Investing BasicsCalculator
Rule of 72 calculator
Estimate how many years it takes money to double at a given yearly rate, and compare the rule of 72 with the exact answer.
Rule of 72 calculator
Result
- Years to double (rule of 72)
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- Years to double (exact)
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- Difference
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Quick answer
The rule of 72 estimates how long money takes to double: divide 72 by the yearly rate in percent. At 6% that gives 12 years; the exact answer is about 11.9. Enter a rate to compare the shortcut with the exact figure.
What is the rule of 72?#
Investor.gov describes it simply: divide 72 by your investment's expected rate of return[1]. Its example is a 9% return, which doubles money about every 8 years. The Federal Reserve Bank of St. Louis gives a lower-rate example: at 2%, doubling takes about 36 years[2].
How accurate is the shortcut?#
The exact doubling time is ln(2) ÷ ln(1 + rate). The rule of 72 is very close for rates between roughly 6% and 10% and drifts further away at very low or very high rates.
Worked example
Rule of 72 vs exact doubling time
Each row compares 72 ÷ rate with the exact formula, rounded to two decimals.
| Yearly rate | Rule of 72 | Exact |
|---|---|---|
| 2% | 36.00 years | 35.00 years |
| 4% | 18.00 years | 17.67 years |
| 6% | 12.00 years | 11.90 years |
| 8% | 9.00 years | 9.01 years |
| 10% | 7.20 years | 7.27 years |
| 12% | 6.00 years | 6.12 years |
Figures computed in code from the stated inputs; rounded to the nearest cent or tenth.
What can you use it for?#
- Comparing rates quickly: at 3% money doubles in about 24 years; at 6%, about 12.
- Seeing the cost of fees: cutting a return from 6% to 5% stretches doubling from about 12 to about 14.4 years — see fee drag.
- Understanding inflation: at 3% inflation, prices roughly double in 24 years — see how inflation affects investments.
For full balances, including monthly additions, use the compound growth calculator.
What mistakes do beginners make?#
Using it for very high or very low rates
The shortcut drifts away from the exact answer at rates far from about 8%. Check the exact column before relying on it.
Plugging in a hoped-for return
The rule only turns a rate into a doubling time. If the rate is optimistic, the doubling time is too.
Forgetting fees and inflation
Use the rate after fees, and remember that doubling in dollars is not doubling in buying power.
What else do beginners ask?#
Does the rule of 72 work for investments that go up and down?
Only as a rough guide to an average rate. Real returns vary year to year, so actual doubling time can be much shorter or longer.
Why 72 and not 70?
72 is easy to divide by many common rates (2, 3, 4, 6, 8, 9, 12). Some people use 70 or 69.3 for low rates; the exact formula avoids the choice.
Sources
Numbers in brackets in the text point here. Grade A = primary source (regulator, statistics agency, law or official document).
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