Body mass index, or BMI, is a number calculated from your weight and height that serves as a quick estimate of whether your body weight falls within a healthy range. For adults, a BMI between 18.5 and 24.9 is considered healthy, while 25 to 29.9 is overweight and 30 or above is classified as obese. It’s the most widely used screening tool in healthcare for flagging weight-related health risks, though it has real limitations that are worth understanding.
How BMI Is Calculated
The math behind BMI is simple: divide your weight by your height squared. If you’re using metric units, that’s your weight in kilograms divided by your height in meters squared. In 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 formula was originally developed in 1832 by Adolphe Quetelet, a Belgian astronomer and mathematician who wasn’t trying to measure obesity at all. He was attempting to define the “average man” using statistics. The formula sat mostly unused for weight assessment until 1972, when American physiologist Ancel Keys and colleagues published a review showing that Quetelet’s ratio accurately tracked with normal human growth patterns across thousands of measurements. From that point on, it became known as BMI and entered routine clinical use.
BMI Categories for Adults
The CDC defines the following BMI ranges for adults age 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 aren’t arbitrary. A large UK study of 3.6 million adults found that the lowest risk of death from all causes occurred in the BMI range of 21 to 25. Above 25, every 5-point increase in BMI raised mortality risk by about 21%. Below 25, mortality actually increased as BMI dropped, creating a J-shaped curve. Being significantly underweight carried risks comparable to obesity: from age 40, underweight men lost an estimated 4.3 years of life expectancy compared to those at a healthy weight, while obese men lost about 4.2 years. For women, the pattern was similar, with underweight women losing 4.5 years and obese women losing 3.5 years.
What BMI Is Used For
Doctors use BMI primarily as a screening tool, not a diagnosis. A high BMI doesn’t mean you’re unhealthy, but it flags the need for further evaluation. It’s fast, free, and doesn’t require any equipment beyond a scale and a tape measure, which makes it practical for tracking population-level trends and for initial assessments in a clinical setting.
The health risks associated with higher BMI are well documented. Nearly 9 in 10 people with type 2 diabetes have overweight or obesity. Elevated BMI raises the risk of high blood pressure, which is the leading cause of strokes. It increases the likelihood of heart disease, fatty liver disease, chronic kidney disease, and sleep apnea. There’s also a cancer connection: men with overweight or obesity face higher rates of colon, rectal, and prostate cancers, while women face elevated risk for breast, uterine, and gallbladder cancers. Adults who gain less weight over time have lower rates of colon, kidney, breast, and ovarian cancers.
BMI is also used to determine eligibility for certain medical treatments, including weight-loss medications and bariatric surgery, and to track changes in weight over time at both the individual and population level.
How BMI Works Differently for Children
BMI means something different for kids and teens because their body composition changes as they grow. Rather than fixed cutoffs, pediatric BMI is expressed as a percentile that compares a child to others of the same age and sex. A child at the 60th percentile, for instance, has a BMI equal to or higher than 60% of children their age and sex.
The categories for children and teens ages 2 through 19 break down like this:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than 85th percentile
- Overweight: 85th to less than 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above, or a BMI of 35 or higher
The American Academy of Pediatrics further divides severe obesity into Class 2 (120% to 140% of the 95th percentile) and Class 3 (140% or above), which helps clinicians identify children who may need more intensive support.
Where BMI Falls Short
The biggest limitation is straightforward: BMI cannot tell the difference between fat, muscle, and bone. A muscular athlete and a sedentary person of the same height and weight will have identical BMI scores despite very different health profiles. This means BMI can overestimate body fat in people with significant muscle mass and underestimate it in older adults who have lost muscle over the years.
BMI also doesn’t account for where fat is stored, which matters a great deal. Fat that accumulates around the organs in your abdomen (visceral fat) is far more metabolically dangerous than fat stored under the skin on your hips or thighs. Two people with the same BMI can have dramatically different levels of visceral fat and, therefore, very different health risks.
Ethnicity is another blind spot. Health organizations have suggested lower obesity cutoffs for adults with Asian ancestry, starting at a BMI of 27.5 rather than 30. This adjustment exists because Asian adults tend to develop conditions like high blood pressure and diabetes at lower body weights compared to white adults. Using the standard cutoff of 30 would miss a significant number of people already at elevated risk.
Measurements That Add Context
Because BMI misses so much about body composition and fat distribution, other measurements can fill in the gaps. Waist circumference is the simplest addition. You measure around your natural waistline, usually near your belly button, and a larger number signals more abdominal fat regardless of what BMI says.
Waist-to-hip ratio goes a step further. You divide your waist circumference by the circumference at the widest part of your hips. Research from Harvard Health found that this ratio is better than BMI at predicting future health problems, most likely because it more accurately reflects levels of dangerous visceral fat. For most men, a healthy waist-to-hip ratio falls below 0.95.
More precise tools exist for measuring actual body fat percentage, including specialized body scans, but these are expensive and not widely available. For most people, combining BMI with a simple waist measurement gives a much more complete picture than either number alone.