BMI Calculator
Calculate your body mass index (BMI) from height and weight, in metric or imperial units, with categories as defined by the World Health Organization.
What BMI is — and what it is not
Body mass index is weight in kilograms divided by the square of height in meters (kg/m²). The World Health Organization classifies adult BMI below 18.5 as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 or above as obesity.
BMI is a quick population-level screening measure, not a diagnosis. It does not distinguish muscle from fat, and it can misclassify athletes, older adults, and people of certain body types. Waist circumference, body composition, and clinical assessment give a fuller picture.
If your BMI falls outside the normal range, the number alone says little about your individual health. Discuss it with a healthcare professional who can interpret it alongside your history, lifestyle, and other measurements.
Sources: WHO — Obesity and overweight fact sheet, CDC — About Adult BMI
Frequently asked questions
How is BMI calculated?
BMI = weight (kg) ÷ height² (m²). In imperial units the formula is weight (lb) ÷ height² (in²) × 703. A person 175 cm tall weighing 70 kg has a BMI of 70 ÷ 1.75² ≈ 22.9.
What are the WHO BMI categories for adults?
Underweight: below 18.5. Normal weight: 18.5–24.9. Overweight: 25–29.9. Obesity: 30 and above. These thresholds apply to adults aged 20 and over.
Does BMI apply to children and teenagers?
Not with adult thresholds. For ages 2–19, BMI is interpreted with age- and sex-specific percentile charts, because healthy body composition changes as children grow.
Is BMI an accurate measure of health?
It is a useful screening tool at the population level but imperfect for individuals. It ignores muscle mass, fat distribution, and ethnicity-specific risk differences. Treat it as one indicator among many, not a verdict.