What is BMI?
Body Mass Index (BMI) is a simple screening measure that uses height and weight to broadly categorize a person as underweight, normal weight, overweight, or obese. It was devised by Belgian mathematician Adolphe Quetelet in the 1830s and remains one of the most widely used indicators of body fat in clinical practice and public health.
While BMI does not directly measure body fat, it correlates reasonably well with more direct measures of body fat such as DEXA scans. It is used by the WHO, CDC, and national health organizations worldwide as a population-level screening tool.
BMI Formula
For imperial units:
BMI Categories
| Category | BMI Range (kg/m²) | Health Risk |
|---|---|---|
| Severe Underweight | < 16.0 | Very high (malnutrition risk) |
| Moderate Underweight | 16.0 – 16.9 | High |
| Mild Underweight | 17.0 – 18.4 | Moderate |
| Normal Weight | 18.5 – 24.9 | Low (optimal) |
| Overweight | 25.0 – 29.9 | Slightly increased |
| Obese Class I | 30.0 – 34.9 | Moderate |
| Obese Class II | 35.0 – 39.9 | High |
| Obese Class III | ≥ 40.0 | Very high |
BMI Scale Diagram
BMI Prime & Ponderal Index
BMI Prime is the ratio of your actual BMI to the upper limit of normal (25.0). A BMI Prime of 1.0 means you are exactly at the threshold of overweight. Values below 1.0 indicate normal weight, above 1.0 indicate overweight.
Ponderal Index (also called Corpulence Index) adjusts for height cubed rather than squared, making it more appropriate for very tall or very short individuals:
Limitations of BMI
- Doesn't distinguish fat vs. muscle: Athletes with high muscle mass may be classified as overweight despite low body fat
- Age-blind: Body composition changes with age; older adults may have higher body fat at the same BMI
- Sex-blind: Women typically have higher body fat percentage than men at the same BMI
- Ethnicity variations: Asian populations may have higher metabolic risk at lower BMI cutoffs (some agencies use 23 as overweight threshold for Asian populations)
- Doesn't measure fat distribution: Central (visceral) obesity is more metabolically dangerous than peripheral fat, but BMI doesn't capture this
Worked Example
A person weighing 75 kg and 180 cm (1.80 m) tall:
This falls within the Normal weight category (18.5–24.9). BMI Prime = 23.15 / 25 = 0.93 (within normal range).
Frequently Asked Questions
What is a healthy BMI?
A healthy BMI is between 18.5 and 24.9 kg/m². This range is associated with the lowest risk of weight-related health problems for most adults, regardless of age.
Does BMI change with age?
The BMI formula itself doesn't change with age for adults. However, body composition changes — older adults tend to have more fat and less muscle at the same BMI. For children and teens, age- and sex-specific percentiles are used instead of fixed cutoffs.
Is BMI accurate for muscular people?
BMI can overestimate body fat in muscular individuals. If you have significant muscle mass, consider supplementary measures like waist circumference, body fat percentage, or waist-to-hip ratio for a more complete assessment.
How is BMI used differently for Asian populations?
Research shows that Asian populations face increased metabolic risks at lower BMI values. The WHO suggests using 23 kg/m² as the overweight threshold and 27.5 kg/m² as the obesity threshold for Asian populations, though these are not universally adopted.