See how a lump sum and monthly deposits grow with compound interest over time.
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What this calculator covers and how the numbers are worked out
This calculator projects how a pot of savings could grow over time when you combine a starting balance, regular monthly deposits and compound interest. It's designed for anyone saving into an easy-access account, a regular saver or a fixed-rate bond who wants a realistic sense of how their balance builds up, rather than a flat guess based on the interest rate alone.
The key idea is compound interest: rather than only ever earning interest on the amount you originally deposited, you earn interest on your growing balance - your deposit plus whatever interest has already been added. Each time interest is calculated, it gets added to the balance, and the next round of interest is worked out on that larger amount. This calculator compounds monthly, which is how most everyday savings accounts work.
As a worked example, imagine you open an account with £5,000 and add £200 every month, at an annual interest rate of 4%. Because interest compounds monthly, roughly a twelfth of that 4% is applied to your balance each month. After 12 months your balance would be around £7,648 - made up of your original £5,000, the £2,400 you've deposited over the year (12 x £200), and about £248 of interest earned on top.
That £248 of interest didn't come purely from your opening balance - part of it came from interest earned on the deposits you added throughout the year too, which is why compound interest grows faster the longer you leave money in an account.
Whether that interest is taxed depends on your Personal Savings Allowance. Basic-rate (20%) taxpayers can earn up to £1,000 of savings interest a year tax-free, higher-rate (40%) taxpayers can earn up to £500 tax-free, and additional-rate (45%) taxpayers get no Personal Savings Allowance at all, so all of their savings interest is potentially taxable. In the example above, £248 of interest would comfortably fit within either the basic-rate or higher-rate allowance.
If your savings are held in an ISA, none of this matters for tax purposes - interest earned inside an ISA is always tax-free, regardless of how much you earn or how much interest you receive, and it doesn't use up any of your Personal Savings Allowance.
Two things to bear in mind: the results here are shown in cash terms and don't account for inflation, so the spending power of your pot may grow more slowly than the number suggests. And interest rates on savings accounts are usually variable and can go up or down, so treat any projection as an illustration of how compounding works rather than a guarantee of what you'll actually end up with.
It's interest calculated on your whole balance - your original deposit plus any interest already added - rather than only on the amount you first put in. Because the balance interest is calculated on keeps growing, the pounds of interest you earn tend to increase over time even if the rate doesn't change.
It compounds monthly, which matches how most everyday savings accounts, including regular savers and many easy-access accounts, calculate and add interest.
It depends on your income. Basic-rate taxpayers have a £1,000 Personal Savings Allowance, higher-rate taxpayers have £500, and additional-rate taxpayers have none, so interest above your allowance is potentially taxable.
Interest earned inside an ISA is always tax-free, no matter how much you earn or how much interest builds up, and it doesn't count towards your Personal Savings Allowance at all.
No. The figures shown are in cash terms only, so while your balance may grow, what that money can actually buy in the future isn't factored in.
Not necessarily. Most savings accounts have variable rates that can rise or fall, so a long-term projection is illustrative rather than a guaranteed outcome - fixed-rate bonds are the exception, as the rate is locked for the term.
The calculator assumes a consistent monthly deposit for the period you choose, so if your real contributions vary or stop, treat the projection as an upper estimate and recalculate with lower or zero monthly deposits to see the effect.
Yes - just set the monthly deposit to zero and enter your starting balance and rate. The calculator will still compound the interest monthly on the lump sum alone.
These results are estimates for general guidance and are not financial advice. Always confirm important figures with an official source.
Rates last reviewed: 2 October 2026 · Built around the 2026/27 UK tax year
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