National Health Institute BMI Calculator
The Body Mass Index (BMI) is a simple numerical measure that classifies a person's weight relative to their height. Developed by Adolphe Quetelet, it is widely used by health professionals, including the National Institutes of Health (NIH), as an initial screening tool to identify potential weight problems for adults. While not a direct measure of body fat, BMI is an inexpensive and easy-to-perform method to categorize individuals into weight status categories: underweight, normal weight, overweight, and obesity.
Understanding Your BMI
The National Institutes of Health (NIH) uses BMI to categorize weight status for adults aged 20 and older. These categories help healthcare providers assess potential health risks associated with weight.
- Underweight: BMI less than 18.5
- Normal weight: BMI between 18.5 and 24.9
- Overweight: BMI between 25.0 and 29.9
- Obesity: BMI of 30.0 or greater
How BMI is Calculated (Imperial Units)
For adults using imperial measurements (pounds and inches), the BMI is calculated using the following formula:
BMI = (Weight in pounds / (Height in inches × Height in inches)) × 703
This formula converts weight from pounds to kilograms and height from inches to meters, then applies the standard metric BMI formula (weight in kg / (height in m)²).
Examples
Let's look at a few examples to illustrate BMI calculation and interpretation:
- Example 1 (Normal Weight): A person weighing 150 lbs and standing 5 feet 7 inches tall.
Height in inches = (5 * 12) + 7 = 67 inches.
BMI = (150 / (67 * 67)) * 703 ≈ 23.5. This falls into the Normal weight category. - Example 2 (Overweight): A person weighing 180 lbs and standing 5 feet 6 inches tall.
Height in inches = (5 * 12) + 6 = 66 inches.
BMI = (180 / (66 * 66)) * 703 ≈ 29.0. This falls into the Overweight category. - Example 3 (Obesity): A person weighing 200 lbs and standing 5 feet 4 inches tall.
Height in inches = (5 * 12) + 4 = 64 inches.
BMI = (200 / (64 * 64)) * 703 ≈ 34.3. This falls into the Obesity category.
Limitations of BMI
While BMI is a useful screening tool, it has limitations. It does not directly measure body fat or distinguish between muscle and fat. For instance, a very muscular athlete might have a high BMI but very little body fat, while an older person with less muscle mass might have a normal BMI but a high percentage of body fat. Other factors like age, sex, ethnicity, and body composition can influence the interpretation of BMI.
Therefore, the NIH emphasizes that BMI should be used as a general indicator and not as the sole diagnostic tool. A healthcare provider can perform further assessments, such as skinfold thickness measurements, waist circumference, and evaluations of diet and physical activity, to determine if a person's weight poses health risks. Always consult with a medical professional for personalized health advice.