BMI: What It Measures, What It Misses and Why Drug Labels Use It

Body mass index (BMI) is your weight divided by your height squared. It’s a quick calculation meant for screening, that is, for spotting who might benefit from a fuller assessment. It doesn’t measure fat, it can’t tell fat from muscle, and it says nothing about where fat sits. Even so, it’s the number drug regulators use to define who the weight-management indications of Wegovy, Zepbound and Mounjaro cover. Here I explain where it comes from, the categories the World Health Organization (WHO) uses, where it falls short and why regulators still rely on it.

If you want to work out yours, the BMI calculator (in Spanish) accepts pounds and feet and inches as well as kilograms and centimeters.

Where BMI comes from

The formula dates from the 19th century. The Belgian statistician Adolphe Quetelet studied human growth and noticed that, outside the growth spurts of early childhood and puberty, weight rises with the square of height. He described it in 1832, according to Eknoyan’s historical review. For more than a century it was called the “Quetelet index.” The current name arrived in 1972: Ancel Keys and his team compared several indices of relative weight in 7,424 men from 12 cohorts in five countries. Weight divided by height squared came out slightly better than the alternatives: it depended less on height and correlated more closely with body fat. They named it “body mass index” and noted that it was easy to calculate and could be applied to any population. It was built to study populations, not to judge one person.

How it’s calculated and the WHO categories

In metric units, BMI = weight (kg) ÷ height (m)². At 70 kg and 1.68 m: 70 ÷ (1.68 × 1.68) = 24.8 kg/m².

In US units, BMI = 703 × weight (lb) ÷ height (in)². At 154 lb and 5 ft 6 in (66 in): 703 × 154 ÷ (66 × 66) = 24.9. That’s roughly the same person; the decimal moves because the conversions are rounded.

The WHO classifies adults with this table, from its Technical Report 894 (2000). The category names are WHO’s, with “obese class” written as “obesity class”:

WHO category	BMI (kg/m²)
Underweight	under 18.5
Normal range	18.5 to 24.9
Pre-obese	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 or more

The WHO fact sheet sums up the cutoffs: overweight is a BMI of 25 or more, and obesity is 30 or more. For children and teens aged 5 to 19, these numbers don’t apply; WHO uses its growth references (BMI-for-age) instead. The 2000 report itself warns that the same BMI may not reflect the same amount of fat in different populations.

What BMI doesn’t measure

Muscle, fat and where it sits

According to the US CDC, BMI doesn’t distinguish between fat, muscle and bone, and doesn’t show where in the body fat is stored. The WHO calls it a “surrogate marker” of fatness.

How far off is it in practice? A 2010 meta-analysis pooled 25 studies, with 31,968 people, that compared BMI with directly measured body fat. The usual obesity cutoffs rarely flagged people who didn’t have excess fat (specificity of 90%). But they caught only half of those who did (sensitivity of 50%). In other words, a high BMI usually does go with more fat, but a “normal” BMI doesn’t rule it out.

Ancestry

In 2004, a WHO expert consultation reviewed the cutoffs for Asian populations. It concluded that a substantial share of people in these populations have a high risk of type 2 diabetes and cardiovascular disease at a BMI below 25. The point where risk appeared ranged from 22 to 25 depending on the population. WHO kept the international classification and proposed additional public health action points at 23, 27.5, 32.5 and 37.5.

Age

With age, people lose muscle and body composition changes. In a meta-analysis of 32 studies with 197,940 people aged 65 or older, followed for an average of 12 years, a BMI of 25 to 29.9 was not associated with higher mortality. Risk rose below 23 and above 33. The authors suggest keeping an eye on weight in older adults with a BMI under 23.

Your waist, as a complement

The WHO notes that other measurements, such as waist circumference, can help diagnose obesity. According to the NHLBI, part of the US National Institutes of Health, carrying fat mostly around your waist raises the risk of heart disease and type 2 diabetes. It sets the threshold at more than 35 inches (about 88 cm) for women and more than 40 inches (about 102 cm) for men. Measure standing up, with the tape just above your hipbones, right after you breathe out.

