Compound Interest Calculator
Project savings growth with regular contributions and compounding.
About the Compound Interest Calculator
Compound interest means earning interest on interest already earned. The difference from simple interest is small over a year and enormous over decades: £10,000 at 7 percent simple becomes £31,000 over 30 years, and compounded it becomes £76,000.
Compounding frequency matters less than people expect. Moving from annual to monthly compounding at 7 percent adds about a fifth of a percentage point to the effective rate; moving from monthly to daily adds almost nothing. The rate and the time horizon are what dominate.
Regular contributions usually matter more than the rate. Over 30 years, adding £200 a month to a £10,000 starting balance contributes far more to the final figure than an extra percentage point of return, and unlike the rate it is something you control.
How it works
Enter your starting balance, what you add each month, the expected annual return and the term.
Choose how often interest compounds — monthly is typical for savings, annually for bonds.
See the final balance split into contributions and growth, year by year.
Frequently asked questions
- What is the difference between simple and compound interest?
- Simple interest is calculated only on the original amount; compound interest is calculated on the balance including interest already earned. Over 30 years at 7 percent, the compounded total is more than double the simple one.
- How often should interest compound?
- More often is better, but the gain shrinks fast. Annual to monthly at 7 percent adds about 0.2 percentage points of effective return; monthly to daily adds a rounding error. The rate and the number of years dominate everything else.
- What is the rule of 72?
- Divide 72 by the annual percentage return to estimate the years until money doubles — 72 divided by 8 is 9 years. It is an approximation that holds well between about 4 and 12 percent and is accurate enough for mental arithmetic.
- Do these figures account for inflation?
- No, they are nominal. At 3 percent inflation, money loses about half its purchasing power over 24 years, so subtract your inflation assumption from the return to see the figure in today's money.
- Is it better to invest a lump sum or contribute monthly?
- A lump sum wins on average, because more money is invested for longer. Monthly contributions reduce the risk of investing everything just before a fall, which is why most people choose them despite the lower expected outcome.
Privacy
Everything happens locally. Your files are read by your own browser, processed on your device, and never uploaded — closing the tab is all it takes to erase them.
Related Calculator tools
Loan & EMI Calculator
Work out monthly payments, total interest and a full amortisation schedule.
Tax Calculator
Estimate income tax and take-home pay from a gross salary, band by band.
BMI Calculator
Calculate body mass index in metric or imperial units, with the healthy range.
Calorie Calculator
Estimate daily calorie needs from BMR and activity level, with macro splits.