Body mass index, or BMI, is a number calculated from your weight and height that estimates where you fall on a spectrum from underweight to severely obese. For adults, 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 real limitations, and understanding both its usefulness and its blind spots will help you interpret your own number.
How BMI Is Calculated
The formula is simple: divide your weight by your height squared. In metric units, that’s your weight in kilograms divided by your height in meters, squared. If you’re working in pounds and inches, you divide your weight by your height in inches squared, then multiply by 703. A person who weighs 70 kg and stands 1.75 m tall, for example, has a BMI of about 22.9.
The formula was originally developed in the 1830s by Belgian mathematician Adolphe Quetelet, who observed that in adults, weight tends to increase proportionally to the square of height. It was known as the Quetelet Index until 1972, when researcher Ancel Keys renamed it the Body Mass Index. The simplicity of the calculation is a big part of why it became so widespread: it requires no special equipment, just a scale and a tape measure.
Adult BMI Categories
The CDC uses the following ranges for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity Class 1: 30 to 34.9
- Obesity Class 2: 35 to 39.9
- Obesity Class 3 (severe): 40 or higher
These thresholds were set based on population-level data linking BMI ranges to disease risk. They’re the same for men and women, which is one of the metric’s known shortcomings since body composition differs between sexes.
How BMI Works Differently for Children
Because children and teens are still growing, a raw BMI number doesn’t mean the same thing it does for an adult. Instead, a child’s BMI is plotted on age-and-sex-specific growth charts and expressed as a percentile. A 10-year-old girl and a 16-year-old boy with the same BMI number could fall into completely different categories.
For children and teens ages 2 through 19, the categories are:
- 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
What a Higher BMI Means for Health Risk
At the population level, the link between elevated BMI and chronic disease is well established. A large-scale genetic analysis published in BMC Medicine found that higher BMI causally increases the risk of type 2 diabetes, with roughly double the odds per unit increase in genetically predicted BMI. The same analysis found increased risk across 14 cardiovascular conditions, with the strongest links to aortic valve stenosis (about double the risk), heart failure (69% higher risk), and hypertension (68% higher risk).
The connections extend beyond the heart. Higher BMI was linked to increased risk of asthma, chronic obstructive pulmonary disease, gallstone disease, gastroesophageal reflux, nonalcoholic fatty liver disease, gout, osteoarthritis, and cancers of the digestive system, uterus, kidney, and bladder. These aren’t just correlations: the study used genetic methods designed to isolate the causal effect of body weight itself.
None of this means a BMI of 26 guarantees health problems. It means that as BMI climbs, the statistical odds of these conditions rise in a dose-dependent way. The steepest increases in risk tend to appear above a BMI of 30.
Why BMI Can Be Misleading
BMI’s biggest flaw is that it can’t distinguish between muscle and fat. A bodybuilder with 6% body fat can register a BMI above 30, landing in the “obese” category despite being exceptionally lean. Research on elite military special forces found that BMI classified highly trained soldiers as overweight even though their actual body fat was lower than that of untrained individuals classified as normal weight. In the other direction, BMI underestimates body fat in older adults and people with very little muscle mass.
BMI also ignores where fat is stored, and location matters enormously. Visceral fat, the kind that packs around your organs deep in the abdomen, 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 amounts of visceral fat, and therefore very different health risk profiles.
Ethnic and Racial Differences
The standard BMI thresholds were developed primarily from data on white European populations, and they don’t apply equally across all ethnic groups. People of Asian descent generally carry a higher percentage of body fat than white people at the same BMI, which means health risks start climbing at lower numbers. A WHO expert consultation found that the BMI at which disease risk increases varies from 22 to 25 across different Asian populations, well below the standard “overweight” cutoff of 25. Some countries in Asia have adopted lower thresholds for clinical decision-making.
Measurements That Fill the Gaps
Because of these blind spots, several complementary measurements can give you a clearer picture of your metabolic health.
Waist circumference is the simplest. Wrap a tape measure around your waist just above your hip bones. For women, 35 inches or more signals elevated risk from visceral fat. For men, the threshold is 40 inches. This takes about 10 seconds and captures something BMI completely misses.
Waist-to-height ratio divides your waist circumference by your height. If that ratio exceeds 0.5 (meaning your waist is more than half your height), your risk of cardiovascular and metabolic disease goes up. A systematic review found that waist-to-height ratio predicts cardiovascular events slightly better than BMI, particularly in people with type 2 diabetes. Each standard-deviation increase in waist-to-height ratio raised the risk of a major cardiovascular event by 16%, compared to 9% for the same increase in BMI.
Waist-to-hip ratio is another option. Divide your waist measurement by the circumference of the widest part of your hips. A ratio above 0.85 for women or 0.90 for men indicates abdominal obesity.
How Doctors Use BMI Today
In 2023, the American Medical Association adopted a policy acknowledging BMI’s “significant limitations” in clinical settings. The AMA now recommends using BMI alongside other measures like waist circumference, body composition, visceral fat estimates, and metabolic markers rather than relying on it alone. The policy also notes that BMI should not be the sole criterion for insurance coverage decisions.
That said, the AMA didn’t abandon BMI entirely. It recognizes that BMI above 35 remains useful for risk stratification, that it works well as an initial screening tool for metabolic health risks, and that its simplicity makes it valuable for tracking obesity trends across populations. Think of it as a starting point: useful for flagging potential concerns, but not the full picture of anyone’s health. If your BMI falls outside the healthy range, the next step is looking at additional measurements and metabolic markers to understand what that number actually means for you.