How to use this calculator
Enter what you are starting with, how much you will add each month, the expected yearly return and how many years you plan to save. The calculator shows your final balance and how much of it comes from interest.
The compound interest formula
For a single deposit, the future value is:
A = P × (1 + r/n)^(n × t)
where P is the starting amount, r the yearly rate as a decimal, n the number of compounding periods per year and t the number of years. With monthly contributions, each deposit grows for the time remaining. This calculator simulates every month so any compounding frequency works correctly.
Why starting early matters
Time is the biggest driver of compound growth. Saving 100 a month at 8% for 10 years grows to about 19,000. Keep the same habit for 30 years and it grows to roughly 150,000, even though you only deposited 36,000. Most of that balance is interest earned on earlier interest.
The Rule of 72
A quick mental shortcut: divide 72 by your yearly interest rate to estimate how many years it takes to double your money. At 8%, money doubles in about 9 years. At 12%, about 6 years.