Rule of 72

What Is Rule of 72?

The Rule of 72 is a mental-math shortcut for estimating how long a sum takes to double at a given annual rate of compound growth. Dividing 72 by the annual interest rate, expressed as a whole number, yields an approximate number of years to double. At about 10 percent a year, for example, 72 divided by 10 gives roughly 7.2 years. The rule is an approximation, not an exact formula; it works best for moderate rates and assumes returns compound steadily. It can also be run in reverse, dividing 72 by a target number of years to estimate the rate needed. The underlying reason it works is that compound growth is exponential, and 72 happens to be a convenient, easily divisible stand-in.

Why It Matters

The Rule of 72 turns the abstract idea of compounding into a quick, intuitive estimate that needs no calculator. Because it links a rate directly to a doubling time, it makes the power of small differences in interest visible: a rate twice as high roughly halves the time to double. The same shortcut illuminates the cost side of borrowing, since a debt left to compound also doubles, and a higher APR shortens how long that takes. It is widely used in financial education precisely because it is memorable and highlights how time and rate interact. As an estimate it drifts from exact figures at very high or very low rates, so it illustrates a relationship rather than producing a precise projection.

Test Your Knowledge

Questions on this topic from the EconRecall fact bank:

• What is the "Rule of 72" used to quickly estimate?
  Roughly how many years it takes an investment to double at a given annual interest rate
• Using the Rule of 72, approximately how many years would it take to double an investment earning about 10% annually?
  About 7.2 years