A body mass index (BMI) of 30 or higher is considered obese for adults. BMI is calculated by dividing your weight in kilograms by your height in meters squared, and most doctor’s offices or online calculators can give you your number in seconds. But BMI is just one way to define obesity, and the thresholds shift depending on your age, sex, and ethnic background.
BMI Categories for Adults
The CDC breaks adult obesity into three classes based on BMI:
- Class 1 obesity: BMI of 30 to 34.9
- Class 2 obesity: BMI of 35 to 39.9
- Class 3 (severe) obesity: BMI of 40 or higher
For reference, a BMI between 25 and 29.9 falls into the overweight category, and 18.5 to 24.9 is considered a healthy weight. To put those numbers in practical terms, a 5’9″ person crosses into the obese range at roughly 203 pounds. A 5’4″ person hits that same threshold at about 175 pounds.
Class 3 obesity, formerly called “morbid obesity,” carries the highest risk of weight-related health problems. That label applies at a BMI of 40 or above, which for that same 5’9″ person would be around 270 pounds.
Lower Thresholds for Asian Populations
The standard BMI cutoffs don’t apply equally across all ethnic groups. People of Asian descent tend to carry more body fat at lower BMIs and develop conditions like type 2 diabetes and heart disease at lighter weights. A WHO expert panel reviewed the data from Asian countries in 2002 and proposed lower cutoffs: a BMI of 23 to 27.4 for overweight and 27.5 or higher for obese. If you’re of East Asian, Southeast Asian, or South Asian descent, these adjusted numbers give a more accurate picture of your health risk than the standard 30 threshold.
How Obesity Is Defined in Children
Children and teens are measured differently because their bodies are still growing. Instead of a fixed BMI number, doctors compare a child’s BMI to others of the same age and sex using growth charts. A child is considered obese if their BMI falls at or above the 95th percentile for their age group. That means they have a higher BMI than 95 out of 100 kids the same age and sex. Overweight for children starts at the 85th percentile.
Waist Circumference as a Risk Marker
Where you carry your weight matters as much as how much you weigh. Fat stored around the midsection, called visceral fat, wraps around internal organs and is more metabolically dangerous than fat stored in the hips or thighs. That’s why waist circumference is used alongside BMI to assess risk.
The most widely used thresholds in the U.S. are a waist measurement over 35 inches for women and over 40 inches for men. Exceeding these numbers signals elevated risk for heart disease, type 2 diabetes, and other complications, even in people whose BMI might not technically reach the obese range.
These thresholds also vary by ethnicity. For example, recommendations for Japanese men set the cutoff at about 33.5 inches, while for Japanese women it’s roughly 35.4 inches. South Asian and Chinese populations use a cutoff of about 31.5 inches for women. If your waist measurement is high relative to your ethnic group’s threshold, it’s a meaningful signal regardless of what the scale says.
Body Fat Percentage
BMI doesn’t actually measure fat. It’s a ratio of height to weight, and it can’t distinguish between fat, muscle, and bone. Body fat percentage offers a more direct measurement. Research comparing body fat levels to metabolic risk suggests that obesity corresponds to roughly 30% body fat in men and 42% in women. Overweight ranges start at about 25% for men and 36% for women.
Body fat can be estimated through methods like skinfold calipers, bioelectrical impedance scales (the kind found in many home scales), or more precise tools like DEXA scans. None of these are as quick as stepping on a scale and plugging your weight into a BMI formula, which is part of why BMI remains the default screening tool despite its limitations.
Why BMI Can Be Misleading
BMI is a screening tool, not a diagnosis. The CDC acknowledges that it doesn’t distinguish between fat, muscle, and bone mass. A muscular athlete with low body fat can register as obese by BMI alone. An older adult who has lost muscle but gained fat might have a “normal” BMI while carrying an unhealthy amount of body fat.
This is one reason the medical field is shifting toward a broader view. The 2025 Standards of Care in Overweight and Obesity, developed through the American Diabetes Association’s Obesity Association, emphasize a “complication-centric” approach. Rather than focusing solely on weight, these guidelines consider the actual health consequences a person is experiencing, things like high blood sugar, joint problems, sleep apnea, or fatty liver disease. Two people with the same BMI can have very different health profiles, and treatment decisions increasingly reflect that.
Putting the Numbers Together
No single measurement captures the full picture. BMI is the quickest way to screen for obesity, but it works best when combined with waist circumference and, when available, body fat percentage. Someone with a BMI of 28 but a large waist circumference and high body fat may face more health risk than someone with a BMI of 31 who carries weight in their legs and has healthy metabolic markers.
If your BMI puts you at or above 30, or your waist circumference exceeds the thresholds for your sex and ethnic background, those are signals worth paying attention to. They don’t define your health on their own, but they point to the areas where closer attention and proactive changes can make the biggest difference.