BMI is a screening tool, not a diagnostic measure. It does not account for muscle mass, bone density, age, sex, or fat distribution. Athletes and muscular individuals may have a high BMI without excess body fat. Consult a healthcare professional for a complete health assessment.
WHO BMI Classification
The World Health Organization (WHO) defines the following BMI categories for adults over 20 years old. These ranges apply regardless of age or sex.
Category
BMI Range (kg/m²)
Severe Thinness
< 16.0
Moderate Thinness
16.0 – 16.9
Mild Thinness
17.0 – 18.4
Normal
18.5 – 24.9
Overweight
25.0 – 29.9
Obese Class I
30.0 – 34.9
Obese Class II
35.0 – 39.9
Obese Class III
≥ 40.0
Health Risks by BMI Category
Risks of Overweight & Obesity
High blood pressure (hypertension)
Type 2 diabetes
Coronary heart disease
Stroke
Sleep apnea
Osteoarthritis
Certain cancers (breast, colon, kidney)
Fatty liver disease
High cholesterol
Risks of Being Underweight
Malnutrition and vitamin deficiencies
Osteoporosis (weakened bones)
Weakened immune system
Anemia
Fertility issues
Growth and development delays
Increased surgical complications
How BMI Is Calculated
BMI is calculated by dividing your weight by the square of your height. The formula differs slightly between metric and imperial units:
Historical note: BMI was invented in the 1830s by Belgian mathematician Adolphe Quetelet — not as a health metric, but as a statistical tool for studying populations. It was never designed to assess individual health.
Metric
BMI = weight (kg) ÷ height (m)²
Example: 70 kg ÷ 1.75² = 70 ÷ 3.0625 = 22.9
Imperial
BMI = 703 × weight (lbs) ÷ height (in)²
Example: 703 × 154 ÷ 69² = 108,262 ÷ 4,761 = 22.7
BMI Prime & Ponderal Index
BMI Prime is the ratio of your BMI to the upper limit of normal (25). A BMI Prime of 1.0 means you are exactly at the boundary. Values below 1.0 are in the normal or underweight range; above 1.0 means overweight or obese.
Ponderal Index (also called Corpulence Index) uses height cubed instead of squared, which makes it more accurate for very tall or very short individuals. Normal range is 11–15 kg/m³. Unlike BMI, the Ponderal Index does not systematically overestimate body fat in tall people or underestimate it in short people.
Limitations of BMI
Athletes beware: Dwayne "The Rock" Johnson (BMI ~34) and most NFL linemen (BMI 30+) would be classified as "obese" by BMI despite having low body fat percentages. BMI cannot distinguish between muscle and fat.
Muscle mass: Athletes and bodybuilders may be classified as overweight despite having low body fat, because BMI cannot distinguish between muscle and fat.
Age: Older adults tend to have more body fat than younger adults at the same BMI. BMI may underestimate body fat in elderly individuals.
Sex: Women typically have more body fat than men at an equivalent BMI.
Ethnicity: Health risks at a given BMI vary across populations. For example, Asian populations may face higher risks at lower BMI thresholds.
Fat distribution: BMI does not indicate where fat is stored. Abdominal (visceral) fat is more dangerous than subcutaneous fat, but BMI cannot measure this. Waist circumference is a better indicator of visceral fat.
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