UK Compound Interest Calculator

How to use the compound interest calculator

Enter how much you are starting with, how much you plan to add each month, your expected annual interest rate, how many years you are saving for, and how often the interest compounds. The calculator instantly shows your final balance, how much of that is your own money, and how much is interest earned on top.

This tool is useful for planning almost any long term savings goal: an emergency fund, a house deposit, or simply seeing what your current savings habit could grow into over time. It also pairs well with our savings goal calculator, which works backwards from a target amount to tell you how long it will take to get there.

What compound interest actually means

Simple interest pays you the same amount each year, based only on your original deposit. Compound interest is different. Once interest is added to your balance, it becomes part of the pot that earns the next round of interest. This means your money grows faster over time, especially over longer periods, because you are earning interest on your interest as well as on your original savings.

The compounding frequency you choose, annually, monthly or daily, affects how often this happens. More frequent compounding means interest starts working for you sooner, though the difference between monthly and daily compounding is usually small compared with the effect of the interest rate itself.

To put a number on that, take £5,000 saved with £200 added every month at 5% for 10 years. Compounding annually gives a final balance of around £39,143. Compounding monthly gives around £39,421. Compounding daily gives around £39,446. The gap between annual and daily compounding here is about £300, real money, but small next to the roughly £10,400 earned in interest overall. The interest rate you are offered matters far more than how often it compounds.

Why time matters more than almost anything else

Time is the most powerful ingredient in compound interest, more powerful than the interest rate in many real world cases. A lump sum of £10,000 earning 5% a year, with no further contributions at all, grows to around £26,533 after 20 years. Left for 30 years instead, it grows to around £43,219. That extra decade very nearly doubles the growth, simply because there is more time for interest to earn interest on itself.

This is why starting early, even with small amounts, tends to beat waiting until you can save more but starting later. A pound saved in your twenties has decades longer to compound than a pound saved in your forties.

Worked example

Say you start with £5,000 and add £200 a month, at an annual interest rate of 5%, compounding monthly, for 10 years.

Over that time you would pay in £29,000 in total (your £5,000 start plus £200 a month for 120 months). But your final balance would be closer to £39,400, meaning you earned around £10,400 in interest, roughly a third on top of what you paid in yourself. That extra money came entirely from compounding, not from your own contributions.

Try changing the term to 20 years in the calculator above with the same numbers. The balance grows to around £96,000, more than double the 10 year figure, even though the monthly contribution has not changed at all. That difference is entirely down to compounding having twice as long to work.

Common mistakes to avoid

A common mistake is assuming interest rates stay fixed for years at a time. Savings rates move with the wider economy, so treat any long term projection as a rough guide rather than a guarantee. It is also easy to forget tax: if your savings sit outside an ISA and your interest goes above your Personal Savings Allowance, some of that interest could be taxed, which this calculator does not account for. Finally, remember that regular contributions matter just as much as the interest rate. In the example above, stopping monthly contributions early would make a much bigger difference to your final balance than a small change in interest rate.

Another mistake worth avoiding is comparing two savings products purely on their headline rate without checking how often they compound and whether the rate is fixed or variable. A slightly lower rate that compounds monthly can sometimes beat a slightly higher rate that only compounds annually, though the gap is usually modest. Always read the small print on whether a rate is guaranteed for a set period or can change at any time, since a rate that drops part way through your savings term will change your real world result compared with this calculator's projection.

Once you know how your savings could grow, it is worth checking your take-home pay to see how much you can realistically afford to save each month, and our income tax guide if you want to understand how tax affects the money you are setting aside in the first place.

Related calculators

If you are saving towards a house, our mortgage calculator and mortgage deposit guide can help you work out how big a deposit you will need, and how a bigger deposit changes your monthly repayment. If you are carrying any debt alongside your savings, it is usually worth clearing higher interest debt first, since the interest rate on credit cards is normally far higher than anything a savings account will pay you.