Understanding Compound Interest
Compound interest is the interest you earn on both your original money and on the interest you have previously accumulated. Unlike simple interest, where you only earn on the principal, compound interest creates exponential growth — your money makes money, then that money makes more money.
The Compound Interest Formula
For a lump sum investment with periodic contributions, the future value is calculated as:
A = P(1 + r/n)nt + PMT × [( (1 + r/n)nt − 1) ÷ (r/n)]
Where A is the future value, P is your initial investment, PMT is your regular monthly contribution, r is the annual interest rate (as a decimal), n is how many times per year interest compounds, and t is the number of years.
Why It Matters
With an 8% average annual return, investing $500 per month starting with $5,000 will grow to over $339,000 in 20 years — with nearly $214,000 of that coming from compound interest alone. Time is the most powerful factor; starting even a few years earlier can mean tens of thousands of dollars more.
Common Questions
What is a good average return rate?
The S&P 500 has historically averaged around 10–11% annually before inflation. A conservative estimate for long-term planning is 7–8% after accounting for inflation.
How often should I compound?
For most investments, compounding frequency (monthly, daily, etc.) has a negligible effect on the final number compared to the interest rate and time. Monthly is the standard assumption for retirement planning.
Is this calculator financial advice?
No. This tool is for educational and estimation purposes only. Always consult a qualified financial advisor before making investment decisions.