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kg
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Underweight
<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
≥30
<18.5 Normal
18.5–24.9 Overweight
25–29.9 Obese
≥30
BMI limitations: what the number doesn't tell you
BMI is a simple ratio of weight to height squared. It's a useful screening tool, but it has well-documented limitations that clinicians, researchers, and the WHO acknowledge.
Muscle mass
BMI cannot distinguish between fat and muscle. Muscular athletes can have a high BMI while carrying very little body fat.
Older adults
As people age, muscle mass decreases and fat mass increases even when weight stays the same. BMI may underestimate adiposity in people over 60.
Fat distribution
Visceral fat (around the abdomen) poses greater cardiovascular risk than subcutaneous fat. BMI does not capture this distinction. Waist circumference is a complementary measure.
Ethnicity
Some research suggests that Asian populations face increased metabolic risk at lower BMI thresholds. Some health organizations use adjusted thresholds for these groups.
BMI is best used alongside other measures — waist circumference, waist-to-hip ratio, body fat percentage, blood markers — for a fuller picture of health.
How BMI is calculated — the formula
BMI uses the World Health Organization (WHO) standard formula:
BMI = weight (kg) ÷ height (m)²
Imperial: BMI = [weight (lb) × 703] ÷ height (in)²
For example: a person 170 cm tall weighing 70 kg has a BMI of 70 ÷ (1.70)² = 24.2 — in the normal range.
WHO BMI categories
| BMI | Category |
|---|---|
| < 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| ≥ 40.0 | Obese (Class III) |
Source: World Health Organization, Global Database on Body Mass Index.
Frequently asked questions
What is a normal BMI?
According to the World Health Organization (WHO), a BMI between 18.5 and 24.9 is considered normal weight for adults. Below 18.5 is underweight; 25–29.9 is overweight; 30 and above is obese.
Is BMI accurate for women?
The same BMI thresholds apply to adult women and men, but research shows that for a given BMI, women typically carry a slightly higher percentage of body fat than men. BMI also does not account for fat distribution, which matters more for cardiovascular risk.
Why might my BMI be wrong?
BMI can overestimate body fat in muscular individuals (e.g., athletes) and underestimate it in older adults who have lost muscle mass. It also ignores where fat is stored, which is a key factor in health risk. Use BMI as one data point, not a diagnosis.
What BMI is considered overweight?
A BMI of 25.0 to 29.9 is classified as overweight by the WHO. A BMI of 30 or above is classified as obese (with subcategories at 35 and 40).
Does BMI apply to children?
No. Adult BMI thresholds do not apply to children and adolescents under 18. For children, the CDC uses age- and sex-specific BMI percentiles. Consult the CDC's pediatric BMI calculator or a healthcare provider.