Understanding Body Mass Index (BMI)
BMI (Body Mass Index) is a widely used screening tool that measures the relationship between a person's weight and height. Developed in the 19th century by Adolphe Quetelet, it provides a simple numerical classification that correlates with body fat and associated health risks across large populations. While not a diagnostic tool, BMI helps identify individuals who may benefit from medical evaluation of their weight status.
BMI Formula and Categories
Metric formula: BMI = weight(kg) ÷ height(m)². Imperial formula: BMI = (weight(lbs) ÷ height(in)²) × 703. The WHO classifies BMI results as: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25.0–29.9), Obese Class I (30.0–34.9), Obese Class II (35.0–39.9), Obese Class III (40.0+).
Healthy Weight Range
Rather than just your current BMI, our calculator also shows the weight range that falls within the "Normal" BMI category for your specific height. This gives you a practical target range for weight management.
Limitations of BMI
BMI does not directly measure body fat percentage. Athletes and muscular individuals may have a high BMI despite being healthy and lean. Elderly individuals may have a normal BMI but still have excess body fat (sarcopenic obesity). BMI thresholds also vary by ethnicity — Asian populations have higher health risks at lower BMI values. Always consult a healthcare professional for personalized guidance.
Using Our Free BMI Calculator
Select metric (kg/cm) or imperial (lbs/inches), enter your weight and height, and click Calculate. You'll see your BMI value, your category, and your healthy weight range — all computed instantly in your browser with no data stored.
Frequently Asked Questions
What is a healthy BMI range?
18.5 to 24.9 is considered the normal/healthy range. Below 18.5 is underweight; 25.0–29.9 is overweight; 30.0+ is obese.
Is BMI accurate for athletes?
Not always. BMI may overestimate body fat for muscular individuals since muscle is denser than fat. A 200lb athlete and a 200lb sedentary person have the same BMI but very different body compositions. Body fat percentage measurements are more accurate for athletes.
How is BMI calculated?
Metric: BMI = weight(kg) ÷ height(m)². Imperial: BMI = (weight(lbs) ÷ height(inches)²) × 703.
What is the BMI difference between men and women?
The standard BMI formula and categories are the same for both sexes, though women naturally have a higher body fat percentage than men at the same BMI. Some health organizations recommend slightly different interpretations by sex and age.