BMI (Body Mass Index)

Deep dive

BMI (Body Mass Index): complete reference

Body Mass Index (BMI) is a numeric value calculated from a person’s weight and height (kg/m² in metric, or lb × 703 / in² in US units) used as a screening tool for weight categories. Classification: underweight (<18.5), normal (18.5-24.9), overweight (25-29.9), obesity class I (30-34.9), class II (35-39.9), and class III (≥40, formerly “morbid obesity”). BMI is the standard threshold for many GLP-1 indications: Wegovy and Zepbound are FDA-approved for adults with BMI ≥30, or BMI ≥27 with a weight-related comorbidity (T2D, hypertension, dyslipidemia, OSA, CVD). BMI has well-documented limitations — it does not distinguish muscle from fat, underestimates body fat in older adults, and overestimates it in athletes with high muscle mass — so clinical evaluation should include additional measures (waist circumference, body composition, comorbidities) for accurate risk stratification. Despite limitations, BMI remains the most widely used tool because it is simple, requires no equipment, and has decades of population-level outcome data.

In practice

A person who is 5'9&quot; (175cm) and weighs 200 lbs (91kg) has BMI 29.5 (overweight); at 210 lbs (95kg) BMI is 31 (obesity class I) — crossing the threshold for FDA-approved weight-loss medication eligibility.

Clinical context

Insurance prior-auth decisions and FDA labels both rely on BMI thresholds. Clinicians document BMI at each visit for both medication eligibility and risk monitoring.