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Estimation · Chapter 5

Leading-Digit Multiplier

Multiply the digits that aren’t zero, then count the zeros. Divide by a friendly neighbor of the awkward number.

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Opens on Chapter 5’s example: Kofi’s 40 × 600 × 50, and 7,000 ÷ 33.

Divide
Enter your numbers, then press Calculate.
From the book, Chapter 5. 40 × 600 × 50: the digits make 120, then four zeros: 1,200,000. 7,000 ÷ 33 ≈ 7,000 ÷ 35 = 200, against an exact 212, 6% low. Rounding both numbers the same way keeps a division close: 7,000 ÷ 30 = 233, while 7,000 ÷ 25 = 280.

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.