Body Mass Index (BMI) is a value calculated from a person's weight and height. It provides a reliable indicator of body fatness for most adults and is used globally to screen for weight categories that may lead to health problems. Anyone tracking their weight, a clinician doing a quick assessment, or an insurer estimating risk may reach for it first.
The formula. BMI = weight (kg) ÷ height (m)², where weight is in kilograms and height in metres. In imperial units the equivalent is BMI = 703 × weight (lb) ÷ height (in)². The result is always expressed in kg/m².
Worked example. A person weighing 70 kg who is 1.75 m tall has a BMI of 70 ÷ (1.75)² = 70 ÷ 3.0625 ≈ 22.9 kg/m², which sits comfortably in the WHO normal-weight range of 18.5–24.9. Their healthy weight range at that height is about 56.7–76.3 kg.
BMI Prime is the ratio of actual BMI to the upper limit of normal BMI (25). A BMI Prime of 1.0 corresponds exactly to a BMI of 25. Values below 0.74 indicate underweight; values above 1.0 indicate overweight.
Where it's used. Because it needs only height and weight, BMI is the standard first-pass screen in clinics, insurance, public-health surveys and fitness apps. The WHO categories above let practitioners flag possible underweight or obesity and decide whether to follow up with measures like waist circumference or body-fat percentage.
Limitations: BMI does not directly measure body fat. Athletes with high muscle mass may have a high BMI without excess fat. It also does not account for age, sex, ethnicity, or body composition. Always consult a healthcare professional for a complete assessment.