Obesity Classification by BMI
Obesity is classified into three distinct classes based on BMI: Class 1 (mild) is BMI 30-34.9 kg/m², Class 2 (moderate) is BMI 35.0-39.9 kg/m², and Class 3 (severe, extreme, or morbid) is BMI ≥40 kg/m². 1
Standard Adult BMI Classification
The American Heart Association provides the following weight classification system for adults 1, 2:
- Underweight: BMI <18.5 kg/m²
- Normal or acceptable weight: BMI 18.5-24.9 kg/m²
- Overweight: BMI 25-29.9 kg/m²
- Obese: BMI ≥30 kg/m²
Obesity Class Breakdown
Once the obesity threshold of BMI ≥30 kg/m² is reached, further stratification into three classes helps guide treatment intensity and predict disease risk 1, 3:
- Class 1 (Mild Obesity): BMI 30-34.9 kg/m² 1, 2
- Class 2 (Moderate Obesity): BMI 35.0-39.9 kg/m² 1, 2
- Class 3 (Severe Obesity): BMI ≥40 kg/m² 1, 2
Class 3 obesity is also referred to as severe, extreme, or morbid obesity and carries extremely high disease risk for cardiovascular disease, type 2 diabetes, and mortality 1, 4.
Extended Classification for Extreme Obesity
Some classification systems extend beyond the standard three classes to account for super-severe obesity 4:
These extended classifications represent the most rapidly growing segments of the obese population and are associated with extremely high disease risk 4.
Clinical Significance of Classification
The obesity class directly correlates with cardiovascular risk and mortality, with each increasing class conferring progressively higher morbidity and mortality risk 3, 4. Women with BMI 30-39.9 have cardiovascular event rates of 9.97 per 1000 person-years compared to 6.37 per 1000 person-years in normal-weight women 3.
Treatment recommendations escalate with obesity class 4:
- Class 1-2: Intensive lifestyle modification with consideration of pharmacotherapy
- Class 3 and above: Bariatric surgery is strongly recommended, particularly for BMI >50 kg/m² regardless of prior conservative interventions 4
Important Caveats
BMI classification has limitations and should be supplemented with waist circumference measurement 2, 3. The American Heart Association recommends measuring waist circumference to assess central adiposity, with thresholds of >102 cm (>40 inches) for men and >88 cm (>35 inches) for women indicating increased cardiovascular risk 2.
Ethnic-specific considerations are critical, particularly for Asian populations who develop obesity-related comorbidities at lower BMI thresholds 2, 3. For Asian populations, overweight may be defined as BMI ≥23 kg/m², with correspondingly lower obesity thresholds 2.
BMI has good specificity (90%) but low sensitivity (50%) for diagnosing excess adiposity and does not distinguish between lean and fat mass, potentially misclassifying individuals with high muscle mass 2.