Body Mass Index (BMI) is a simple calculation using height and weight to estimate body fat. The formula is: BMI = weight (kg) / height (m)². In imperial units: BMI = (weight in pounds × 703) / (height in inches)². For example, a person who is 5'10" (70 inches) and weighs 170 pounds has a BMI of (170 × 703) / (70²) = 24.4.

BMI was developed in the 1830s by Belgian mathematician Adolphe Quetelet as a simple way to measure obesity at the population level. It remains widely used because it's easy to calculate and requires only basic measurements. Healthcare providers use BMI as an initial screening tool to identify potential weight-related health risks.

The World Health Organization defines BMI categories as: Underweight (below 18.5), Normal weight (18.5-24.9), Overweight (25-29.9), and Obese (30 and above). Obesity is further divided into Class I (30-34.9), Class II (35-39.9), and Class III (40+). These categories help assess risk for conditions like heart disease, diabetes, and high blood pressure.

Different BMI ranges are associated with different health risks. Underweight individuals (BMI below 18.5) may face nutritional deficiencies, weakened immune function, and bone density issues. This category requires medical evaluation to rule out underlying conditions.

Normal weight (BMI 18.5-24.9) is associated with the lowest health risks from weight-related conditions. However, BMI doesn't account for body composition, so someone in this range could still have unhealthy body fat levels if they lack muscle mass.

Overweight (BMI 25-29.9) and obese (BMI 30+) categories carry increased risks for type 2 diabetes, cardiovascular disease, certain cancers, and joint problems. However, some studies show that people in the overweight category with good fitness levels may have lower mortality than sedentary people in the normal range, a phenomenon sometimes called the "obesity paradox."

BMI is a useful screening tool but has significant limitations. It doesn't distinguish between muscle and fat. Athletes and bodybuilders often have "overweight" or "obese" BMIs despite having low body fat and excellent health. Conversely, someone with little muscle but high body fat might have a "normal" BMI.

Age and sex affect body composition in ways BMI doesn't capture. Women naturally have more body fat than men at the same BMI. Older adults tend to have more fat and less muscle than younger adults at the same BMI. Ethnic differences also matter — Asian populations may have higher health risks at lower BMI levels than European populations.

For a more complete picture, consider additional measurements: waist circumference (risk increases above 40 inches for men, 35 inches for women), waist-to-hip ratio, body fat percentage, or more sophisticated measures like DEXA scans. Use BMI as one data point among many rather than a definitive health indicator.