How BMI is calculated
Body mass index is your weight in kilograms divided by the square of your height in meters (kg/m²). The imperial version multiplies pounds divided by inches squared by 703. This calculator does the unit conversion for you, so you can enter feet and inches with pounds, or centimeters with kilograms, and get the same standardized number. It then compares that number to the WHO cut-offs to name your category.
What the categories mean
The WHO bands are: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obesity from 30 up, split into class I (30–34.9), class II (35–39.9), and class III (40+). These thresholds are population-level screening bands, not personal diagnoses. The healthy weight range shown for your height is simply the weights that would put your BMI between 18.5 and 24.9, which is often more actionable than the index itself.
Where BMI falls short
BMI measures mass, not body composition, so it cannot tell muscle from fat. Muscular athletes often score as overweight or obese despite low body fat, while older adults who have lost muscle can score "normal" while carrying excess fat. It also does not account for where fat is stored — abdominal fat carries more health risk than fat on the hips. For these reasons BMI is best used as a rough screen alongside measures like waist circumference and body fat percentage.
A worked example
Someone 175 cm tall weighing 75 kg has a BMI of 24.5, at the upper end of the normal range, with a healthy weight range of roughly 56.7 to 76.3 kg for that height. Add 10 kg and the BMI rises to 27.8 — overweight. Switch to imperial and the same person entered as 5 ft 9 in and 165 lb returns essentially the same 24.4, confirming the units are equivalent.
Health disclaimer
This calculator provides general guidance only and is not medical advice. BMI is a screening tool, not a diagnosis. Discuss your weight and health with a qualified healthcare provider, who can consider your muscle mass, fat distribution, medical history, and other factors that a single number cannot capture.