The Lancet commission on clinical obesity (2025) goes further. It says BMI should be used only for screening and for studying populations, not as an individual measure of health. It proposes confirming excess fat with a direct measurement or another body measure, such as waist circumference, waist-to-hip ratio or waist-to-height ratio. Only above a BMI of 40 does it assume excess fat without further checks.

Why drug labels use it

A practical reason: it’s the measure the trials used to choose their participants, and an approved indication reflects who was studied.

In the United States, the labels for Wegovy and Zepbound don’t put a number in the indication. They cover adults with obesity, or adults with overweight in the presence of at least one weight-related comorbid condition, alongside a reduced-calorie diet and more physical activity. The clinical studies section defines those groups with BMI cutoffs: obesity as a BMI of 30 or more, and overweight as a BMI of 27 to under 30 with at least one weight-related condition, such as high blood pressure or abnormal blood fats. Mounjaro, the same molecule as Zepbound, is approved in the US for type 2 diabetes, not for weight management.

The EU prescribing information for Wegovy and Mounjaro, where Mounjaro is also authorized for weight management, writes the numbers into the indication: adults with a BMI of 30 or more, or of 27 to under 30 with at least one weight-related comorbidity. As examples it lists high blood pressure, abnormal blood fats, obstructive sleep apnea, cardiovascular disease, and prediabetes or type 2 diabetes.

For teens, the US Wegovy label covers pediatric patients aged 12 years and older with obesity, and its trial enrolled adolescents with a BMI at or above the 95th percentile for age and sex. The EU prescribing information for Wegovy adds a weight floor: from age 12, with a BMI at or above the 95th percentile for age and sex and a body weight above 60 kg (132 lb).

Whether your BMI sits above or below these numbers doesn’t decide anything on its own. Your prescriber weighs the indication against your medical history, your other conditions and the medications you take. What each drug is approved for is in my guide to what GLP-1 medications are.

Who got into the trials

The three trials behind these indications used BMI as the entry gate:

Trial	Participants	BMI criterion
STEP 1 (semaglutide 2.4 mg, 68 weeks)	1,961 adults without diabetes	30 or more, or 27 or more with at least one weight-related condition
SURMOUNT-1 (tirzepatide, 72 weeks)	2,539 adults without diabetes	30 or more, or 27 or more with at least one weight-related complication
STEP TEENS (semaglutide 2.4 mg, 68 weeks)	201 adolescents aged 12 to under 18	95th percentile or more; or 85th or more with a weight-related condition

In SURMOUNT-1, the average starting BMI was 38.0, and 94.5% had a BMI of 30 or more. The results of these trials, and why they vary so much from person to person, are in how much weight you can lose on GLP-1 medications. And if you lose weight, holding on to muscle is what matters: I cover it in protein and muscle on GLP-1 medications.

A number, not a verdict

BMI squeezes two measurements into one figure. It doesn’t know whether you lift weights, how you sleep, what your lab results say or how you feel. It says nothing about your worth as a person.

If checking your BMI makes you anxious, or you notice it pushing you to eat less and less or skip meals, talk to your doctor or a mental health professional. MedlinePlus describes eating disorders as serious mental health conditions that can be treated, and you don’t have to wait until things are severe to ask for help.

Frequently asked questions

Can BMI diagnose obesity?

Not on its own. The WHO calls it a surrogate marker, and the 2025 Lancet commission asks for excess fat to be confirmed with another measure, such as waist circumference, except at a BMI above 40.

Why is my BMI high if I work out a lot?

Because BMI can’t tell muscle from fat, as the CDC points out. With a lot of muscle, weight goes up without body fat going up. Waist circumference helps fill in the picture.

Are the cutoffs the same for everyone?

The WHO keeps the same international cutoffs, identical for women and men. But its 2004 consultation found risk at a BMI of 22 to 25 in some Asian populations, and in people over 65 a BMI of 25 to 29.9 wasn’t linked to higher mortality.

Can I get Wegovy or Zepbound with a BMI of 27 or 30?

The labels use those cutoffs to define who the weight-management indication covers, but it isn’t automatic: your prescriber decides based on your medical history. BMI is only one piece of information.

Does BMI work for children and teens?

Not with the adult categories. From 5 to 19, the WHO uses its growth references (BMI-for-age), and the Wegovy labels for teens use the 95th percentile for age and sex.