Compound Interest Calculator
Project the future value of an investment with compound interest plus optional monthly contributions. See growth year by year.
About this tool
This compound interest calculator projects the future value of an investment including the effect of regular monthly contributions. Enter an initial amount, a monthly addition, an expected annual return and a horizon in years — the result shows the final balance and how much of it came from interest.
It is the fastest way to make the power of compounding visible: modest monthly sums at 8% become six-figure balances over 20–30 years, and the interest portion grows to dominate contributions.
FAQ
What is the compound interest formula?
Future value = P x (1 + r/n)^(n x t), where r is the annual rate, n the compounding frequency and t the years. Monthly contributions are added to the balance every period.
How fast does money double?
Use the rule of 72: divide 72 by the annual return. At 8% money doubles roughly every 9 years.
Is the return guaranteed?
No. Stock-market returns are volatile and average 7–10% historically. This tool illustrates compounding, not a promise.