BMI Categories
The World Health Organization (WHO) defines the following BMI categories for adults aged 20 and older:
| Category | BMI Range | Health Risk |
|---|---|---|
| Severe Thinness | < 16.0 | High |
| Moderate Thinness | 16.0 – 16.9 | Moderate |
| Mild Thinness | 17.0 – 18.4 | Low |
| Normal Weight | 18.5 – 24.9 | Minimal |
| Overweight | 25.0 – 29.9 | Increased |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
BMI Formula
Body Mass Index is calculated by dividing your weight by the square of your height. The formulas differ depending on whether you use metric or imperial units:
Metric Formula
Example: A person who weighs 70 kg and is 175 cm (1.75 m) tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9 (Normal weight).
Imperial Formula
Example: A person who weighs 154 lbs and is 5'9" (69 inches) tall has a BMI of (154 × 703) ÷ (69 × 69) = 108,262 ÷ 4,761 = 22.7 (Normal weight).
Limitations of BMI
While BMI is a widely used screening tool, it has several important limitations:
Muscle Mass
BMI does not differentiate between muscle and fat. Athletes and individuals with high muscle mass may be classified as overweight or obese despite having low body fat percentages. For example, many professional athletes have BMIs above 25 while being in excellent health.
Age
Older adults tend to have more body fat than younger adults at the same BMI. Additionally, they may lose muscle mass with age, so a “normal” BMI might still correspond to higher-than-ideal body fat levels in seniors.
Gender
Women generally have more body fat than men at the same BMI. The standard BMI categories do not account for these differences, which can lead to under- or over-estimation of health risks.
Ethnicity
Research shows that health risks associated with a given BMI can vary across ethnic groups. For example, Asian populations may face higher health risks at lower BMI values, leading the WHO to suggest lower cutoff points for overweight and obesity in these groups.
Body Fat Distribution
BMI does not indicate where fat is stored. Visceral fat (around organs) is more dangerous than subcutaneous fat (under the skin). Waist circumference and waist-to-hip ratio are better indicators of fat distribution.
BMI for Different Age Groups
Adults (20+)
For adults, BMI is interpreted using the standard categories: underweight (<18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30+). These categories apply regardless of age or sex, though the limitations described above should be considered.
Children and Teens (2–19)
For children and teenagers, BMI is calculated the same way but interpreted using age- and sex-specific percentile charts. The CDC growth charts are used in the United States. A BMI at the 85th to 94th percentile is considered overweight, and the 95th percentile or above is obese. Children’s body composition changes as they grow, so raw BMI numbers are not directly comparable to adult categories.
Older Adults (65+)
Some research suggests that a slightly higher BMI (25–27) may be associated with lower mortality in older adults. This “obesity paradox” may be because a small amount of extra weight provides reserves during illness. However, very high BMI still increases health risks in this age group.
Pregnant Women
BMI is not a useful measure during pregnancy. Pre-pregnancy BMI is used instead to guide recommended weight gain during pregnancy. The Institute of Medicine provides weight gain recommendations based on pre-pregnancy BMI category.