How BMI Is Calculated
Learn the BMI formula, category ranges, limitations, and when to use alternative health metrics.
5 min read · Last updated: July 13, 2026
The BMI formula
BMI = weight (kg) ÷ height (m)². In imperial units: BMI = (weight in lbs ÷ height in inches²) × 703. The result is a unitless ratio used to categorize body weight relative to height.
BMI categories (adults)
WHO adult ranges: Underweight (<18.5), Normal (18.5–24.9), Overweight (25–29.9), Obese Class I (30–34.9), Class II (35–39.9), Class III (≥40). These are population-level screening thresholds, not individual health diagnoses.
Limitations of BMI
BMI doesn't distinguish fat from muscle — athletes often score 'overweight.' It doesn't account for age, sex, ethnicity, or fat distribution. Waist circumference and body composition tests provide additional context.
Healthy weight range
For a given height, the healthy BMI range (18.5–24.9) translates to a weight range. Our BMI Calculator shows this range automatically so you can set realistic goals with professional guidance.