What is BMI?
Body Mass Index (BMI) is a person's weight in kilograms divided by the square of height in meters. BMI is an inexpensive and easy screening method for weight category—underweight, healthy weight, overweight, and obesity.
BMI Formula
Metric Units: BMI = weight (kg) ÷ (height (m))²
Imperial Units: BMI = 703 × weight (lbs) ÷ (height (in))²
Why is BMI important?
BMI can be a screening tool, but it does not diagnose body fatness or health. Healthcare providers use BMI to screen for weight categories that may lead to health problems.
| BMI | Weight Status |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Healthy weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and above | Obese |
BMI Prime
BMI Prime is a simple modification of BMI where the normal weight range is from 0.74 to 1.0. It is calculated as the ratio of a person's BMI to the upper limit of normal BMI (25).
For example, a person with BMI 27 has a BMI Prime of 27/25 = 1.08.
BMI Limitations
While BMI is useful as a quick screening tool, it has limitations:
- May overestimate body fat in athletes and others with muscular builds
- May underestimate body fat in older persons and others who have lost muscle
- Does not directly measure body fat
- Does not account for differences in body composition between individuals
- May not be equally accurate across different ethnic groups
Other Measurements
For a more complete assessment, healthcare providers may also consider:
- Waist circumference
- Skinfold thickness measurements
- Dietary and physical activity assessments
- Family history
- Other appropriate health screenings