Rule of 72 Calculator
Quickly estimate how long it takes an investment to double at a given interest rate, or what rate is needed to double it by a target year — with the exact compound-interest answer alongside.
From interest rate → years to double
Rule of 72 estimate: years
Exact (compound interest): years
From target years → required rate
Rule of 72 estimate: %
Exact (compound interest): %
How it works
The Rule of 72 is a quick mental-math shortcut: divide 72 by the annual interest rate to estimate years to double (or divide 72 by years to estimate the rate needed). It's most accurate for rates between roughly 6% and 10% — the exact compound-interest answer is shown alongside for comparison.