Monthly Compounding
Compound Interest Investment Calculator
Model long-term investment returns using compound interest. The S&P 500 has historically returned approximately 10% annually before inflation. Enter your starting amount, monthly contributions, and expected rate to see your projected balance.
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Final Balance
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after monthly compounding
Total Contributions
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Interest Earned
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Exponential Growth
Year-by-Year Growth
| Period | Balance | Total Contributions | Interest This Year | Total Interest |
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Monthly compound interest explained
Investing $10,000 in an index fund returning 10% annually, with $500 monthly contributions, grows to approximately $1,057,000 over 30 years — with over $887,000 coming from compound growth. This is the mathematical foundation of long-term investing.