure above and the result moves with it. A payslip can differ from any calculator because payroll works pay period by pay period, so a tax-code change, a bonus, arrears or a refund can land in one month and not the next. See how each deduction is worked out.
Because the balance is smaller. Interest is a percentage of what is already there, so early months earn little on a small balance while later months earn interest on everything deposited so far plus all the interest already added. That acceleration is the whole case for starting early, and the chart above shows it: each step after the first plots a year-end checkpoint, and the rise from one year to the next grows every year. Between checkpoints the balance is growing month by month; the chart samples it once a year.
The AER. It is the figure accounts advertise and the one this calculator compounds from. If you only have the gross monthly-paying rate, it sits slightly below the AER for the same account — the section above shows the relationship.
The projection reports interest before any tax. Inside an ISA it is tax-free without limit; outside one, allowances cover most people's interest in full, and the savings hub explains the order they apply in and when tax starts.
Last updated 2026-08-28. Checked by Sean Elsmore. Rates last checked against GOV.UK on 2026-08-07. Estimates are for planning and should be checked against official records where the decision matters.