Rule of 72 Calculator
A quick mental-math shortcut for compound growth. Enter a rate to see how long money takes to double (and triple), or a number of years to find the return you need, compared with the exact figure.
Rule of 72 estimate
| Rate | Rule of 72 | Exact years to double |
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How to use the Rule of 72
- Choose whether you know the rate or the number of years.
- Enter it, and the calculator divides 72 by it.
- Compare the shortcut with the exact answer in the results and the table.
Why the Rule of 72 works
Money growing at r% a year doubles in ln(2) ÷ ln(1 + r) years. For typical rates that is very close to 72 ÷ r, and 72 is easy to divide by 2, 3, 4, 6, 8, 9 and 12, so it became the standard shortcut. At 8% money doubles in about 9 years; at 6%, about 12.
The rule is most accurate between about 6% and 10%. For continuous compounding, 69.3 is exact; for very high rates, 72 understates the time. Similar rules estimate tripling (114) and quadrupling (144).
It works for anything that compounds: investment returns, debt growing at a credit card rate, or inflation eroding purchasing power. At 3% inflation, prices double in about 24 years.
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Frequently asked questions
What is the Rule of 72?
A shortcut: divide 72 by the annual percentage rate to estimate how many years an amount takes to double.
How accurate is it?
Within a few percent for rates between about 4% and 12% with annual compounding.
What is the Rule of 69?
The exact version for continuous compounding, since ln(2) ≈ 0.693.
Can I use it for debt?
Yes. A credit card balance at 24% doubles in about 3 years if unpaid.