BMI Formula (Body Mass Index)

How it works

BMI divides mass by the square of height to give a number that is roughly independent of how tall someone is, which makes it useful for comparing populations. The square is the whole point: body mass scales with volume, and volume rises faster than height, so dividing by height alone would still leave tall people looking heavier. If you work in pounds and inches, multiply by 703 rather than converting first - that constant is simply the unit conversion folded in, since a pound is 0.45359237 kg and an inch is 0.0254 m. What BMI does not do is distinguish muscle from fat, or account for where fat sits on the body, and it was never designed to assess an individual. It was devised in the 1830s by Adolphe Quetelet as a statistical tool for describing groups.

Variables

• BMIBody Mass Index (kg/m²)
• weightBody weight in kilograms
• heightBody height in meters

Worked examples

Practical applications

• Population and public-health statistics, which is what the measure was built for
• A rough first screen before more specific measurements such as waist circumference or body-fat percentage
• Tracking a change in your own figure over time, where the limitations stay constant and mostly cancel out

Common mistakes

• Using centimeters instead of meters for height. 175 cm must be entered as 1.75.
• Mixing systems - pounds with meters, or kilograms with inches.
• Reading a single BMI figure as a diagnosis. Muscular people routinely land in the overweight band, and the standard adult ranges do not apply to children, pregnant women, or the very elderly.

Use it in a tool

FAQs

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