BMI Calculator

Understanding Body Mass Index (BMI)

Body Mass Index (BMI) is a widely-used screening tool that categorizes adults into underweight, normal weight, overweight, or obese based on their height and weight. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and remains the standard first-pass health metric in clinical settings worldwide.

BMI is calculated by dividing your weight in kilograms by the square of your height in meters. The resulting number falls into one of four standard categories defined by the World Health Organization:

• Below 18.5: Underweight
• 18.5 to 24.9: Normal weight
• 25.0 to 29.9: Overweight
• 30.0 and above: Obese

BMI Limitations and Context

BMI is a useful population-level screening tool but has known limitations for individuals. It does not distinguish between muscle mass and fat mass, so athletes and very muscular people may register as "overweight" or "obese" despite being in excellent health. Conversely, older adults or sedentary individuals may have a "normal" BMI despite carrying excess body fat.

BMI also does not account for bone density, ethnicity, age, or sex-based differences in body composition. For these reasons, doctors typically use BMI as one of several health indicators rather than a definitive measure of health.

BMI FAQ

Is BMI different for men and women?

The standard BMI formula is the same for adult men and women. However, body composition differs between sexes, so BMI cutoffs are sometimes interpreted differently. Women naturally carry a higher percentage of body fat than men at the same BMI.

Should children use adult BMI categories?

No. Children and teens (ages 2-20) should use BMI-for-age percentiles specific to their age and sex. The CDC provides growth charts used by pediatricians to assess whether a child's weight is appropriate for their height and developmental stage.

What's a healthy BMI for my age?

For adults, the standard 18.5-24.9 range applies regardless of age. However, some research suggests that the "ideal" BMI may shift slightly upward for older adults (closer to 25-27), as slight extra weight in older age is associated with better outcomes in some studies.