Compound Interest Calculator — Calculate Investment Growth Over Time
Free compound interest calculator with monthly contributions. See how your investment grows over 5 10 20 or 30 years with the power of compounding.
Albert Einstein reportedly called compound interest the eighth wonder of the world. When your investment earns interest and that interest earns more interest your money grows exponentially rather than linearly. This is why starting to invest early is so powerful. A person who invests $500 per month from age 25 will have significantly more at 65 than someone investing $1000 per month from age 35 despite investing less total money. Use our calculator to see the dramatic difference time makes.
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Compound Interest Calculator
Key Information
| Parameter | Details |
|---|---|
| S&P 500 Average Return | 10.5% annual (historical) |
| Savings Account Rate | 4.5% - 5% (2026) |
| Bond Fund Average | 4% - 6% annual |
| Rule of 72 | Divide 72 by return rate = years to double |
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Use Calculator NowFrequently Asked Questions
How does compound interest work?
Compound interest means you earn interest on your original investment plus on all the interest that has already been added. With simple interest you only earn on the original amount. For example $10000 at 10% simple interest earns $1000 per year. With compound interest year 1 earns $1000 but year 2 earns $1100 because you are earning 10% on $11000 not just $10000.
What is the Rule of 72?
The Rule of 72 is a quick way to estimate how long it takes for an investment to double. Divide 72 by the annual interest rate. At 6% your money doubles in about 12 years. At 10% it doubles in about 7.2 years. At 12% it doubles in just 6 years. This simple rule helps you compare different investment options quickly.
Should I choose monthly or annual compounding?
Monthly compounding gives slightly higher returns than annual compounding because interest is calculated and added more frequently. For example $10000 at 10% for 5 years gives $16105 with annual compounding but $16453 with monthly compounding. The difference grows larger with higher amounts and longer time periods.
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Last updated: 24 March 2026