BMI: what it measures, and where the label thresholds come from

Body-mass index is a nineteenth-century ratio that survived because it is cheap, reproducible and correlates with health risk across populations. It is used in the GLP-1 labels and in every trial entry criterion, which is why it is worth understanding exactly what it does and does not measure.

The formula

BMI is weight in kilograms divided by height in metres squared. Someone 1.75 m tall weighing 85 kg has a BMI of 85 / (1.75 x 1.75) = 27.8. In imperial units, convert first: pounds times 0.4536 gives kilograms; inches times 2.54 gives centimetres. The calculator does the conversion and also inverts the formula, showing the weight at your height that corresponds to each cut point.

The categories

The WHO adult classification, used by the CDC with the same cut points:

The boundaries are conventions chosen for population statistics. Nothing biological happens at 29.9 versus 30.0.

Where 27 and 30 come from

The Wegovy and Zepbound prescribing information describe the products for chronic weight management in adults with an initial BMI of 30 or more, or 27 or more with at least one weight-related condition (the labels list examples such as hypertension, type 2 diabetes and dyslipidaemia), as an adjunct to a reduced-calorie diet and increased physical activity. Those thresholds mirror the entry criteria of the pivotal trials, STEP 1 and SURMOUNT-1.

That is label wording. It describes the population studied. It is not an eligibility rule that a calculator can apply, because the same labels carry contraindications and warnings that only a prescriber can weigh against your history. The calculator shows the two thresholds as weights at your height so the label text becomes concrete; it does not tell you whether you qualify.

What BMI cannot tell you

• Composition. Muscle and fat weigh the same. A muscular person can sit in the overweight range with little body fat, and someone in the normal range can carry a high fat percentage.
• Distribution. Where fat sits matters for metabolic risk; BMI is blind to it. Waist measurement is a common, if imperfect, complement.
• Change under treatment. Weight lost on a GLP-1 is not all fat. Trials that measured composition found a meaningful lean-mass component, which is one reason the protein target and resistance training get attention.
• Populations. Some clinical bodies use different cut points for some ethnic groups, and BMI is not used the same way in children, in pregnancy, or in older adults with sarcopenia.

Using the calculator

Enter your weight and height. Read the BMI and the category, then look at the weight table: it converts every category boundary into pounds or kilograms at your height, so you can see how far you sit from each one. Treat those weights as coordinates, not as goals; a goal is something you and a clinician set with more information than one ratio.