Years to Double
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Exact (compound)
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Doubled Amount
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When Should You Use This?
Use the Rule of 72 for quick mental math on any investment. At 8% return, your money doubles in 9 years. At 12%, it doubles in 6 years. This simple rule helps you evaluate opportunities instantly.
How It Works
1
Enter Return Rate
Input the expected annual return rate. The S&P 500 averages about 10%.
2
See the Result
Divide 72 by your return rate. At 6% return, money doubles in 12 years. At 10%, it doubles in 7.2 years.
3
Think in Doubles
If you're 30 with $10,000 at 10% return: double at 37 ($20K), double at 44 ($40K), double at 51 ($80K), double at 58 ($160K).
Frequently Asked Questions
72 is a convenient approximation because it's divisible by many numbers (2, 3, 4, 6, 8, 9, 12). The exact formula uses ln(2) = 0.693, but 72 is close enough and easier for mental math.
It's most accurate for rates between 6-10%. For very low rates (< 4%), use 70 instead. For very high rates (> 20%), use 78. The calculator also shows the exact compound answer.
Yes. Credit card debt at 18% doubles in 4 years (72/18). This shows why high-interest debt is so dangerous.
Absolutely. At 3% inflation, prices double in 24 years (72/3). This means $100 today buys only $50 worth of goods in 24 years.
69.3 (= ln(2) x 100) is mathematically exact for continuous compounding. 72 is used because it's easier to calculate mentally and close enough for annual compounding.