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 Rule of 72 turns an interest rate into a doubling time with a single division:
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.
Full version: the full Rule of 72 calculator — in the CalculateMine tools library.
Want to run your own version of this number? Thousands of free tools live in the CalculateMine tools library.
This is a sample story from the Mother Lode proof edition — the standfirst above and the math are the story so far. The full-length version arrives with the first regular edition.
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THE MOTHER LODE — proof edition. This story is a demonstration article; the mathematics in it is real and checkable.