Compound Interest Calculator
See exactly how your money grows over time with compounding — the most powerful force in personal finance.
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Why compound interest is called the eighth wonder of the world
Compound interest means you earn interest on your interest — not just your principal. A $10,000 investment growing at 7% annually doubles to $20,000 in about 10 years, and reaches $40,000 in 20 years. The key insight: time matters more than the amount you invest.
Adding regular monthly contributions dramatically accelerates growth. Contributing $200/month to that $10,000 over 20 years at 7% yields over $150,000 — roughly 4x what the same money would be without compounding.
Frequently Asked Questions
What is the Rule of 72?
The Rule of 72 lets you estimate how long to double your money: divide 72 by the annual interest rate. At 7% return, your money doubles in ~10.3 years. At 6%, it takes ~12 years.
What is a realistic long-term stock market return?
The S&P 500 has historically returned approximately 10% annually before inflation, or about 7% inflation-adjusted. Individual results vary significantly year to year.