How BMI is calculated
Body mass index compares weight with height. In metric units, BMI is weight in kilograms divided by height in meters squared. In US units, the same idea is converted from pounds, feet, and inches.
Metric formula
BMI = kg / m2
US formula
BMI = 703 x lb / in2
Adult BMI categories
Below 18.5
Underweight
18.5 to 24.9
Healthy weight
25.0 to 29.9
Overweight
30.0 and above
Obesity
When BMI can be misleading
BMI does not directly measure body fat, muscle mass, bone density, pregnancy, age-related body composition changes, or where weight is carried. Athletes and highly muscular people may have a higher BMI without the same health risk as someone with less muscle.
- Use waist size or body measurements for more context.
- Look at weight trends instead of one isolated result.
- Pair BMI with energy, sleep, fitness, and lab markers.
- Ask a professional for personal medical advice.
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