Free Rule of 72 Calculator
Use the Rule of 72 to estimate how long an investment takes to double at a given annual rate, or the rate needed to double in a set number of years.
At 8% a year, a value roughly doubles every 9.0 years.
The Rule of 72 is an approximation. For the exact figure, use the CAGR calculator.
What the Rule of 72 is
The Rule of 72 is a shortcut for thinking about compounding without a calculator. Divide 72 by an annual growth rate and you get roughly the number of years it takes for a value to double. At 8% a year, money doubles in about nine years; at 12%, in about six. The same arithmetic runs backwards: divide 72 by a number of years to find the rate required to double in that time.
How to use this calculator
- Use Years to double to turn an annual rate into a doubling time.
- Use Rate to double to find the rate needed to double in a target number of years.
- Treat the answer as an estimate. For the exact figure, use a compound growth calculation.
Why doubling time matters
Compounding is the engine of long-term returns, and doubling time makes a rate feel concrete. The gap between an 8% and a 12% compounder looks minor over one year, but the Rule of 72 reframes it as nine years to double versus six. Over a few decades that is the difference between a handful of doublings and many, which is most of why small differences in rate produce enormous differences in outcome. The rule also works for inflation: 72 divided by the inflation rate is roughly how long until prices double and purchasing power halves.
How accurate it is
The Rule of 72 is closest to exact for rates in the range of about 5% to 12%, which covers most long-term investing assumptions. At very high or very low rates it drifts from the precise answer. When the difference matters, compute the exact doubling time from the compound growth formula rather than the shortcut.
Related calculators
- CAGR calculator — compute the exact compound growth rate and doubling time between two values.
- DCF calculator — put a growth rate to work inside a full intrinsic value model.
Frequently asked questions
- What is the Rule of 72?
- The Rule of 72 is a mental-math shortcut for compounding. Divide 72 by the annual growth rate and you get the approximate number of years it takes for a value to double. At 8 percent a year, money doubles in about 9 years; at 6 percent, about 12 years. It works in reverse too: 72 divided by the years gives the rate needed to double.
- How accurate is the Rule of 72?
- It is an approximation, and a good one for rates in the range of roughly 5 to 12 percent. At very high or very low rates it drifts from the exact figure. When precision matters, compute the exact doubling time with a compound growth formula rather than the shortcut.
- Why does doubling time matter for investing?
- Compounding is the engine of long-term returns, and doubling time makes the rate tangible. The gap between an 8 percent and a 12 percent compounder looks small in a single year, but the Rule of 72 shows it as 9 years to double versus 6, which over decades is the difference between a few doublings and many.
- Can the Rule of 72 be used for inflation?
- Yes. Dividing 72 by an inflation rate estimates how long it takes for prices to double, or equivalently for purchasing power to halve. At 3 percent inflation, prices double in about 24 years. It is a quick way to see why even modest inflation matters over a long horizon.
This calculator is for education and research only. It is not investment advice and it does not recommend buying or selling any security. The output depends entirely on the assumptions you enter.