What Is Compound Interest?
Compound interest is interest earned on both your original investment and the interest that has already accumulated. Unlike simple interest, which only applies to the initial principal, compound interest creates exponential growth — your money earns interest on interest, accelerating wealth building over time.
The compound interest formula is A = P(1 + r/n)nt, where P is the principal (starting amount), r is the annual interest rate, n is the number of times interest compounds per year, and t is the number of years. This calculator applies this formula automatically with your inputs.
How Does Compound Interest Work? (Example)
Compound interest works by adding earned interest back to your balance, so each new interest calculation uses a larger amount. For example, $10,000 invested at 7% compounded monthly grows to $40,387 after 20 years — even without adding any extra money. You earn $30,387 in interest alone.
If you also contribute $500 per month, the same investment grows to approximately $301,706 after 20 years. Of that total, $130,000 comes from your contributions and $161,706 is pure compound interest earned. This demonstrates why consistent contributions combined with compound interest are the most powerful wealth-building strategy.
Compound Interest vs. Simple Interest: What's the Difference?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all previously earned interest. Over long periods, the difference is dramatic.
For example, $10,000 at 7% simple interest earns a flat $700 every year — $14,000 in interest over 20 years, totaling $24,000. The same investment with compound interest (monthly compounding) earns $30,387 in interest, totaling $40,387. That's $16,387 more, simply from interest compounding on itself.
How Often Should Interest Compound?
More frequent compounding produces higher returns, but the difference diminishes as frequency increases. Daily compounding earns slightly more than monthly, which earns more than quarterly or annually.
For $10,000 at 7% over 20 years: daily compounding yields $40,648, monthly yields $40,387, quarterly yields $39,927, and annually yields $38,697. The difference between daily and monthly is only $261 over 20 years — so for most practical purposes, monthly compounding captures nearly all the benefit.
How to Use This Compound Interest Calculator
Enter your initial investment, monthly contribution, annual interest rate, investment period, and compounding frequency — then click "Calculate Growth" to see your results instantly.
The calculator shows three key results: your total future value, total interest earned, and total amount contributed. Below, an interactive chart visualizes growth over time (switch between area and bar views), a donut chart breaks down the proportion of principal, contributions, and interest, and a year-by-year table shows exactly how your investment grows each year.
Disclaimer: This calculator is for educational and informational purposes only. It does not constitute financial advice. Results are estimates based on the inputs provided and assume a constant interest rate. Actual investment returns may vary. Consult a qualified financial advisor before making investment decisions.