The BMI formula
BMI is calculated as weight in kilograms divided by height in metres squared: BMI = kg / m². In imperial units, the formula is: BMI = (lbs / in²) × 703. For example, a person who weighs 75 kg and stands 1.75 m tall has a BMI of 75 / (1.75 × 1.75) = 75 / 3.0625 ≈ 24.5.
The result is a unitless number. The same formula applies regardless of sex, age (for adults), or ethnicity, though some health organisations publish adjusted ranges for specific populations — particularly South and East Asian adults, for whom health risks associated with excess body fat appear at lower BMI values.
BMI categories
The World Health Organization defines four standard adult categories: Underweight is a BMI below 18.5. Normal weight is 18.5–24.9. Overweight is 25.0–29.9. Obese is 30.0 and above, with further subdivisions at 35 and 40 for Class II and Class III obesity.
These thresholds were established based on population-level statistical correlations with cardiovascular disease risk, type 2 diabetes, and all-cause mortality. They are screening guidelines, not clinical diagnoses.
What BMI does not measure
The most significant limitation of BMI is that it does not distinguish between fat mass and lean mass. A strength athlete with a high muscle mass may have a BMI in the overweight range while carrying very little body fat. Conversely, someone with a normal BMI may carry a high proportion of visceral (abdominal) fat, which is more strongly associated with metabolic risk than subcutaneous fat.
BMI also does not account for fat distribution (waist circumference is a better proxy for visceral fat), age-related changes in body composition, sex differences in fat distribution, or bone density. Clinicians typically use BMI alongside other assessments rather than in isolation.
Practical uses
Despite its limitations, BMI remains useful as a low-cost initial screen because it requires no specialist equipment and is highly reproducible. It is a reasonable starting point for tracking changes over time in an individual — a falling or rising BMI trend carries more signal than any single reading.
If your BMI is outside the normal range and you want a more complete picture, body fat percentage calculators (using tape measurements or skinfold calipers) and waist-to-height ratio are practical next steps that do not require a gym or clinic.