Obesity is most commonly calculated using Body Mass Index (BMI), a simple formula that uses your weight and height. A BMI of 30 or higher classifies an adult as obese. But BMI is just one method, and it has well-known blind spots. Waist circumference, body fat percentage, and newer formulas like Relative Fat Mass can give you a more complete picture.
The BMI Formula
BMI divides your weight by the square of your height. The result is a single number that places you into a weight category. Here are the two versions of the formula depending on your units:
- Metric: BMI = weight in kilograms ÷ (height in meters × height in meters)
- Imperial: BMI = (weight in pounds ÷ (height in inches × height in inches)) × 703
For example, if you weigh 200 pounds and stand 5 feet 8 inches tall (68 inches), the math looks like this: 200 ÷ (68 × 68) = 0.04325, then multiply by 703 to get a BMI of about 30.4. That falls just into the obesity range.
BMI Categories for Adults
The CDC uses these BMI ranges for adults:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or higher
These classes matter because health risks increase with each tier. Someone with a BMI of 31 faces different statistical risks than someone at 42, even though both fall under the obesity umbrella.
Adjusted Thresholds for Asian Populations
The standard BMI cutoffs were developed primarily from data on white European populations. People of Asian descent tend to develop obesity-related health problems at lower BMI levels, so a WHO expert panel proposed different thresholds: overweight starts at 23 (instead of 25), and obesity starts at 27.5 (instead of 30). If you’re of East Asian, Southeast Asian, or South Asian descent, these lower cutoffs give a more accurate picture of your metabolic risk.
How BMI Is Calculated for Children
BMI works differently for kids and teens ages 2 through 19. The formula itself is the same, but instead of fixed cutoffs, the result is compared to growth charts that account for age and sex. A child’s BMI is expressed as a percentile, showing where they fall relative to other children of the same age.
- Healthy weight: 5th to 84th percentile
- Overweight: 85th to 94th percentile
- Obesity: 95th percentile or above
- Severe obesity: 120% of the 95th percentile or above
A 10-year-old boy and a 16-year-old girl with the same BMI number could fall into completely different categories because their reference populations differ. This is why pediatric BMI always requires a growth chart rather than a simple number comparison.
Where BMI Falls Short
BMI cannot tell the difference between fat, muscle, and bone. It also says nothing about where your body stores fat. This creates real problems for certain groups. A muscular athlete can easily land in the “obese” BMI range while carrying very little body fat. On the other end, an older adult who has lost significant muscle mass might have a “normal” BMI while carrying a high proportion of body fat, particularly around the organs.
Because of these limitations, BMI works best as a starting point. Combining it with at least one other measurement gives you a much clearer picture.
Measuring Waist Circumference
Waist circumference captures something BMI misses entirely: how much fat you carry around your midsection. Abdominal fat wraps around internal organs and is more metabolically active than fat stored in your hips or thighs, which makes it a stronger predictor of heart disease and type 2 diabetes risk.
To measure correctly, place a tape measure at the top of your hip bone (the bony ridge you can feel at your side) or at the midpoint between your lowest rib and that hip bone. Stand up straight, breathe out normally, and read the tape. Don’t pull it tight enough to compress the skin.
The general thresholds that signal elevated health risk for white adults are a waist circumference above 40 inches (102 cm) for men and above 35 inches (88 cm) for women. These thresholds vary by ethnicity. For Chinese men and women, the cutoff is lower at about 31.5 inches (80 cm). For Korean men, it’s about 35.4 inches (90 cm), and for Korean women, about 33.5 inches (85 cm). For people of Asian Indian descent, the thresholds are approximately 35.4 inches for men and 31.5 inches for women.
Waist circumference also becomes more useful when paired with your BMI category. A normal-weight man with a waist of 90 cm or more already faces elevated cardiovascular risk, even though his BMI looks fine. A person with Class 1 obesity (BMI 30 to 34.9) doesn’t hit the higher-risk waist threshold until 110 cm for men or 105 cm for women, because some abdominal size is already expected at that BMI.
Body Fat Percentage
Body fat percentage directly measures what BMI only estimates: the proportion of your body that is fat tissue. Obesity is generally defined as body fat above 25% for men and above 30% for women. This is a more precise measurement than BMI, but it’s harder to get accurately at home.
Consumer-level methods include bioelectrical impedance scales (the kind that send a small electrical current through your feet) and skinfold calipers. Both can give you a rough trend over time, but individual readings can swing by several percentage points depending on your hydration, when you last ate, or who is taking the measurement. Clinical methods like DEXA scans, which use low-dose X-rays, are far more precise and can also show you exactly where fat is distributed throughout your body.
Relative Fat Mass: A Newer Alternative
Introduced in 2018, Relative Fat Mass (RFM) estimates your body fat percentage using only your height and waist circumference, with no scale required. The formula is:
RFM = 64 − (20 × height/waist circumference) + (12 × sex), where sex equals 0 for males and 1 for females. Height and waist circumference use the same unit (both in meters or both in inches).
RFM has two practical advantages over BMI. First, because it uses waist circumference, it captures abdominal fat more accurately. Second, because it doesn’t use body weight at all, it avoids the muscle mass problem that trips up BMI in athletic or heavily muscled individuals. Studies comparing RFM to DEXA scans show strong agreement, making it a surprisingly reliable estimate from just a tape measure and a height measurement.
RFM doesn’t yet have universally adopted obesity cutoffs the way BMI does, but because it outputs an estimated body fat percentage, you can interpret the result using the same body fat thresholds: roughly above 25% for men and above 30% for women suggests obesity.
Which Method to Use
For a quick personal check, BMI is the easiest calculation and the one your doctor is most likely to reference. Adding a waist circumference measurement takes about 30 seconds and meaningfully improves the picture, especially if your BMI lands in the overweight or Class 1 obesity range where the health implications are less clear-cut. If you want a single number that better reflects actual body fat without needing specialized equipment, RFM is worth calculating.
No single metric captures the full story. Someone with a BMI of 29, a large waist circumference, and high body fat percentage faces greater health risks than their BMI alone suggests. Someone with a BMI of 31 who carries that weight as muscle and has a moderate waist measurement is in a very different situation. Using two or three of these tools together gives you the most honest assessment.