Complete BMI Guide & Methodology
Body Mass Index (BMI) is a screening tool used to identify potential weight problems. At YouBMI.com, we complement this with the Ponderal Index and BMI Prime for a more precise health profile.
1. The Ponderal Index (PI) - Accuracy for All Statures
The Ponderal Index (Rohrer's Index) is superior to BMI for people with extreme heights. While BMI assumes human growth is two-dimensional ($kg/m^2$), the PI recognizes that body volume is three-dimensional ($kg/m^3$).
Ponderal Index Reference Values:
| Classification | PI Range (kg/m³) |
|---|---|
| Severe Thinness | < 10.0 |
| Normal (Healthy) Range | 11.0 - 15.0 |
| Overweight | 15.0 - 17.0 |
| Obese | > 17.0 |
2. BMI Table for Adults (WHO Standards)
Detailed classification for adults (20+) according to the World Health Organization:
| Classification | BMI range - kg/m² |
|---|---|
| Severe Thinness | < 16.0 |
| Moderate Thinness | 16.0 - 17.0 |
| Mild Thinness | 17.0 - 18.5 |
| Normal Weight | 18.5 - 25.0 |
| Overweight | 25.0 - 30.0 |
| Obese Class I (Moderate) | 30.0 - 35.0 |
| Obese Class II (Severe) | 35.0 - 40.0 |
| Obese Class III (Morbid) | > 40.0 |
3. BMI Prime
BMI Prime is the ratio of your measured BMI to the upper limit of the normal range (25). It shows how far you are from the healthy threshold.
- 0.74 - 1.00: Normal range.
- 1.00 - 1.20: Overweight.
- > 1.20: Obese.
4. Risks Associated with Weight Status
Overweight/Obesity: Increased risk of Type II diabetes, hypertension, coronary heart disease, and sleep apnea.
Underweight: Risk of malnutrition, osteoporosis, and weakened immune system.
5. BMI for Children & Teens (Percentiles)
Children are assessed using percentiles to account for rapid growth and developmental changes:
| Category | Percentile |
|---|---|
| Underweight | < 5th percentile |
| Healthy weight | 5th - 85th percentile |
| Overweight | 85th - 95th percentile |
| Obese | > 95th percentile |
6. Limitations & Formulas
Note: BMI does not distinguish between muscle and fat. Athletes may have high BMIs despite low body fat.
PI: mass(kg) / height(m)³
US Units (BMI): 703 × lbs / inches²