BMI (Body Mass Index): definition, formula, and what it doesn't tell you

Definition

Body Mass Index (BMI) is a numerical ratio of a person’s weight to their height squared:

BMI = weight in kilograms ÷ (height in meters)²

Or in US units: BMI = (weight in pounds × 703) ÷ (height in inches)²

WHO classification

Range	Category
<18.5	Underweight
18.5 – 24.9	Normal weight
25.0 – 29.9	Overweight
30.0 – 34.9	Obesity class I
35.0 – 39.9	Obesity class II
≥40	Obesity class III (“severe”)

For East Asian populations, the WHO recommends lower cutoffs (overweight ≥23, obese ≥27) due to differential disease risk at lower BMI values.

What BMI is good for

BMI was developed by Adolphe Quetelet in the 1830s as a population statistics tool. It’s still useful for that purpose: comparing populations, tracking trends, large-scale epidemiology. It’s also a reasonable first-pass screen at the clinical level for adults outside the extremes.

What BMI doesn’t tell you

BMI fails as an individual health metric in predictable ways:

• It doesn’t distinguish muscle from fat. Athletes with significant muscle mass routinely register as “overweight” or “obese” by BMI despite low body fat.
• It doesn’t account for fat distribution. Two people with identical BMI can have dramatically different visceral fat — and visceral fat is what drives most cardiometabolic risk. See our visceral fat measurement guide.
• It misclassifies older adults. Sarcopenia (age-related muscle loss) can produce “normal BMI” with significant fat percentage.
• It was developed in European populations. Cutoffs translate imperfectly to populations with different body proportions.
• “Normal BMI” doesn’t guarantee metabolic health. Roughly 30% of US adults with normal BMI have at least one cardiometabolic risk factor [^2].

Better individual metrics

For individual cardiometabolic risk assessment, more useful than BMI:

• Waist-to-height ratio (waist circumference should be less than half your height)
• Waist circumference (>35” women, >40” men → elevated risk)
• Body composition via DEXA scan
• Cardiometabolic labs (fasting glucose, A1c, lipid panel, blood pressure)

Related glossary entries

• Visceral fat
• Insulin resistance