A BMI of 30 or higher is classified as obese for adults. BMI, short for body mass index, is a number calculated from your weight and height. The CDC breaks obesity into three classes based on severity, each carrying progressively higher health risks.
BMI Ranges for Obesity
The standard adult BMI categories start at age 20. Here’s how obesity breaks down:
- Class 1 Obesity: 30.0 to 34.9
- Class 2 Obesity: 35.0 to 39.9
- Class 3 (Severe) Obesity: 40.0 or higher
For context, the overweight range sits just below at 25.0 to 29.9. A “normal” BMI falls between 18.5 and 24.9. These thresholds were set based on population-level data linking BMI ranges to disease risk, though they don’t work equally well for everyone (more on that below).
How to Calculate Your BMI
BMI divides your weight by the square of your height. If you’re using pounds and inches, the formula is: weight in pounds divided by height in inches squared, then multiplied by 703. In metric units, it’s simply weight in kilograms divided by height in meters squared.
As a quick example: a person who is 5’9″ (69 inches) and weighs 203 pounds has a BMI of 30.0, right at the obesity threshold. That same person would need to weigh under 169 pounds to reach a “normal” BMI. Online calculators from the CDC do the math instantly if you’d rather skip the arithmetic.
What Health Risks Rise With an Obese BMI
The class distinctions aren’t just labels. Each step up in BMI correlates with measurably higher rates of conditions like type 2 diabetes, high blood pressure, heart disease, sleep apnea, and certain cancers. A large electronic health records study found that the risk of developing type 2 diabetes roughly quadrupled above a BMI of about 31, jumping from 0.6 new cases per 100 people per year below that point to 2.5 cases above it. For high blood pressure, elevated risk kicked in even earlier, around a BMI of 28.4, which is still technically in the overweight range.
This is important because it shows that health risks don’t flip on like a switch at exactly 30. They increase on a gradient. Someone at 29 isn’t dramatically healthier than someone at 31. The cutoff of 30 is a useful clinical marker, but your personal risk depends on other factors too, including where you carry your weight, your family history, and your metabolic health.
Why BMI Doesn’t Tell the Whole Story
BMI measures total body mass relative to height. It cannot distinguish between muscle, fat, bone, and water. This creates two well-known blind spots.
First, muscular people often register as overweight or obese despite having low body fat. A 200-pound person with significant muscle mass and a 200-pound person with excess body fat will get the same BMI, even though their health profiles look very different.
Second, BMI can miss health risks in older adults. As people age, they tend to lose muscle and gain fat simultaneously. Because those changes can offset each other on the scale, BMI stays stable while body composition shifts in an unhealthy direction. Researchers call this combination of low muscle mass and high fat mass “sarcopenic obesity,” and it’s linked to frailty and metabolic problems that BMI alone won’t flag.
Waist Circumference as an Added Measure
Because BMI can’t tell you where fat is stored, waist circumference provides a useful second data point. Fat carried around the midsection (visceral fat) is more strongly linked to heart disease and diabetes than fat stored in the hips or thighs.
General thresholds that signal increased health risk are about 40 inches (102 cm) for men and 35 inches (88 cm) for women. However, research suggests these thresholds actually vary by BMI category. For someone already in the Class 1 obesity range, a waist circumference above 43 inches (110 cm) for men or 41 inches (105 cm) for women indicates elevated risk beyond what BMI alone captures. Using both measurements together gives a more accurate picture than either one on its own.
Different Thresholds for Asian Populations
The standard BMI cutoff of 30 was developed primarily from data on white European populations. People of Asian descent tend to develop obesity-related metabolic problems at lower BMIs. The WHO’s Western Pacific Regional Office uses a lower threshold: a BMI of 25 or higher is classified as obese, and overweight starts at 23. These adjusted cutoffs better predict conditions like diabetes and high blood pressure in East Asian and South Asian populations, where dangerous visceral fat accumulation occurs at lighter body weights.
How Childhood Obesity Is Measured Differently
For children and teens ages 2 through 19, obesity isn’t defined by a fixed BMI number. Instead, a child’s BMI is compared to other children of the same age and sex using growth charts. A child at or above the 95th percentile is classified as having obesity. Severe obesity is defined as 120% or more of the 95th percentile value, or a BMI of 35 or higher, whichever is lower. This percentile-based approach accounts for the fact that healthy body composition changes dramatically as children grow.