BMI Calculator

How BMI Works

Body Mass Index (BMI) is a widely used screening measure developed by Belgian mathematician Adolphe Quetelet in the 19th century. The formula is straightforward: BMI = weight (kg) / height (m)². For example, a person who is 5'10" (1.78 m) and weighs 170 lbs (77 kg) would calculate their BMI as 77 / (1.78)² = 24.3, placing them in the upper end of the normal weight range. In imperial units, the formula is BMI = (weight in lbs × 703) / (height in inches)².

According to the World Health Organization (WHO), BMI is a useful population-level measure for classifying adults into weight categories. The WHO defines four primary classifications: Underweight (BMI below 18.5), Normal weight (BMI 18.5 to 24.9), Overweight (BMI 25.0 to 29.9), and Obese (BMI 30.0 or above). The obesity category is further divided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (40.0 and above). These thresholds are used globally by healthcare providers to identify potential weight-related health risks.

The U.S. Centers for Disease Control and Prevention (CDC) notes that BMI correlates with body fatness at the population level and is associated with metabolic and disease outcomes. Research published in The Lancet has shown that a BMI in the overweight or obese range is linked to increased risk of type 2 diabetes, cardiovascular disease, certain cancers, and reduced life expectancy. Conversely, a BMI below 18.5 is associated with nutritional deficiency, weakened immunity, and other health concerns.

However, BMI has well-documented limitations. It does not distinguish between muscle mass and fat mass — a muscular athlete and a sedentary individual can share the same BMI while having vastly different body compositions. BMI also does not account for factors such as age, gender, ethnicity, or fat distribution. For example, the WHO has acknowledged that certain Asian populations may face elevated health risks at lower BMI thresholds, leading some countries to adopt adjusted cutoff points.

For these reasons, medical professionals recommend using BMI as a starting point rather than a definitive diagnosis. Complementary measures such as waist circumference, waist-to-hip ratio, body fat percentage, and blood markers provide a more complete picture of an individual's health. You may also want to check your daily calorie needs or TDEE for a fuller understanding of your health profile. If your BMI falls outside the normal range, consult a qualified healthcare provider who can evaluate your specific circumstances and recommend an appropriate course of action.