BMI stands for body mass index, a numerical value that estimates whether your weight is proportional to your height. It’s calculated by dividing your weight by the square of your height, producing a single number that healthcare providers use as a quick screening tool for potential weight-related health risks. A BMI of 25, for example, is the standard threshold where overweight begins for most adults.
Despite its widespread use, BMI is not a diagnostic tool. It can’t tell you how much body fat you actually carry, where that fat sits, or whether your weight comes from muscle, bone, or fat. It’s a starting point, not a final answer.
How BMI Is Calculated
The formula is straightforward. If you’re using metric units, divide your weight in kilograms by your height in meters squared. In U.S. units, divide your weight in pounds by your height in inches squared, then multiply by 703. A person who weighs 170 pounds and stands 5’7″ (67 inches) would calculate it as: 170 ÷ (67 × 67) × 703 = 26.6.
The formula dates back to 1832, when Belgian mathematician Adolphe Quetelet observed that in most adults, weight increases proportionally to the square of height. It was originally called the Quetelet Index. In 1972, American physiologist Ancel Keys validated it against other measures of relative weight and gave it the name “body mass index,” which stuck.
Standard BMI Categories for Adults
For adults 20 and older, the CDC defines four main categories:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity: 30 or higher
Obesity is further divided into three classes. Class 1 ranges from 30 to 34.9, Class 2 from 35 to 39.9, and Class 3 (sometimes called severe obesity) is 40 or above. These thresholds matter because health risks, particularly for heart disease, type 2 diabetes, and stroke, increase as BMI rises. Weight gain triggers a cascade of metabolic changes: insulin resistance, elevated blood pressure, and disruptions to blood fat levels that collectively raise cardiovascular risk.
How BMI Is Used Differently for Children
BMI works differently for anyone under 20. Because children’s body composition changes dramatically as they grow, a raw BMI number means nothing on its own. Instead, a child’s BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile, showing how they compare to other children of the same age and sex.
The categories for children and adolescents aged 2 and older are:
- Underweight: below the 5th percentile
- Healthy weight: 5th to below the 85th percentile
- Overweight: 85th to below the 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: at or above 120% of the 95th percentile, or a BMI of 35 or higher
A BMI of 22 might be perfectly healthy for a 16-year-old boy but could signal a concern for a 6-year-old girl. The percentile system accounts for these differences in a way a flat number cannot.
Why BMI Thresholds Vary by Ethnicity
The standard cutoffs of 25 for overweight and 30 for obesity were developed primarily from data on non-Hispanic White populations. For people of Asian descent, those thresholds are too high. At the same BMI, Asian populations tend to carry a higher percentage of body fat and face a greater risk of cardiovascular disease and diabetes than White populations.
A WHO expert consultation concluded that significant health risks appear in Asian adults at BMIs well below 25. The recommended cutoffs for Asian populations are 23 for overweight and 27.5 for obesity, with a narrower “normal” range of 18.5 to 22.9. This means an Asian adult with a BMI of 24 could already be at elevated metabolic risk, even though that number falls in the “healthy” range on standard charts.
Where BMI Falls Short
BMI’s biggest limitation is that it treats all weight the same. It cannot distinguish between fat, muscle, and bone. A competitive athlete with significant muscle mass can easily register a BMI in the overweight range while carrying very little body fat. On the flip side, someone with a BMI in the healthy range can still have high levels of body fat, particularly if they have low muscle mass. This is sometimes called “normal weight obesity” and it carries its own health risks.
BMI also reveals nothing about where fat is stored. Fat that accumulates around the organs in your midsection (visceral fat) is far more metabolically dangerous than fat stored under the skin on your hips or thighs. Two people with identical BMIs can have very different risk profiles depending on their fat distribution.
In 2023, the American Medical Association formally acknowledged these limitations in a policy update. The AMA recognized that BMI “loses predictability when applied on the individual level” and that its cutoffs don’t account for differences across racial and ethnic groups, sexes, and age. The policy recommends that BMI be used alongside other measures rather than as a standalone metric. At the same time, the AMA noted that BMI remains useful as an initial screener for metabolic health risks and is valuable for tracking obesity trends at the population level due to its simplicity.
Measurements That Complement BMI
Because BMI can’t capture the full picture, healthcare providers often pair it with other measurements. Waist circumference is the most common addition. Measured slightly above the hipbones, it gives a rough estimate of visceral fat. Health risks increase as waist size grows, regardless of what your BMI says. For most adults, a waist circumference above 35 inches for women or 40 inches for men signals elevated risk.
Other tools the AMA recommends considering include body composition analysis (which separates fat mass from lean mass), waist-to-hip ratio, and metabolic markers like blood sugar and cholesterol levels. These paint a more complete picture of whether your weight is actually affecting your health. BMI tells you one thing: whether your weight is roughly proportional to your height. What you do with that information depends on everything else your body is telling you.