What is the body mass index (BMI) range for obesity in an adult patient?

BMI Definition for Obesity in Adults

Obesity is defined as a body mass index (BMI) of 30 kg/m² or greater in adults. 1, 2, 3

Standard BMI Classification

The World Health Organization and major medical societies use the following BMI categories for adults 1, 2, 3:

• Normal weight: BMI 18.5-24.9 kg/m²
• Overweight: BMI 25-29.9 kg/m²
• Obesity: BMI ≥30 kg/m²

Obesity Subclassification by Severity

Obesity is further subdivided into three classes based on increasing health risk 1, 3:

• Class 1 (Mild): BMI 30-34.9 kg/m²
• Class 2 (Moderate): BMI 35-39.9 kg/m²
• Class 3 (Severe/Morbid): BMI ≥40 kg/m²

BMI Calculation Methods

BMI is calculated using one of two formulas 1, 2:

• Metric: Weight (kg) ÷ [Height (m)]²
• Imperial: [Weight (pounds) ÷ Height (inches)²] × 703

Ethnic-Specific Adjustments

For Asian populations, lower BMI thresholds should be used because they have greater adiposity and higher comorbidity risks at lower BMI levels 1, 2, 3:

• Overweight in Asians: BMI ≥23-25 kg/m²
• Obesity in Asians: BMI ≥25-27.5 kg/m²

Critical Limitations of BMI

BMI has good specificity (90%) but poor sensitivity (50%) for diagnosing excess adiposity, meaning it misses more than half of people with excess body fat 2, 3, 4. The American Heart Association notes that BMI does not distinguish between lean and fat mass, potentially misclassifying individuals with high muscle mass as overweight or obese 2, 3.

Essential Additional Measurement: Waist Circumference

Waist circumference must be measured in addition to BMI to assess central adiposity and cardiovascular risk 1, 2, 3:

• Men: >102 cm (>40 inches) indicates increased cardiovascular risk
• Women: >88 cm (>35 inches) indicates increased cardiovascular risk
• Asian men: ≥90 cm indicates abdominal obesity 3
• Asian women: ≥80 cm indicates abdominal obesity 3

Note: Waist circumference thresholds are not reliable for patients with BMI >35 1.

Clinical Treatment Thresholds

Weight loss treatment is recommended based on the following algorithm 2, 3:

• BMI 18.5-24.9 without risk factors: Maintain current weight, avoid weight gain
• BMI 25-29.9 without cardiovascular risk factors: Avoid further weight gain through regular weight monitoring and physical activity
• BMI ≥25 with cardiovascular risk factors OR BMI ≥30: Weight loss treatment recommended with initial goal of 10% body weight reduction

Pharmacotherapy with anti-obesity medications should be considered for any patient with BMI ≥30 kg/m² or BMI ≥27 kg/m² with metabolic comorbidities 1, 5, 6.

Bariatric surgery should be considered for patients with BMI ≥35 kg/m² or BMI ≥30 kg/m² with obesity-related comorbidities 1, 3.