Calculators
How to Calculate BMI
AllinOne Editorial · Published 2026-09-09 · Updated 2026-09-16
Body mass index is a height-and-weight ratio, not a diagnosis. Here is the formula this calculator uses.
Body mass index (BMI) is weight in kilograms divided by the square of height in metres. It is widely used in clinics as a screening number. It does not measure body fat, muscle or health on its own.
Formula
BMI = kg ÷ m². Imperial inputs convert first: 1 inch = 0.0254 m and 1 lb = 0.45359237 kg. The BMI Calculator on this site uses those exact factors.
Adult categories (WHO)
- Underweight: BMI under 18.5
- Normal weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity: 30.0 and above
Worked example
A person who is 170 cm (1.70 m) and 68 kg has BMI 68 ÷ 2.89 ≈ 23.5, in the normal-weight range. The healthy weight band for that height is the weight that produces BMI 18.5–24.9, about 53.5–72.0 kg.
High muscle mass can push BMI up without the same health meaning. Treat the number as a screen, not a diagnosis.
Frequently asked questions
Can I use BMI for children?+
Not with adult cut-offs. Child BMI uses age- and sex-specific percentiles, which this adult tool does not apply.
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