Compound Interest Calculator

How to Use the Compound Interest Calculator

Enter your initial investment, annual interest rate, and time period. Optionally add monthly contributions and select your compounding frequency. The calculator shows your future balance, total interest earned, and a visual breakdown of contributions versus earnings.

Understanding the Results

The green bar in the results shows what portion of your final balance came from interest earnings versus your actual contributions. The larger the green section, the more compound interest worked in your favor. This visual clearly demonstrates why starting early and investing consistently is so powerful.

Compound Interest Formula

The core formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual rate as a decimal, n is how many times interest compounds per year, and t is the number of years. For $10,000 at 7% compounded monthly for 10 years: 10000 × (1 + 0.07/12)^(12×10) ≈ $20,097. When you add regular deposits, a future-value-of-an-annuity term is layered on top to compound each contribution from the date it is made.

Why Compounding Frequency and Time Matter

Two levers make compounding powerful: how often interest is added, and how long it runs. More frequent compounding helps a little — daily beats annual — because interest starts earning its own interest sooner. But time is the dominant force. The curve is exponential, not linear, so the last decade of a 30-year horizon adds far more than the first. This is why financial advisors stress starting early: a saver who begins at 25 can end up ahead of one who invests twice as much starting at 40.