ToolWrenCompound interest is often called the most powerful force in personal finance — it is how modest savings grow into large sums over decades, and also how debt can snowball. Understanding it is one of the highest-value things you can learn about money.
Simple vs compound interest
With simple interest, you earn a fixed amount on your original deposit each period. With compound interest, you earn interest on your deposit and on all the interest already added — so each period grows a little faster than the last. Compare the two side by side with our compound interest calculator and simple interest calculator.
A worked example
Put £1,000 in at 5% a year. With simple interest you earn £50 every year — £500 over 10 years, giving £1,500. With interest compounded annually, year one earns £50, but year two earns 5% of £1,050 (£52.50), and so on. After 10 years you have about £1,629 — roughly £129 more, purely from interest earning interest. Over 30 years the gap becomes dramatic.
Why compounding frequency matters
Interest can compound annually, monthly, or daily. The more often it compounds, the more you earn, because interest starts earning its own interest sooner. Monthly compounding beats annual; daily beats monthly — though the differences shrink at higher frequencies. Our compound interest calculator lets you set the frequency and see the effect instantly.
The role of time (and regular contributions)
Time is the biggest lever in compounding — starting five years earlier can outweigh contributing more later. Adding a regular monthly amount accelerates growth further, because every contribution starts compounding the moment it lands. Try projecting different timeframes and contribution amounts to see how your savings could grow.
It works against you too
The same maths applies to debt. Credit cards and loans compound interest on any unpaid balance, which is why balances can grow alarmingly fast. If you are borrowing, model the real cost first with our loan calculator, and budget repayments against your take-home pay using the salary calculator. This is general information, not financial advice — consult a qualified adviser for personal decisions.