Calculate your Body Mass Index from height and weight. Free BMI calculator.
Body Mass Index is a widely used screening metric that estimates whether an adult's weight is proportional to their height. Originally developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI remains the most common first-line assessment tool recommended by the World Health Organization and national health agencies worldwide. This calculator supports both metric (kilograms and centimeters) and imperial (pounds, feet, and inches) measurements, giving you your BMI score and its corresponding WHO category instantly. While BMI has well-known limitations for athletes and certain populations, it remains a useful starting point for understanding your weight status.
The formula divides your weight by the square of your height. In metric units, BMI equals weight in kilograms divided by height in meters squared. In imperial units, the formula multiplies weight in pounds by 703, then divides by height in inches squared. The calculator automatically converts feet and inches to total inches before applying the formula. Results are categorized using WHO standard thresholds: under 18.5 is underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese.
Health-conscious individuals use BMI as a quick weight screening tool. Doctors and nurses calculate BMI during routine checkups to flag potential weight-related health risks. Fitness enthusiasts track BMI alongside other metrics like body fat percentage and waist circumference. Insurance companies sometimes reference BMI ranges for policy assessments. Public health researchers use population-level BMI data to study obesity trends and design interventions.
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BMI is a useful screening tool but has important limitations. It does not distinguish between muscle mass and fat mass, so athletes with significant muscle may have a high BMI despite low body fat. It also does not account for fat distribution, bone density, age, or sex differences. Always consult a healthcare professional for a complete assessment.
The World Health Organization defines a normal BMI as 18.5 to 24.9 for most adults. However, optimal ranges can vary by ethnicity. Some Asian populations have higher health risks at lower BMI values, leading some guidelines to use 23 as the overweight threshold for Asian adults.
For children and teens aged 2 to 19, BMI is calculated the same way but interpreted differently. Pediatric BMI uses age- and sex-specific percentile charts from the CDC or WHO rather than fixed adult thresholds, because body composition changes significantly during growth.
Muscle tissue is denser than fat tissue. A muscular person may weigh more at the same height as a sedentary person, producing a higher BMI even though their body fat percentage is low. This is why BMI should be used alongside other metrics like waist circumference or body fat measurements for physically active individuals.
A BMI of 40 or higher is classified as class III obesity, sometimes called morbid or severe obesity. Some medical definitions also include individuals with a BMI of 35 or higher who have obesity-related health conditions such as type 2 diabetes or hypertension.