BMI

Quick definition. BMI is a number calculated from your weight and height that sorts adults into weight categories. It is the single most common gate insurers use to decide whether a GLP-1 medication will be covered.

What BMI Measures

Body mass index is a screening ratio that relates a person's weight to their height. It does not measure body fat directly, but across large populations it correlates well enough with body fat to serve as a practical first-pass indicator of whether someone is underweight, at a healthy weight, overweight, or has obesity. Because it requires only a scale and a tape measure, BMI has become the default metric in clinical guidelines, insurance policies, and the eligibility criteria written into GLP-1 prescribing labels.

How BMI Is Calculated

BMI is weight in kilograms divided by height in meters squared (kg/m²). In US units, the same result comes from weight in pounds divided by height in inches squared, multiplied by 703. A person who is 5 feet 6 inches (1.68 m) and weighs 185 lb (84 kg), for example, has a BMI of about 29.8 — at the top of the overweight range. The math is deliberately simple so that the same threshold means the same thing across clinics and insurers.

BMI Categories for Adults

The World Health Organization and US clinical guidelines use these cutoffs for adults:

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity Class I: 30.0 to 34.9
• Obesity Class II: 35.0 to 39.9
• Obesity Class III: 40.0 and above

These bands feed directly into the more granular obesity classification used for treatment decisions.

How BMI Determines GLP-1 Eligibility

For weight-management GLP-1 medications such as Wegovy, Zepbound, and Saxenda, the FDA labels and most insurers set the same two-part bar: a BMI of 30 or greater, or a BMI of 27 or greater accompanied by at least one weight-related condition such as type 2 diabetes, high blood pressure, or obstructive sleep apnea. This threshold is usually the first item checked during prior authorization, which is why patients near the cutoff are sometimes asked to document a qualifying comorbidity.

The Limitations of BMI

BMI is a population screening tool, not a diagnosis. It cannot distinguish muscle from fat, so a muscular athlete can register as "overweight" while carrying very little body fat. It also says nothing about where fat is distributed, even though visceral (abdominal) fat carries more metabolic risk than fat elsewhere. The standard cutoffs were derived largely from European-ancestry populations; the WHO notes that many Asian populations face elevated cardiometabolic risk at lower BMI values, with overweight risk emerging closer to 23 and obesity-level risk near 27.5. For these reasons, clinicians treat BMI as a starting point and combine it with waist circumference, lab values such as A1C, and overall health history before recommending treatment.