What BMI Measures
BMI (Body Mass Index) is a screening tool that measures the relationship between a person's weight and height, calculated as weight in kilograms divided by height in meters squared (kg/m²), and is used to assess total body adiposity and classify individuals into weight categories. 1
Calculation Methods
BMI can be calculated using three different formulas depending on the units of measurement available:
- Metric formula: weight (kg) / [height (m)]² 1
- Metric alternative: weight (kg) / height (cm) / height (cm) × 10,000 1
- Imperial formula: weight (lb) / height (in) / height (in) × 703 1
Classification Categories
The standard BMI classifications for adults are 1:
- Underweight: BMI < 18.5 kg/m²
- Normal weight: BMI 18.5-24.9 kg/m²
- Overweight: BMI 25.0-29.9 kg/m²
- Obesity Class I: BMI 30.0-34.9 kg/m²
- Obesity Class II: BMI 35.0-39.9 kg/m²
- Obesity Class III (Severe Obesity): BMI ≥ 40.0 kg/m² 2
What BMI Actually Measures vs. What It Doesn't
BMI measures total body weight indexed to height, but it does NOT distinguish between lean mass (muscle) and fat mass. 1 This is a critical limitation because:
- The numerator in the BMI calculation is "total" body weight without differentiating tissue types 1
- Individuals with normal weight but excess body fat may not be diagnosed as overweight or obese 1
- Adults with high levels of lean body mass (such as athletes) may be misclassified as overweight or obese 1
- BMI does not measure body fat distribution, particularly dangerous abdominal fat versus gluteofemoral fat 3
Accuracy as a Screening Tool
BMI has high specificity (90-99%) but poor sensitivity (36-50%) for detecting obesity, meaning it correctly identifies most non-obese individuals but misses approximately half of those with excess body fat. 1, 4
Specific accuracy data shows 1:
- In men: 95% specificity but only 36% sensitivity for detecting obesity (defined as ≥25% body fat)
- In women: 99% specificity but only 49% sensitivity for detecting obesity (defined as ≥35% body fat)
- 30% of men and 46% of women with BMI < 30 kg/m² actually have obesity levels of body fat 5
Clinical Utility Despite Limitations
BMI remains the most practical and widely recommended screening tool in clinical practice because it is easy to measure, highly reliable, and strongly correlated with adverse health outcomes. 1 The correlation with body fat mass is high (R² = 0.95 in men; R² = 0.98 in women) 1, and multiple epidemiological studies demonstrate that every 5-unit increase in BMI above 25 kg/m² is associated with a 30% increase in all-cause mortality 1.
Important Caveats for Specific Populations
BMI interpretation varies by 1:
- Age: In elderly patients, BMI correlates less strongly with body fat percentage due to higher proportions of internal fat 1
- Sex: At similar BMI levels, women typically have higher percentages of body fat than men 1
- Race/Ethnicity: Hispanic women have higher body fat percentages than Black and White women at similar BMIs, while Black women have lower body fat percentages than White women at the same BMI 1
- Children and adolescents: BMI must be evaluated against age- and gender-specific reference charts, with overweight defined as ≥85th percentile and obesity as ≥95th percentile 1
When Additional Measurements Are Needed
For patients with BMI < 30 kg/m², direct measurement of body fat is more appropriate to accurately assess obesity, as BMI has the lowest predictive accuracy in this range. 5 Additionally, waist circumference should be measured to assess central adiposity, with increased cardiovascular risk defined as >102 cm (>40 inches) in men and >88 cm (>35 inches) in women 1.