The World Health Organization classifies adult BMI into four main categories: underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or above). These thresholds, based on body weight relative to height, are used worldwide as a screening tool for weight-related health risks. They apply to adults of all ages, though separate standards exist for children and different cutoffs have been proposed for certain ethnic groups.
How BMI Is Calculated
BMI is your weight divided by your height squared. In metric units, that means your weight in kilograms divided by your height in meters, squared. In imperial units, you divide your weight in pounds by your height in inches squared, then multiply by 703. A person who weighs 70 kg and stands 1.75 m tall, for example, has a BMI of about 22.9.
The number itself has no units you need to worry about. It’s simply expressed as kg/m², and it lands you in one of the categories below.
The Standard Adult Categories
- Underweight: BMI below 18.5
- Healthy weight: BMI 18.5 to 24.9
- Overweight: BMI 25 to 29.9
- Obesity Class 1: BMI 30 to 34.9
- Obesity Class 2: BMI 35 to 39.9
- Obesity Class 3 (severe obesity): BMI 40 or above
The line between healthy weight and overweight sits at 25, and the line between overweight and obese sits at 30. These two thresholds are the most widely referenced in public health guidelines, clinical screening, and insurance assessments. The three obesity classes exist because health risks, including the likelihood of heart disease, type 2 diabetes, and certain cancers, rise substantially as BMI climbs further above 30.
Lower Cutoffs for Asian Populations
The standard thresholds were developed largely from data on European populations. A WHO expert consultation found that Asian adults face increased risks of type 2 diabetes and cardiovascular disease at lower BMI values. The reason: Asian populations tend to carry a higher percentage of central body fat at any given BMI compared to non-Asian populations.
Based on this evidence, the WHO’s Western Pacific Regional Office proposed adjusted cutoffs for Asian adults: overweight starting at 23 (instead of 25) and obesity starting at 25 (instead of 30). Obesity expert groups from South and Southeast Asia have endorsed the same thresholds. If you’re of Asian descent, these lower cutoffs may give a more accurate picture of your metabolic risk than the standard chart.
How BMI Works for Children and Teens
BMI means something different for kids because body composition changes rapidly during growth. For children and adolescents aged 5 to 19, the WHO uses BMI-for-age charts that compare a child’s BMI to a reference population of the same age and sex. Instead of fixed cutoff numbers, the system uses standard deviations from the median.
A child more than one standard deviation above the median is classified as overweight; more than two standard deviations, obese. On the other end, less than two standard deviations below the median counts as thinness, and less than three is severe thinness. These thresholds are calibrated so that by age 19, the overweight and obesity cutoffs align with the adult values of 25 and 30.
What BMI Does Not Tell You
BMI measures excess weight, not excess fat. It cannot distinguish between fat, muscle, and bone mass, and it says nothing about where fat is distributed on your body. That distinction matters because fat stored around the abdomen (visceral fat) poses greater health risks than fat stored in the hips or thighs.
This creates well-known blind spots. A highly trained athlete with significant muscle mass can register as overweight or even obese by BMI despite carrying very little body fat. Conversely, an older adult who has lost muscle mass might fall in the healthy range while actually carrying a high proportion of body fat. Age, sex, and ethnicity all influence the relationship between BMI and actual body fatness, which is why clinicians treat BMI as a starting point rather than a diagnosis.
Waist circumference is one common supplement. WHO identifies a waist measurement above 88 cm (about 34.6 inches) for women or above 102 cm (about 40.2 inches) for men as a marker of increased metabolic risk, regardless of BMI. Combining both measurements gives a better picture of where you stand than either number alone.
Why These Thresholds Still Matter
Despite its limitations, BMI remains the most widely used population-level screening tool for weight-related health risk. It requires no special equipment, no blood test, and no clinical visit. At the 2022 World Health Assembly, WHO member states adopted new recommendations for obesity prevention and management that continue to rely on BMI classification as the foundation for identifying who needs further assessment.
The practical takeaway: if your BMI falls in the overweight or obese range, it’s a signal that additional evaluation, such as waist circumference, blood pressure, blood sugar, and cholesterol levels, can clarify your actual risk. If it falls in the healthy range but you have other risk factors, such as a family history of diabetes or a large waist measurement, those deserve attention too. BMI is a useful first filter, not the final word.