The Rule of 72 tells you how long an investment takes to double: divide 72 by the annual interest rate.
TL;DR: Years to double = 72 ÷ rate. At 8 percent, that is 72 ÷ 8 = 9 years. The Rule of 72 calculator handles both directions of the formula.
The two ways to use it
The forward version gives doubling time. Money growing at 8 percent a year doubles in about 9 years, because 72 ÷ 8 = 9. At 6 percent it takes 12 years.
You can flip the formula to find the rate you need. If you want to double your money in 6 years, divide 72 by 6 to get 12 percent. So you would need a 12 percent annual return to hit that goal.
The Rule of 72 calculator does both, so you can type in either a rate or a target number of years.
How accurate it is
The rule is an approximation, and it is most accurate in the 6 to 10 percent range. At 8 percent the rule says 9 years, and the exact compound figure is about 9.01 years. Very close.
The gap grows at higher rates. At 24 percent the rule says 3 years, but the exact doubling time is closer to 3.22 years. Above 20 percent, treat the rule as a rough sketch and run the precise compound interest math when the number matters.
Inflation and the Rule of 70
The same trick estimates how inflation erodes money. Divide 72 by the inflation rate to find the years until buying power halves. At 3 percent inflation, 72 ÷ 3 is 24 years for a dollar to lose half its value.
You may also see the Rule of 70. It works the same way, just with 70 instead of 72. The number 70 is slightly more accurate at low rates, while 72 divides cleanly by more numbers (2, 3, 4, 6, 8, 9, 12), which is why it stays popular for mental math. This guide is informational and not financial advice.