Calculate your Body Mass Index instantly in metric or imperial units.
BMI vs body fat percentage in practice: If you have access to body composition testing — through a gym, medical clinic, or smart scale — body fat percentage is a more actionable metric than BMI. Healthy body fat ranges are approximately 10–20% for men and 20–30% for women. Professional athletes may have body fat as low as 6–13% (men) or 14–20% (women) while maintaining excellent health, even though their BMI may classify them as overweight.
Body Mass Index (BMI) is a simple numerical measure of body size derived from a person's weight and height. First developed by Adolphe Quetelet in the 1830s and adopted by the World Health Organization as a screening tool, BMI provides a quick, cost-free way to categorise weight status in large populations. However, it has important limitations that anyone using it should understand.
How BMI is calculated: In metric units, BMI = weight(kg) ÷ height(m)². In imperial units, BMI = 703 × weight(lbs) ÷ height(inches)². The standard WHO categories are: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), and Obese (30.0 and above). Obesity is further sub-classified into Class I (30–34.9), Class II (35–39.9), and Class III, sometimes called morbid obesity (40 and above).
BMI limitations and criticisms: BMI does not distinguish between muscle and fat. Athletes and bodybuilders often have BMIs in the "overweight" range due to high muscle mass, despite having very low body fat percentages. Conversely, "normal weight obesity" — where someone has a healthy BMI but high body fat — is possible. BMI also does not account for where fat is stored: visceral fat around the abdomen carries higher health risks than subcutaneous fat. Additionally, BMI thresholds were historically calibrated on Western European populations and may not be equally accurate for all ethnicities — some Asian health agencies use lower cutoff points.
Better metrics to use alongside BMI: Waist circumference is a strong predictor of cardiovascular disease risk: health organisations recommend keeping waist circumference below 94cm (37 inches) for men and 80cm (31.5 inches) for women. Waist-to-height ratio (ideally below 0.5) is increasingly recommended as a more accurate single metric. Body fat percentage measured via DEXA scan, hydrostatic weighing, or bioelectrical impedance gives a more accurate picture than BMI, though these methods are more expensive.
What a high or low BMI actually means for your health: Research consistently shows that a BMI above 30 is associated with increased risk of type 2 diabetes, heart disease, certain cancers, and sleep apnoea. A BMI below 18.5 is associated with malnutrition, osteoporosis, and immune deficiency. However, BMI is a risk indicator, not a diagnosis. Many people with high BMIs live long, healthy lives, and many people with normal BMIs develop serious health conditions. BMI should be one data point in a broader health conversation with a medical professional.