Doubling and Halving Time: the Rule of 72 Checked

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Opens on Chapter 15’s example: Joaquin’s mailing list growing 9% a month, and a $900 phone losing 20% a year.

Growth

Growth a period %

Loss

Loss a period %

Starting value

Periods to show

Doublings

From

Folding paper

Sheet thickness (mm) an assumption

Folds

Enter your numbers, then press Calculate.

From the book, Chapter 15. At 9% a month: 72 ÷ 9 = 8 months; 1.09⁸ = 1.99; exactly 8.0. Losing 20% a year halves in about 3.6 years by the rule of 72 (3.5 by the rule of 70), 3.1 exactly: the rule runs long for shrinking. A $900 phone is worth $720 after a year and $294.91 after five. From one to 8 billion takes 33 doublings (2³³ = 8,589,934,592). A 0.1 mm sheet folded 42 times would be 439,805 km thick.

Rounding is half up, the way the book rounds. Numbers can be typed as 1,450, $38, 12%, 1/20, 2.5 million or 3 × 10⁷. Everything stays in your browser: no sign-up, nothing stored or sent.