Understanding Body Mass Index (BMI)
The Body Mass Index (BMI) is a widely used screening tool for estimating body fat levels. While it does not measure body fat directly, BMI provides a reliable indicator of body fatness for most people and is used to screen for weight categories that may lead to health problems.
Our calculator goes beyond standard BMI by also calculating BMI Prime (which adjusts for the upper limit of "healthy" weight) and the Ponderal Index (often used for assessing leanness in very tall or short individuals).
BMI Formula
Metric Units: BMI = weight (kg) / [height (m)]2
US Units: BMI = 703 x weight (lbs) / [height (in)]2
BMI Table for Adults
The World Health Organization (WHO) classifies adults (age 20+) into the following categories:
| Classification |
BMI Range (kg/m²) |
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
Risks Associated with Being Overweight
Carrying excess weight increases the risk of several serious health conditions. According to the CDC, these risks include:
- High blood pressure (Hypertension)
- High LDL cholesterol ("bad" cholesterol) and low HDL cholesterol
- Type 2 diabetes
- Coronary heart disease and Stroke
- Gallbladder disease
- Osteoarthritis (joint degeneration)
- Sleep apnea and breathing problems
- Certain cancers (endometrial, breast, colon, kidney)
- Mental health issues such as depression and anxiety
Limitations of BMI
BMI is a general indicator and has limitations. It does not distinguish between muscle and fat. For example, athletes may have a high BMI due to muscle mass without having excess body fat. Similarly, older adults tend to have more body fat than younger adults at the same BMI.
For a complete health assessment, BMI should be used alongside other measures such as waist circumference, body fat percentage, and physical fitness levels.