The formula
In metric: BMI = weight in kilograms ÷ (height in metres)². In imperial: BMI = 703 × weight in pounds ÷ (height in inches)². The 703 is a conversion constant, not a magic number — it folds the kg-to-lb and m-to-in conversions into one step.
One rule before you start: never mix units. Kilograms with centimetres, or pounds with feet, will give a confident-looking wrong answer. Convert first, then divide.
Worked example: metric
A person weighs 70 kg and is 1.75 m tall. Step one, square the height: 1.75 × 1.75 = 3.0625. Step two, divide: 70 ÷ 3.0625 = 22.857…, which rounds to 22.9. That sits in the normal band.
If your height is in centimetres, convert first: 175 cm = 1.75 m. Squaring 175 directly and dividing 70 by it gives 0.0023 — the classic unit-mix mistake.
Worked example: imperial
A person weighs 154 lb and is 5 ft 9 in tall. Step one, height in inches: 5 × 12 + 9 = 69 in. Step two, square it: 69 × 69 = 4,761. Step three: 703 × 154 = 108,262. Step four: 108,262 ÷ 4,761 = 22.74…, about 22.7. Same person, same answer as the metric version — as it should be.
Reading the result
The World Health Organization bands: below 18.5 is underweight, 18.5–24.9 is normal, 25–29.9 is overweight, 30 and above is obesity. These describe population-level risk, not a personal diagnosis — muscle, age, and build all shift what a number means for you specifically. For the full honest version, read Is BMI Accurate? What the Number Can't Tell You.
Common mistakes that skew the number: weighing yourself after a meal or in heavy clothes (adds 1–2 kg), measuring height in shoes (adds 2–3 cm), and rounding height before squaring, which compounds the error. Measure morning, barefoot, light clothes.