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Problem
Calculating how long it takes for an investment to double at a given interest rate requires compound interest formulas.
Solution
Divide 72 by the annual interest rate to get the approximate number of years to double your money.
Benefit
Instant mental calculation for financial planning — no calculator or compound interest formula needed.
Example
Years to double = 72 ÷ Interest Rate
At 6% interest:
72 ÷ 6 = 12 years to double
At 9% interest:
72 ÷ 9 = 8 years to double
At 3% interest:
72 ÷ 3 = 24 years to double
Works in reverse too:
To double in 6 years, you need 72 ÷ 6 = 12% return