The BMI Formula
Body Mass Index is weight divided by height squared. The metric version:
BMI = weight (kg) ÷ height (m)²
For imperial units — pounds and inches — a conversion factor is added:
BMI = (weight (lbs) × 703) ÷ height (in)²
Example: a person who weighs 185 lbs and stands 5′10″ (70 inches) has a BMI of (185 × 703) ÷ 70² = 130,055 ÷ 4,900 = 26.5.
BMI Categories
The World Health Organization defines four ranges for adults 20 and older:
These cut-offs are the same for men and women under the standard WHO classification, though they don't adjust for age or body composition.
Asian BMI Cut-Offs
The WHO also recognizes that the standard thresholds may underestimate health risk in some Asian populations. Several countries and health authorities use lower cut-offs:
At the same BMI, people of East or South Asian descent tend to carry a higher percentage of body fat and show metabolic risk factors at lower weights than the population data that set the standard thresholds.
What BMI Doesn't Measure
BMI is a ratio of weight to height. It has no information about where weight comes from — muscle, fat, bone density, or fluid. This produces well-known anomalies: most elite athletes score "overweight" or even "obese" by standard BMI. A 6′3″ professional with 5% body fat and 240 lbs of lean mass lands at BMI 30. The number is correct; the label is not meaningful for that person.
The metric also doesn't adjust for age (older adults carry more fat at the same BMI), sex (women generally have higher body fat percentages at the same BMI), or fat distribution (abdominal fat carries more cardiovascular risk than hip or thigh fat).
How to Interpret Your Result
A number outside the normal range is a prompt to look further — not a diagnosis. Normal-weight BMI with a large waist circumference, poor diet, or sedentary lifestyle carries real risk. An overweight BMI with high muscle mass, normal blood pressure, and good metabolic markers carries far less.
If your BMI is outside the normal range and you're concerned, the next steps are a conversation with a doctor — ideally including waist circumference, blood pressure, fasting glucose, and lipid panel — rather than treating the BMI alone as actionable.