BMI, or Body Mass Index, is a number calculated from your height and weight that places you into a general weight category. It’s one of the most common screening tools in medicine, used to flag whether someone may be underweight, at a healthy weight, overweight, or obese. But it’s a starting point, not a diagnosis. BMI can’t tell you how much of your weight is fat versus muscle, where your body stores fat, or how healthy you actually are.
How BMI Is Calculated
The math behind BMI is simple. You 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 the result.
For example, someone who weighs 170 pounds and stands 5’7″ (67 inches) would calculate it as: 170 ÷ (67 × 67) × 703, which comes out to about 26.6. You don’t need to do this by hand. Free calculators from the CDC and other health organizations will do it instantly.
What the Numbers Mean for Adults
For adults 20 and older, BMI falls into these standard categories:
- Underweight: less than 18.5
- Healthy weight: 18.5 to less than 25
- Overweight: 25 to less than 30
- Class 1 obesity: 30 to less than 35
- Class 2 obesity: 35 to less than 40
- Class 3 (severe) obesity: 40 or greater
These ranges were developed primarily from data on non-Hispanic white populations, which is one reason they don’t apply equally well to everyone. More on that below.
BMI Categories Work Differently for Children
Children and teens between ages 2 and 20 are evaluated differently. Because their body composition changes rapidly as they grow, a fixed number like 25 doesn’t carry the same meaning it does for an adult. Instead, a child’s BMI is compared to other children of the same age and sex using percentile charts.
A child at the 60th percentile, for instance, has a BMI higher than 60% of children the same age and sex. The categories break down like this:
- Underweight: below the 5th percentile
- Healthy weight: 5th to less than the 85th percentile
- Overweight: 85th to less than the 95th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above, or a BMI of 35 or greater
Your child’s pediatrician tracks these percentiles over time. A single reading matters less than the trend across multiple visits.
Health Risks Linked to High BMI
At the population level, higher BMI correlates with a greater risk of several serious conditions. Adults with obesity face increased risk of high blood pressure, type 2 diabetes, and respiratory problems like asthma and obstructive sleep apnea. These same risks show up in children and adolescents with obesity, sometimes appearing years earlier than they otherwise would. A child with a BMI in the obesity range is more likely to develop prediabetes, high blood pressure, and breathing difficulties.
Being underweight carries its own risks, including weakened immunity, nutritional deficiencies, and bone loss. The “healthy weight” range of 18.5 to 25 represents the zone where these population-level risks tend to be lowest, though individual health depends on far more than a single number.
Why BMI Can Be Misleading
BMI treats all weight the same. It doesn’t distinguish between fat, muscle, and bone. A competitive athlete with dense muscle tissue can easily land in the “overweight” or “obese” range while carrying very little body fat. An older adult who has lost muscle mass over the years might fall in the “healthy” range despite having a high percentage of body fat. Both get misclassified.
BMI also can’t tell you what kind of fat you have or where it sits on your body. This matters because fat stored around your midsection (visceral fat) is more metabolically active than fat around the hips and thighs. Visceral fat is closely tied to insulin resistance and raises the risk of heart disease, stroke, and diabetes. Two people with the same BMI can have very different risk profiles depending on where their fat is distributed.
In 2023, the American Medical Association adopted a new policy acknowledging these shortcomings. The AMA noted that BMI loses predictive power when applied to individuals rather than large groups, and that the standard categories were built on data that didn’t represent the full diversity of body types across racial and ethnic groups. The organization recommended that BMI be used alongside other measures rather than on its own.
BMI Thresholds Vary by Ethnicity
The standard cutoff points don’t capture risk equally across all populations. Research on Asian populations shows that metabolic risks like type 2 diabetes and cardiovascular disease begin rising at lower BMI levels. A WHO expert consultation identified potential action points at a BMI of 23 for increased risk and 27.5 for high risk in Asian populations, compared to 25 and 30 in the standard categories. The specific cutoff for observed risk varies between 22 and 25 depending on the population studied.
This means a person of South Asian or East Asian descent with a BMI of 24 may face health risks comparable to those of a person of European descent with a BMI of 27 or higher. Some healthcare systems have already begun using these adjusted thresholds in clinical practice.
Measures That Can Tell You More
If BMI is a rough sketch, other measurements add detail. Waist circumference is one of the simplest and most useful. Fat around the waist is more strongly linked to heart attacks, stroke, and diabetes than fat elsewhere on the body. Women with a waist larger than 35 inches and men with a waist larger than 40 inches face higher risk for heart disease and type 2 diabetes, according to the National Heart, Lung, and Blood Institute.
Waist-to-hip ratio may be even more informative. A study published in the Journal of the American Heart Association found that waist-to-hip ratio predicted heart attack risk better than BMI for both men and women. The advantage is that this measurement captures where fat is concentrated rather than just how much total weight you carry.
Other tools include body composition scans that separate fat mass from lean mass, and relative fat mass calculations that estimate body fat percentage from waist and height measurements. These are less commonly available than a simple scale and tape measure, but they give a fuller picture when precision matters.
How to Think About Your Own BMI
BMI is useful the way a weather forecast is useful: it gives you a general sense of conditions, not a guarantee of what you’ll experience. If your BMI is in the healthy range and you have no concerning symptoms, it’s one reassuring data point among many. If it’s elevated, it’s a signal to look deeper, not a verdict on your health.
The most practical approach is to pair your BMI with a waist measurement and pay attention to trends over time rather than fixating on a single number. A BMI that’s been creeping upward over several years tells a different story than one that has been stable. Context from your blood pressure, blood sugar, cholesterol levels, and family history fills in the rest. No single metric captures the full picture, and BMI was never designed to.