Compound Interest Calculator
See the power of compound interest and how your investments grow over time.
What Is Compound Interest?
Compound interest is interest earned on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which is calculated only on the principal, compounding causes your money to grow exponentially over time. Einstein reportedly called it the eighth wonder of the world. This calculator shows you the dramatic long-term impact of consistent investing with compound returns.
How to Use This Calculator
Enter Your Initial Investment
Input any lump sum you are starting with. Even $0 is fine if you are beginning from scratch with monthly contributions.
Set Monthly Contributions
Enter how much you will add each month. Consistent contributions are the engine of long-term wealth building.
Choose an Expected Return
The S&P 500 has averaged about 10% nominal (7% inflation-adjusted) annually since 1926. Use 7% for conservative estimates, 10% for nominal projections.
Set the Time Horizon
Longer periods amplify compounding dramatically. See how 20 years vs 30 years can double or triple your total returns.
Key Concepts
Rule of 72
Divide 72 by your annual return rate to estimate how many years it takes for your money to double. At 7% returns, money doubles roughly every 10.3 years.
Time Is the Key Variable
$500/month invested at 7% for 30 years becomes $567,000. For 20 years: $246,000. The last 10 years generate more than the first 20 combined.
Inflation Adjustment
For real purchasing power, subtract 2-3% (historical inflation average) from your expected return rate. A 10% nominal return is approximately 7% real.
Tax-Advantaged Accounts
Investing through 401(k)s, IRAs, and Roth accounts lets compounding work without annual tax drag on dividends, interest, and capital gains.
Expert Insights
Starting 10 years earlier with half the monthly contribution almost always beats starting later with double. Time is the most powerful variable in compound growth.
Market returns are not linear. You will experience years with -20% returns and years with +30%. Over 20+ year periods, the historical average holds remarkably steady around 7-10%.
Fees matter enormously over long periods. A 1% annual fee can reduce your ending balance by 20-25% over 30 years. Stick to low-cost index funds with expense ratios under 0.10%.
Frequently Asked Questions
This calculator provides estimates for educational purposes only. Actual results depend on your specific financial situation, lender terms, and market conditions. Consult a qualified financial advisor before making major financial decisions.
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Start Investing Today
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Did You Know?
The average credit card interest rate hit 22.76% in 2025 — the highest since tracking began in the early 1990s.
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