BMI Calculator

What Is BMI?

Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was originally called the Quetelet Index. BMI provides a simple screening method to categorize adults as underweight, normal weight, overweight, or obese. The formula is: BMI = weight (kg) / height² (m²), or for imperial units: BMI = weight (lbs) × 703 / height² (inches²).

BMI Categories

The World Health Organization (WHO) 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 or "severe obesity" (40+). These categories help healthcare providers assess health risks associated with body weight, though BMI alone is not a definitive measure of health.

Limitations of BMI

BMI has significant limitations. It doesn't distinguish between muscle and fat — a muscular athlete may have an "overweight" BMI despite low body fat. It doesn't account for age, sex, ethnicity, or body composition. Asian populations may face health risks at lower BMIs than Western populations. BMI also doesn't measure where fat is stored — visceral fat (around organs) is more dangerous than subcutaneous fat, but BMI can't tell the difference.

BMI vs Other Measures

Several alternatives to BMI exist. Body fat percentage (measured by DEXA scans, calipers, or bioelectrical impedance) directly measures fat tissue. Waist-to-hip ratio identifies central obesity. Waist circumference alone (over 40 inches for men, 35 for women) is a strong predictor of metabolic disease risk. The ABSI (A Body Shape Index) combines waist circumference with BMI and height for better risk prediction.

BMI in Clinical Practice

Despite its limitations, BMI remains widely used in clinical practice because it's quick, free, and requires no special equipment. Doctors use it as an initial screening tool alongside other measurements. A BMI over 30 often triggers further evaluation including blood pressure, cholesterol, blood sugar, and physical examination. BMI trends over time are more useful than a single measurement — gradual increases may signal health risks before symptoms appear.