Body mass index, or BMI, is a number calculated from your weight and height that serves as a rough estimate of body fat. You divide your weight in kilograms by the square of your height in meters. A BMI between 18.5 and 24.9 is considered a healthy weight for most adults, though the number has well-known limitations and is best understood as one data point among several.
How BMI Is Calculated
The formula is simple: take your weight in kilograms and divide it by your height in meters, squared. Someone who weighs 70 kg and stands 1.75 m tall would calculate 70 ÷ (1.75 × 1.75), which equals a BMI of about 22.9.
If you’re working in pounds and inches, you can multiply your weight in pounds by 703, then divide by your height in inches squared. Most people skip the math entirely and use an online calculator, but understanding the formula helps explain why BMI works the way it does: it’s purely a ratio of weight to height. It doesn’t measure fat directly, distinguish between muscle and fat, or account for where your body stores weight.
Standard BMI Categories for Adults
For adults 20 and older, the CDC uses these fixed ranges:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25.0 to 29.9
- Obesity (Class 1): 30.0 to 34.9
- Obesity (Class 2): 35.0 to 39.9
- Severe obesity (Class 3): 40.0 or higher
These thresholds were set based on population-level data showing that health risks rise as BMI increases, particularly above 30. They apply broadly but don’t account for individual differences in body composition, age, sex, or ethnicity.
How BMI Works Differently for Children
Children and teens are still growing, so a fixed number like 25 doesn’t mean the same thing for a 10-year-old as it does for a 40-year-old. Instead, pediatric BMI is expressed as a percentile, comparing a child’s number to others of the same age and sex. A child at the 60th percentile has a higher BMI than 60% of children their age.
The categories shift accordingly. Healthy weight falls between the 5th and 85th percentiles. Overweight is the 85th to just under the 95th percentile. Obesity begins at the 95th percentile, and severe obesity is defined as 120% of the 95th percentile or higher. These percentile-based cutoffs are used for anyone aged 2 through 19.
What a High BMI Means for Health
Carrying excess body fat, especially over long periods, creates chronic inflammation and metabolic changes that raise the risk of a wide range of conditions. The list is extensive: type 2 diabetes, high blood pressure, heart disease, stroke, many types of cancer, chronic kidney disease, fatty liver disease, osteoarthritis, sleep apnea, asthma, and depression. Risks increase with both the amount of excess weight and how long a person carries it.
Being underweight (below 18.5) carries its own risks, including weakened immune function, bone loss, and nutritional deficiencies. BMI at either extreme signals that something may need attention, but the number alone doesn’t tell you what’s going on inside the body.
Why BMI Doesn’t Tell the Full Story
BMI’s biggest weakness is that it can’t distinguish between muscle, fat, bone, and water. A lean, muscular person can easily land in the “overweight” range despite having low body fat. An older adult who has lost muscle mass over the years might have a normal BMI while carrying a higher percentage of body fat than the number suggests. The metric is a blunt instrument.
Ethnicity matters too. Asian populations tend to have a higher percentage of body fat than white populations at the same BMI, and they face elevated risks of type 2 diabetes and cardiovascular disease at lower numbers. A WHO expert consultation identified a BMI of 23.0 as a potential action point for Asian populations, well below the standard overweight threshold of 25. This means a BMI of 24 might look “healthy” on a standard chart but already warrant closer monitoring depending on your background.
In 2023, the American Medical Association adopted a policy stating that BMI should not be used as a sole measure of health. The AMA recommended pairing it with other indicators like waist circumference, body composition, and metabolic factors. The policy also specifically noted that differences across race, ethnicity, sex, and age need to be considered when interpreting BMI, and that the number should not be used as the only criterion for insurance coverage decisions.
Better Ways to Assess Body Fat
If BMI is a starting point, waist circumference and waist-to-hip ratio fill in important details. Waist-to-hip ratio, calculated by dividing your waist measurement (at the narrowest point near your belly button) by your hip measurement (at the widest point), appears to be a better predictor of future health problems than BMI alone. The reason is straightforward: it reflects levels of abdominal fat, including the visceral fat that wraps around internal organs and drives metabolic disease. A healthy waist-to-hip ratio for most men is below 0.95.
Waist circumference on its own is also useful. For most adults, a waist measurement above 40 inches for men or 35 inches for women signals increased metabolic risk, regardless of what BMI says. These measurements take 30 seconds with a tape measure and capture something BMI misses entirely: where your body stores fat.
Body composition testing, which estimates your actual ratio of fat to lean tissue, gives the most detailed picture. Methods range from simple skinfold calipers to more precise tools like DEXA scans. These aren’t routine in most checkups, but they’re worth asking about if your BMI seems inconsistent with your overall fitness level or if you’re trying to track changes in muscle versus fat over time.
How to Use Your BMI Practically
Think of BMI as a screening tool, not a diagnosis. If your number falls in the healthy range and you have no other risk factors, it offers some reassurance. If it’s elevated, it’s a signal to look deeper: check your waist circumference, consider your family history, and pay attention to blood pressure, blood sugar, and cholesterol at your next checkup. A BMI of 27 in someone who exercises regularly, has normal metabolic markers, and carries weight in their hips tells a very different story than the same number in someone with prediabetes and high blood pressure.
For tracking your own health over time, BMI is most useful as a trend line. A gradual climb of several points over a few years is worth paying attention to even if you’re still technically in the “healthy” range. The direction matters as much as the number itself.