What Is the Rule of 72?

The Rule of 72 is a mathematical shortcut for estimating the time required to double an investment at a given compound annual growth rate. Simply divide 72 by the annual interest rate to get the approximate number of years to double your money. At 6% annual return: 72 / 6 = 12 years to double. At 8%: 72 / 8 = 9 years. At 12%: 72 / 12 = 6 years. The rule works because of the mathematical properties of compound growth — it is an approximation that is accurate within about 1% for interest rates between 6% and 10%.

Why 72?

The number 72 was chosen because it is divisible by many common interest rates — 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, and 36 — making mental math easy. The mathematically precise number for the rule would be 69.3 (the natural logarithm of 2, multiplied by 100), but 72 is a better practical approximation for typical interest rates because of its divisibility properties. Some financial educators use the "Rule of 69" or "Rule of 70" for continuous compounding, but 72 remains the most widely known and taught version.

Practical Applications

The Rule of 72 is useful for quickly evaluating investment options, understanding the impact of inflation, and communicating financial concepts to non-technical audiences. At 3% annual inflation, the Rule of 72 tells you that purchasing power halves in approximately 24 years — money saved today will buy roughly half as much in 24 years. At a 24% credit card APR, the debt doubles in 3 years if you make no payments. At a 7% stock market return, investments double approximately every 10 years.

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Our free Rule of 72 calculator at cookiescursor.com calculates doubling time from any interest rate, and reverse calculates the required rate from a target doubling time. It also shows tripling and 10x time at the same rate. No signup required.

Frequently Asked Questions

Is the Rule of 72 exact?
No. It is an approximation accurate to within about 1% for rates between 6% and 10%. At very low or very high rates, the error increases.

Can I use Rule of 72 for inflation?
Yes. Divide 72 by the inflation rate to find how many years until prices double or purchasing power halves.

What does the Rule of 72 tell you about credit card debt?
At 24% APR, 72/24 = 3 years to double. A $5,000 credit card balance with no payments becomes $10,000 in 3 years.

Does the Rule of 72 work for monthly rates?
Yes, but the result will be in months rather than years. Divide 72 by the monthly rate to get months to double.

What is the Rule of 115?
The Rule of 115 estimates tripling time: 115 / rate = years to triple. Similarly, the Rule of 144 estimates quadrupling time.

How does compound frequency affect the Rule of 72?
The Rule of 72 assumes annual compounding. With more frequent compounding (monthly, daily), your money actually doubles slightly faster than the Rule of 72 suggests.

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