Compound interest calculator
ReadySee how an initial investment plus monthly contributions grows with monthly or yearly compounding, year by year, and how long it takes to double (rule of 72).
How to use
- Enter your initial investment and the amount you add at the end of each month.
- Set the annual return, the number of years and whether interest compounds monthly or yearly.
- You’ll see the final balance, total contributions, total gain and a year-by-year table.
Formula
- Monthly compounding: balance × (1 + annual rate ÷ 12) + contribution, every month
- Yearly compounding: start-of-year balance × (1 + annual rate) + this year’s contributions with simple interest for the months remaining
- Rule of 72: years to double ≈ 72 ÷ annual rate (%)
Worked example
Starting with £10,000 and adding £500 a month at 7% a year, compounded monthly, for 20 years: you contribute £130,000 and end up with about £300,851 before tax and fees. By the rule of 72 your money doubles in about 10.3 years.
FAQ
What’s the difference between monthly and yearly compounding?
With the same annual rate, monthly compounding adds interest more often, so it ends slightly higher. At 7% a year, monthly compounding is an effective 7.23% a year.
Are taxes, fees and inflation included?
No. The result is before tax, fund fees and inflation, and assumes the same return every year. Real returns vary from year to year.