A body mass index (BMI) of 30 or higher qualifies as obese for adults. BMI is calculated by dividing your weight in kilograms by your height in meters squared, and it remains the standard screening tool used by doctors worldwide. But the number 30 is only the starting point. Obesity is divided into classes, the thresholds shift for children and certain ethnic groups, and newer measures like waist circumference can change the picture entirely.
Adult BMI Categories
The CDC defines obesity for adults age 20 and older using four weight categories above normal:
- Overweight: BMI of 25 to less than 30
- Class 1 Obesity: BMI of 30 to less than 35
- Class 2 Obesity: BMI of 35 to less than 40
- Class 3 Obesity (Severe): BMI of 40 or higher
To put these numbers in practical terms, a person who is 5’9″ would cross into Class 1 obesity at roughly 203 pounds, Class 2 at about 236 pounds, and Class 3 at around 270 pounds. The World Health Organization uses the same BMI of 30 as its global threshold.
Class 3 obesity was previously called “morbid obesity,” a term that many medical organizations have moved away from because of its stigma. The clinical criteria haven’t changed: a BMI of 40 or higher, or in some cases a BMI of 35 or higher combined with a serious weight-related health condition like type 2 diabetes or sleep apnea.
Why BMI Doesn’t Tell the Whole Story
BMI is a ratio of weight to height. It doesn’t distinguish between muscle, fat, bone, and water. A heavily muscled person can have a BMI over 30 with relatively low body fat, while someone with a normal BMI can carry excess fat around their organs. This is why clinicians increasingly look at additional measurements rather than relying on BMI alone.
Two of the most useful supplementary measures are waist circumference and waist-to-height ratio. According to the National Heart, Lung, and Blood Institute, a waist circumference greater than 35 inches for women or greater than 40 inches for men increases risk for heart disease, type 2 diabetes, and other obesity-related conditions, even if your BMI falls below 30. The NHS recommends keeping your waist measurement to less than half your height as a general rule. If you’re 5’8″ (68 inches), that means your waist should stay under 34 inches.
These measurements capture something BMI misses: where your body stores fat. Fat concentrated around the abdomen (visceral fat) wraps around internal organs and is far more metabolically dangerous than fat stored in the hips and thighs. Two people with the same BMI can have very different health risks depending on their fat distribution.
Lower Thresholds for Asian Populations
The standard BMI cutoff of 30 was developed primarily from data on European-descent populations. People of Asian descent tend to develop obesity-related health problems at lower BMIs. The World Health Organization has proposed lowering the overweight threshold for Asian populations to a BMI of 23, compared to 25 for the general population. Many clinicians treating Asian American patients already use these adjusted cutoffs.
The reason is biological. At the same BMI, people of Asian descent generally carry a higher proportion of body fat and more visceral fat than people of European descent. This means the metabolic consequences of excess weight, including insulin resistance and cardiovascular risk, show up at a lower weight on the scale.
How Obesity Is Defined in Children
Children and teens are assessed differently from adults because their bodies are still growing and their body composition changes naturally with age. Instead of fixed BMI cutoffs, the CDC uses age-and-sex-specific growth charts for anyone between 2 and 19 years old.
A child or teen is considered obese if their BMI falls at or above the 95th percentile for their age and sex. Severe obesity in children is defined as a BMI at or above 120% of the 95th percentile, or a BMI of 35 or higher, whichever is lower. The American Academy of Pediatrics further breaks severe obesity into Class 2 (120% to less than 140% of the 95th percentile) and Class 3 (140% of the 95th percentile or above).
These percentile-based definitions mean that a 10-year-old boy and a 16-year-old girl could have the same BMI number but fall into completely different categories. The comparison is always against other children of the same age and sex, not against a single universal number.
Metabolically Healthy Obesity
Not everyone who qualifies as obese by BMI has the metabolic problems typically associated with excess weight. Researchers have identified a subgroup sometimes called “metabolically healthy obesity,” where a person has a BMI of 30 or above but normal blood pressure, normal blood sugar, healthy cholesterol and triglyceride levels, and no signs of insulin resistance.
This concept is real but comes with a significant caveat. Long-term studies consistently show that many people who appear metabolically healthy at one point in time go on to develop metabolic problems within 5 to 10 years. The condition appears to be more of a transitional phase than a permanent state for most people. Still, it highlights an important point: BMI alone doesn’t predict individual health. Someone with a BMI of 32, normal blood pressure, good cholesterol numbers, and an active lifestyle is in a very different situation from someone with the same BMI who has elevated blood sugar and high triglycerides.
Which Measurement Matters Most
No single number captures whether someone is obese in a way that perfectly predicts their health. BMI is the most widely used measure because it’s simple and requires only a scale and a tape measure. Waist circumference adds information about where fat is stored. Waist-to-height ratio performs well as a quick screen for metabolic risk across different body types and ethnic groups.
If you’re trying to figure out where you stand, start with BMI as a baseline. Then measure your waist at the narrowest point above your hip bones (usually at the level of your navel). If your BMI is between 25 and 35, waist circumference becomes especially informative, because it can distinguish between people in that range who carry most of their weight around their middle and those who don’t. For people with a BMI above 35, the additional health risks are generally present regardless of where the fat sits.
Your doctor may also look at blood pressure, fasting blood sugar, and cholesterol panels to get a fuller picture. The obesity classification on its own is a starting point for understanding risk, not a complete diagnosis.