Body mass index, or BMI, is a number calculated from your weight and height that serves as a quick estimate of where you fall on a spectrum from underweight to obese. For adults 20 and older, a BMI between 18.5 and 24.9 is considered a healthy weight. It’s one of the most widely used screening tools in medicine, but it has significant limitations, and most health organizations now recommend using it alongside other measurements rather than on its own.
How BMI Is Calculated
The formula is straightforward: divide your weight by the square of your height. In metric units, that means your weight in kilograms divided by your height in meters squared. If you’re using pounds and inches, you do the same division and then multiply by 703 to convert. A person who weighs 70 kg and stands 1.75 m tall, for example, has a BMI of about 22.9.
The concept dates back to 1832, when Belgian statistician Adolphe Quetelet observed that in adults, weight tends to increase proportionally to the square of height. His ratio was called the Quetelet Index for nearly 140 years until nutritional scientist Ancel Keys confirmed its usefulness and renamed it the Body Mass Index in 1972.
Adult BMI Categories
The CDC defines the following BMI ranges for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or higher
These thresholds were originally developed using data from predominantly white European populations, which is one reason they don’t fit everyone equally well.
BMI Works Differently for Children
For children and teens ages 2 through 19, raw BMI numbers aren’t compared to fixed cutoffs. Instead, a child’s BMI is plotted against age-and-sex-specific growth charts, producing a percentile that shows how they compare to other children of the same age and sex. A child below the 5th percentile is considered underweight, the 5th to 84th percentile is healthy weight, the 85th to 94th is overweight, and the 95th percentile or above is classified as obesity. Severe obesity in children is defined as reaching 120% of the 95th percentile value or a BMI of 35 or higher.
This percentile approach exists because children’s body composition shifts dramatically as they grow, especially during puberty. A BMI of 22 means something very different in a 7-year-old than in a 15-year-old.
Why BMI Can Be Misleading
BMI treats all weight the same. It cannot distinguish between fat, muscle, and bone mass. It doesn’t tell you what type of fat you carry or where on your body that fat sits. This matters because someone who is heavily muscular, like a competitive athlete or manual laborer, can land in the “overweight” or even “obese” range while carrying very little body fat. Conversely, an older adult who has lost significant muscle mass might have a “normal” BMI while actually carrying a high percentage of body fat.
Where your body stores fat also makes a big difference in health risk. Fat carried deep around the organs in the abdomen (visceral fat) is far more metabolically dangerous than fat stored under the skin on your hips or thighs. BMI captures none of that distinction. Two people with identical BMIs can have vastly different risk profiles depending on their body composition and fat distribution.
BMI Thresholds Vary by Ethnicity
Research has consistently shown that people of Asian descent develop conditions like type 2 diabetes at lower BMI levels than people of European descent. In response, a joint recommendation from the World Health Organization’s Western Pacific office and international obesity task forces set the overweight threshold for Asian populations at a BMI of 23 rather than 25, and the obesity threshold at 25 rather than 30. In China specifically, studies found that a BMI of 24 offered the best balance of sensitivity and specificity for identifying metabolic risk factors, with 28 recommended as the obesity cutoff. India uses 23 as its overweight threshold.
The American Diabetes Association reflects this in its screening guidelines, recommending diabetes testing for all Asian American adults with a BMI of 23 or higher. These population-level differences are a core reason why a single set of universal cutoffs falls short.
How Doctors Use BMI Today
In 2023, the American Medical Association adopted a policy explicitly stating that BMI should not be used as the sole measure when evaluating patients for overweight or obesity. The AMA noted that body shape and composition vary across race, ethnicity, sex, and age, and that relying on BMI alone had led to inequitable treatment. The policy also stated that BMI should not be used as the only criterion for denying insurance coverage.
Instead, the AMA recommended pairing BMI with other assessments: waist circumference, body composition estimates, relative fat mass, and metabolic markers like blood sugar and cholesterol levels. BMI remains useful as a starting point because it’s fast, free, and requires no special equipment. But it’s a screening tool, not a diagnosis. A high BMI signals that further evaluation is worth doing. It doesn’t, on its own, tell you or your doctor how healthy you are.
Waist Circumference as a Complement
Of the additional measurements doctors can use, waist circumference is the most practical one to check at home. According to the National Heart, Lung, and Blood Institute, women with a waist size greater than 35 inches and men with a waist larger than 40 inches face higher risk for heart disease and type 2 diabetes, regardless of their BMI. Research from the American Heart Association has found that waist size actually predicts heart attacks better than BMI does, especially in women.
To measure accurately, wrap a tape measure around your bare abdomen just above the top of your hip bones, level with your navel. Keep the tape snug but not compressing the skin, and read the number at the end of a normal exhale. Tracking this measurement over time can give you a clearer picture of changes in abdominal fat than stepping on a scale alone.