What BMI Measures

The ratio

BMI is kilograms divided by metres squared. A person at 70 kg and 175 cm is 70 ÷ (1.75 × 1.75) ≈ 22.9. Pounds and inches are converted first; that path is the same arithmetic as 703 × lb / in². The BMI calculator does that division and then places the result on an adult band.

The bands used here are the WHO adult cut-offs: under 18.5 underweight, 18.5–24.9 a range WHO describes for a healthy weight in adults, 25–29.9 overweight, 30 and above obesity. This page does not split obesity into classes, and it does not apply child centiles.

What the number is for

The ratio is a quick screen: a form that asks for BMI, a chart that uses the same bands, or a check that the weight you typed lands where you expected. The healthy weight calculator rearranges the same 18.5 and 24.9 cut-offs into a kilogram range for one height. It is not a different medical standard.

What it does not measure

BMI cannot see muscle, bone, fluid, pregnancy or fat distribution. An athlete can land in the overweight band with little fat. An older adult can sit in the middle band with low muscle. A tape estimate answers a different question: the body fat calculator uses the U.S. Navy circumference equations, not kg/m².

Ethnicity-specific thresholds that some clinics discuss are not applied here. A tenth of a point is not a verdict. Scales differ, and height changes a little through the day. The formula is exact for the inputs, not for the person.

This is not medical advice. If a clinician has told you to ignore BMI or to use another index, follow that.