How BMI Is Calculated
BMI uses one of two formulas depending on the unit system. In metric units: BMI = weight (kg) divided by height (m) squared. In US units: BMI = 703 times weight (lb) divided by height (in) squared. This calculator converts your inputs automatically and applies the correct formula.
What the Normal Weight Range Shows
The normal weight range shows the minimum and maximum weight corresponding to a BMI of 18.5 and 24.9 at your height. The "Prime BMI" weight shown is the midpoint of the normal range, corresponding to a BMI of approximately 21.7, which is sometimes cited in research as the lowest-risk point within the normal range.
Frequently Asked Questions
BMI, or Body Mass Index, is a numerical value calculated from a person's height and weight. It is used as a screening tool to identify potential weight-related health risks. BMI is calculated by dividing weight in kilograms by height in meters squared, or by dividing weight in pounds by height in inches squared and multiplying by 703.
The standard BMI categories for adults, as defined by the CDC and WHO, are: Underweight (BMI below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), and Obese (30.0 and above). Some guidelines further divide the obese category into Class 1 (30-34.9), Class 2 (35-39.9), and Class 3 (40 and above).
BMI is a useful population-level screening tool but has well-documented limitations as an individual measure. It does not distinguish between muscle and fat, so muscular athletes often have high BMIs without excess body fat. It also does not account for age, sex, ethnicity, or fat distribution. BMI should be interpreted alongside other indicators such as waist circumference, blood pressure, cholesterol, and blood glucose.
For most adults, a BMI between 18.5 and 24.9 is considered normal or healthy weight. However, research suggests that optimal health risk starts to increase gradually above 23 in some Asian populations, which is why some health organizations use lower thresholds for certain ethnic groups. Your doctor is the best person to interpret your BMI in the context of your overall health.
BMI is calculated the same way for children and adolescents, but the categories are interpreted differently. Instead of fixed thresholds, pediatric BMI is expressed as a percentile for age and sex using CDC growth charts. A BMI at or above the 95th percentile is considered obese in children. This calculator is designed for adults aged 18 and older.
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