Savings Calculator
Rule of 72 Calculator
A quick mental-math shortcut for how long it takes an investment to double, next to the exact answer.
This is an estimate for education, not investment advice. Real returns vary year to year; this assumes a constant annual rate. Nothing you type is sent anywhere.
How the rule of 72 works
Divide 72 by the annual rate of return (as a whole number, not a decimal) to get roughly how many years it takes an investment to double. At 6%, that's 72 / 6 = 12 years. It's a mental-math approximation; the exact formula uses logarithms, shown alongside it here. The rule of 72 is reasonably accurate for rates between about 6% and 10%, and drifts a bit further from exact outside that range.
Frequently asked questions
Does this work for debt too?
Yes, the same math shows how fast debt doubles at a given interest rate if left unpaid, which is a useful way to see how costly high-rate debt can be.
Why use 72 instead of another number?
72 has many small divisors (1, 2, 3, 4, 6, 8, 9, 12...), which makes the mental division easy for common rates, and it happens to approximate the exact logarithmic answer well in the middle range.
Related: the savings calculator, the FIRE calculator and the debt payoff calculator.