BMI, or body mass index, is a measure of body size based on your weight relative to your height. It estimates whether you fall into an underweight, healthy, overweight, or obese category, but it does not directly measure body fat. Instead, it uses a simple math formula as a proxy, which is why it works well for tracking trends across large populations but can be misleading for any single person.
How BMI Is Calculated
The formula divides your weight by the square of your height. In metric units, that’s your weight in kilograms divided by your height in meters squared. In pounds and inches, you divide your weight by your height in inches squared, then multiply by 703 to convert to the same scale. A 170-pound person who stands 5’7″ (67 inches), for example, has a BMI of about 26.6.
The result is a single number, expressed in kg/m², that slots you into one of several categories. For adults 20 and older, the CDC defines them this way:
- 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
What BMI Can and Cannot Tell You
BMI correlates with body fat across large groups of people, and that correlation is the reason it became the standard screening tool. It’s cheap, fast, and requires nothing more than a scale and a tape measure. For population-level health tracking, it remains useful.
At the individual level, though, BMI loses much of its predictive power. The core problem is that the formula treats all weight the same. It cannot distinguish between muscle, bone, water, and fat. A competitive athlete and a sedentary office worker can share an identical BMI while having dramatically different amounts of body fat and very different health risks. Research comparing BMI categories to direct body fat percentage measurements found that the two methods agreed on whether someone was in a healthy or unhealthy range only about 60% of the time. That 40% gap is significant: it means BMI misclassifies a large share of people in both directions, flagging some muscular individuals as overweight while giving a clean bill of health to others who carry excess fat at a normal weight.
This second group, sometimes called “normal weight obesity,” is particularly concerning. These individuals have a BMI under 25 but elevated body fat, putting them at meaningfully higher risk for metabolic problems, type 2 diabetes, and cardiovascular disease without any red flag from a standard BMI screening.
Why Fat Location Matters More Than Total Weight
Not all body fat carries the same health risk. Fat stored around the abdomen, deep inside the abdominal cavity surrounding your organs (visceral fat), is far more metabolically active than fat stored around the hips and thighs. Visceral fat plays a large role in inflammation and insulin resistance, which is why it’s more strongly linked to heart disease, stroke, and diabetes. BMI makes no distinction between these two types of fat.
A study published in the Journal of the American Heart Association found that waist-to-hip ratio was a better predictor of heart attack risk than BMI for both men and women. The reason is straightforward: two people can have the same BMI but carry their weight in completely different places. The person with more fat concentrated around their waist faces higher cardiovascular risk, and BMI alone won’t reveal that. Waist circumference, waist-to-hip ratio, and waist-to-height ratio all capture this distinction in ways BMI cannot.
Different Thresholds for Different Populations
The standard BMI cutoffs were developed primarily from data on white European populations, and they don’t apply equally to everyone. A WHO expert consultation found that Asian populations face elevated metabolic risk at lower BMI values and recommended narrowing the “normal” range to 18.5 to 23 for these groups, compared to the standard upper limit of 25. This means a person of South Asian or East Asian descent with a BMI of 24 may already face health risks that the standard categories would classify as perfectly healthy.
The reasons are partly physiological. At the same BMI, people of Asian descent tend to carry proportionally more visceral fat than people of European descent. Using the standard cutoffs without adjustment can delay early intervention for a substantial portion of the global population.
How BMI Works Differently for Children
For children and teens ages 2 through 19, BMI is calculated the same way but interpreted differently. Because kids are still growing, their BMI is plotted on age- and sex-specific growth charts and expressed as a percentile rather than a raw number. A 10-year-old boy with a BMI at the 70th percentile, for instance, has a higher BMI than 70% of boys his age.
- Underweight: below the 5th percentile
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above
This percentile-based system accounts for the fact that body composition shifts naturally as children develop, making a fixed number meaningless without context.
How the Medical Community Uses BMI Now
The American Medical Association adopted a policy stating that BMI should not be used as a standalone measure. The AMA’s position is that BMI works best when paired with other indicators: waist circumference, body composition assessments, relative fat mass, and metabolic markers like blood sugar and cholesterol levels. The policy also specifies that BMI should not be the sole criterion for denying insurance coverage.
The formula was never designed for individual diagnosis in the first place. It was created in the 1800s by Belgian statistician Adolphe Quetelet, who was interested in defining the “average man” across populations. He had no interest in obesity. His goal was to show that physical characteristics, like height and weight, followed a bell-curve distribution across large groups. The index was repurposed for clinical use much later, and that original population-level intent helps explain why it struggles with individual accuracy.
More Useful Measures to Know
If you want a clearer picture of your health risk beyond BMI, a few additional measurements are worth tracking. Waist circumference is the simplest: measured at the navel, values above 40 inches for men or 35 inches for women are associated with higher risk for metabolic problems. Waist-to-hip ratio adds another layer by comparing where you store fat. Body fat percentage, measured through methods like bioelectrical impedance scales or a DEXA scan, directly quantifies how much of your weight is fat versus lean tissue.
None of these measures is perfect on its own either, which is the broader point. BMI is one data point in a larger picture. It can signal that something is worth investigating, but it was never meant to be the final word on whether you’re healthy.