BMI Calculator
What Is BMI?
Body Mass Index (BMI) is a numerical screening value calculated from a person's weight and height. The formula is simple: BMI = weight (kg) ÷ height² (m²). In imperial units: BMI = (weight in lbs × 703) ÷ height² (in inches). Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI has become the most widely used population-level screening tool for weight classification worldwide and is endorsed by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC).
BMI Categories (WHO Standard)
The World Health Organization defines four standard BMI classifications for adults (18 years and older):
- Underweight (BMI below 18.5): May indicate malnutrition, eating disorders, or other health concerns. Underweight individuals may face risks including weakened immune function, bone density loss, and anemia.
- Normal weight (18.5–24.9): Associated with the lowest risk of weight-related health conditions for most adults. This is the range most health guidelines aim for.
- Overweight (25–29.9): May increase risk of cardiovascular disease, type 2 diabetes, hypertension, and joint problems. Lifestyle modifications are typically recommended.
- Obese (30 or above): Clinically significant health risk. Obesity is further divided into Class I (30–34.9), Class II (35–39.9), and Class III/Severe (40+). Medical consultation is strongly recommended.
Limitations of BMI
BMI is a useful screening tool, but it has well-documented limitations. Because it only accounts for weight and height, it cannot distinguish between fat mass and muscle mass. A powerlifter and a sedentary person of identical weight and height will have the same BMI, despite very different body compositions and health profiles. Similarly, BMI does not differentiate between visceral fat (stored around organs, higher health risk) and subcutaneous fat (stored under the skin, lower risk).
Additional limitations include: BMI thresholds were originally derived from data on predominantly European populations, and research suggests that health risks begin at lower BMI values in Asian populations; sex differences in fat distribution are not captured; and age-related changes in body composition are not accounted for. Always consult a qualified healthcare professional for a complete health assessment.
How to Use This Calculator
Select your preferred unit system — Metric (kg and cm) or Imperial (lbs and inches) — using the toggle at the top. Then use the sliders to set your weight and height. Click Calculate BMI to see your result displayed on the gauge along with your weight category. For the most accurate result, measure your height without shoes and your weight without heavy clothing, preferably in the morning before eating.
Frequently Asked Questions
Is BMI accurate for children? No — for people under 18, BMI is interpreted differently using age- and sex-specific percentile charts (CDC growth charts), not the adult categories above.
What BMI is considered healthy for women vs men? The same WHO categories apply to both sexes for adults, but women naturally carry more body fat than men at the same BMI due to hormonal differences.
Can I have a normal BMI and still be unhealthy? Yes. This is called "normal weight obesity" — where body fat percentage is high despite a normal BMI. Waist circumference is a useful complementary measurement (risk increases above 35 inches for women and 40 inches for men).
⚠️ Medical Disclaimer: This tool is for informational and educational purposes only and does not constitute medical advice. BMI is a screening tool, not a diagnostic measure. Consult a qualified healthcare professional for a complete health assessment.