BMI Calculator

Understanding BMI: Body Mass Index Explained

Body Mass Index (BMI) is a simple screening tool that estimates body fat based on height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and is still widely used by healthcare providers as a quick assessment tool.

How BMI Is Calculated

The formula is straightforward:

• Metric: BMI = weight (kg) / height (m)²
• Imperial: BMI = (weight (lbs) × 703) / height (in)²

BMI Categories

The World Health Organization defines four main categories:

• Underweight: BMI below 18.5
• Normal weight: BMI 18.5 to 24.9
• Overweight: BMI 25.0 to 29.9
• Obese: BMI 30.0 and above (further divided into Class I, II, and III)

What BMI Does Not Measure

BMI is a useful population-level metric, but it has significant limitations at the individual level:

• Muscle vs. fat: BMI does not distinguish between muscle mass and fat mass. A muscular athlete may have a "overweight" BMI despite having low body fat.
• Fat distribution: Where fat is stored matters. Visceral fat (around organs) poses greater health risks than subcutaneous fat (under the skin), but BMI does not account for this.
• Age and sex: Women naturally carry more body fat than men. Older adults tend to have more fat and less muscle. Standard BMI categories do not adjust for these differences.
• Ethnicity: Research shows that health risks associated with a given BMI differ across ethnic groups. For example, some Asian populations face increased health risks at lower BMI thresholds.

Better Alternatives to BMI

For a more complete picture of health, consider these additional metrics: waist circumference, waist-to-hip ratio, body fat percentage (measured via DEXA scan, bioelectrical impedance, or skinfold calipers), and waist-to-height ratio. Many health professionals now use BMI as a starting point alongside other assessments rather than as a standalone diagnostic tool.