What is Body Mass Index (BMI)?
Body Mass Index is a simple ratio of weight to height used to categorize a person's tissue mass relative to their stature. It doesn't measure body fat directly, but offers a useful screening signal for leanness versus excess tissue mass across large populations.
Adult BMI Classification Table
World Health Organization (WHO) classifications for adults:
| Classification | BMI Range (kg/m²) |
|---|---|
| Underweight | < 18.5 |
| Normal | 18.5 – 25 |
| Overweight | 25 – 30 |
| Obese | > 30 |
Health Risks Associated with BMI Status
Risks of Being Overweight
- High blood pressure and elevated LDL cholesterol
- Type 2 diabetes and insulin resistance
- Coronary heart disease and stroke
- Sleep apnea and breathing issues
- Osteoarthritis and joint stress
- Certain cancers (breast, colon, kidney, liver)
Risks of Being Underweight
- Nutritional deficiencies and anemia
- Weakened immune function
- Osteoporosis and decreased bone density
- Fertility issues and irregular menstruation
- Growth and developmental issues in younger adults
Limitations of the BMI Formula
BMI treats every kilogram identically, regardless of whether it's fat or lean tissue. Athletes and people with high muscle mass often score as "overweight" or "obese" despite low body fat. Older adults naturally lose muscle and bone mass, so a "normal" BMI can mask elevated body fat. BMI also doesn't account for fat distribution — visceral fat around the abdomen carries different health risks than subcutaneous fat — nor for differences across sex, ethnicity, or frame size.
The Mathematical BMI Formulas Used
BMI = weight (kg) ÷ [height (m)]²
BMI = 703 × weight (lb) ÷ [height (in)]²