Everyday Math
72
The only money rule worth memorizing
Divide 72 by your interest rate and you know when your money doubles. Bankers do not hate how simple this is. They use it too.
The Math
The Rule of 72 turns an interest rate into a doubling time with a single division:
Years to double ≈ 72 ÷ rate
At 8% a year: 72 ÷ 8 = 9 years. The exact answer — ln 2 ÷ ln 1.08 — is 9.01. A centuries-old shortcut, accurate to a rounding error.
Try this — double it
Full version: the full Rule of 72 calculator — in the CalculateMine tools library.
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THE MOTHER LODE — the mathematics in this story is real and checkable.