Rule of 72

Definition:

A formula used to determine the amount of time it will take for invested money to double at a given compound interest rate, which is 72 divided by the interest rate. The logic is as follows. The time for an amount A to double is given by 2A=A(1+i)^t where ^ represents exponent and i is the interest rate, e.g. .05 is 5%. The A term cancels from both sides of the question. Solve for t by taking the natural log of both sides of the equation. Hence, t= [ln(2) over {ln(1+i)}], which is approximately equal to 0.72 over i. Hence the rule of 72.

Investing Essentials


Copyright © 2011 Campbell R. Harvey, Professor of Finance, Fuqua School of Business at Duke University

Term of the Day

Historical Cost Accounting Convention

An accounting technique that values an asset for balance sheet purposes at the price paid for the asset at the time of its acquisition.

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