What Is BMI?
BMI (Body Mass Index) is a numerical value calculated from a person's height and weight. It is a simple screening tool used by healthcare providers worldwide to categorize weight status and identify potential health risks. BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has been used clinically since the 1970s.
The BMI Formula
BMI = weight (kg) / height (m)^2
Imperial formula:
BMI = (weight (lbs) / height (inches)^2) * 703
Examples:
Person: 70 kg, 1.75 m tall
BMI = 70 / (1.75)^2 = 70 / 3.0625 = 22.9
Person: 154 lbs, 5'9" (69 inches) tall
BMI = (154 / 69^2) * 703 = (154 / 4761) * 703 = 22.7
BMI Categories (WHO)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Possible nutritional deficiency, osteoporosis |
| 18.5 - 24.9 | Normal weight | Low health risk |
| 25.0 - 29.9 | Overweight | Moderate health risk |
| 30.0 - 34.9 | Obesity Class I | High health risk |
| 35.0 - 39.9 | Obesity Class II | Very high health risk |
| 40.0 and above | Obesity Class III | Extremely high health risk |
BMI for Different Populations
Adults (20+)
The standard WHO categories above apply. The same cutoffs apply to both men and women.
Children and Teenagers (2-19)
BMI is interpreted differently for young people. Instead of fixed cutoffs, BMI percentile for age and sex is used:
- Below 5th percentile: Underweight
- 5th to below 85th percentile: Healthy weight
- 85th to below 95th percentile: Overweight
- 95th percentile and above: Obese
Asian Populations
Research suggests that Asian populations have higher health risks at lower BMI values. Some health organizations use modified cutoffs:
- Overweight: BMI 23.0 and above
- Obese: BMI 27.5 and above
Elderly (65+)
A slightly higher BMI (22-27) may be protective in older adults. Low BMI in elderly is associated with greater mortality risk than in younger adults.
Limitations of BMI
BMI is a useful population-level screening tool but has significant limitations for individual assessment:
Does not measure body fat directly: A muscular athlete may have a high BMI (classified as overweight) while having low body fat. Conversely, a "normal" BMI person may have high body fat percentage.
Doesn't account for fat distribution: Visceral fat (around organs) is more dangerous than subcutaneous fat (under skin). Waist circumference is a better predictor of metabolic risk.
Gender differences: Women typically have more body fat than men at the same BMI.
Ethnic variations: Different ethnic groups have different relationships between BMI and health risks.
Age variations: Body composition changes with age even if BMI stays the same.
Better Alternatives to BMI
For a more complete picture of health:
- Waist circumference: Risk threshold >88cm (35") for women, >102cm (40") for men
- Waist-to-height ratio: Waist circumference / Height. Values >0.5 indicate increased risk
- Body fat percentage: Measured by DEXA scan, hydrostatic weighing, or bioimpedance
- Waist-to-hip ratio: Measures fat distribution pattern
Despite its limitations, BMI remains the most widely used weight screening tool due to its simplicity and availability.
Using This Tool
Enter your height and weight to instantly calculate your BMI and see where it falls in the standard categories. You can switch between metric (kg/cm) and imperial (lbs/inches) units.
-> Try the BMI Calculator