How Our BMI Calculator Works
What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a widely used screening tool that estimates whether a person has a healthy body weight for their height. Developed by Adolphe Quetelet in the 19th century, it provides a simple numeric metric that the CDC and WHO use to categorize health risks associated with underweight, overweight, and obesity.
While it is an excellent population-level tool, it is important to note that BMI does not directly measure body fat. It cannot distinguish between muscle mass, bone density, and fat mass. Therefore, highly muscular athletes may be classified as "overweight" or "obese" despite having very low body fat percentages.
The BMI Equation
Our calculator uses the standard mathematical formulas established by the National Institutes of Health (NIH). Depending on the unit system you use, the equations look as follows:
BMI = [Weight (lbs) ÷ (Height (in) × Height (in))] × 703
BMI = Weight (kg) ÷ (Height (m) × Height (m))
BMI Calculator – a practical example
Let's use a practical example. We will calculate the BMI for Michael, who is 5 feet 10 inches tall and weighs 185 pounds.
First, we convert his height entirely into inches:
Next, we square his height in inches:
Finally, we divide his weight by the squared height and multiply by the imperial conversion factor of 703:
Michael's BMI is 26.5. According to standard CDC guidelines, this places him in the Overweight category. If Michael is an avid weightlifter, his physician might also check his waist circumference or use calipers to confirm if this excess weight is muscle or adipose tissue.
Standard BMI Categories
The World Health Organization (WHO) and the CDC use the following standardized ranges to classify BMI for adults (age 20 and older):
| Category | BMI Range |
|---|---|
| Underweight | < 18.5 |
| Normal Weight | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obesity (Class I) | 30.0 - 34.9 |
| Obesity (Class II) | 35.0 - 39.9 |
| Extreme Obesity (Class III) | ≥ 40.0 |