BMI Calculator
Calculate BMI in metric (kg and cm) or imperial (lb and inches).
Returns WHO classification from underweight to obese and healthy weight range for height.
Body Mass Index (BMI) is a screening number derived from height and weight. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is widely used in medicine as a quick population-level health indicator.
The formula:
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) × 703) / height (inches)²
What each variable means:
- Weight — measured in kilograms (metric) or pounds (imperial); use the same unit system throughout
- Height — measured in meters (metric) or inches (imperial); square the height value
- 703 — the conversion factor that adjusts pounds/inches to produce the same result as the metric formula
Worked example (metric): A person weighs 75 kg and is 1.72 m tall.
BMI = 75 / (1.72)² = 75 / 2.9584 = 25.35
Classification: slightly overweight (BMI 25–29.9 = Overweight category).
Worked example (imperial): A person weighs 165 lbs and is 5 ft 8 in (68 inches) tall.
BMI = (165 × 703) / 68² = 116,000 / 4,624 = 25.09
WHO BMI classification:
- Under 18.5: Underweight
- 18.5–24.9: Normal/Healthy weight
- 25.0–29.9: Overweight
- 30.0–34.9: Obese Class I
- 35.0–39.9: Obese Class II
- 40.0 and above: Obese Class III (severe)
Known limitations of BMI: BMI does not distinguish between fat and muscle mass. Athletes with high muscle mass may be classified as overweight despite being very healthy. For a fuller picture, pair BMI with waist circumference and body fat percentage measurements.