Body Mass Index (BMI) Calculator
Assess your weight status using standardized World Health Organization (WHO) adult categories.
Understanding Body Mass Index (BMI)
Body Mass Index (BMI) is a simple index of weight-to-height commonly used to classify underweight, normal weight, overweight, and obesity in adult men and women.
Threshold classifications are calibrated according to the World Health Organization (WHO) Technical Report Series 854 and the Centers for Disease Control and Prevention (CDC) adult BMI guidelines.
Originally created in the 1830s by Belgian astronomer, mathematician, and statistician Adolphe Quetelet, it is widely utilized by health organizations worldwide—including the World Health Organization (WHO) and the NHS—as a preliminary screening tool to identify possible weight categories that may lead to health problems.
WHO Standard Adult BMI Classifications
| BMI Range (kg/m²) | Classification | Health Risk Profile |
|---|---|---|
| < 18.5 | Underweight | Increased risk of nutritional deficiency, weakened immune function, and osteoporosis. |
| 18.5 – 24.9 | Normal Weight | Lowest statistical risk of cardiovascular diseases and obesity-related metabolic disorders. |
| 25.0 – 29.9 | Overweight | Moderate risk of hypertension, dyslipidemia, and type 2 diabetes. |
| 30.0 – 34.9 | Obesity Class I | High risk of developing cardiovascular diseases and joint stress. |
| 35.0 – 39.9 | Obesity Class II | Very high risk of chronic metabolic and vascular complications. |
| ≥ 40.0 | Obesity Class III | Extremely high health risk (clinically severe or morbid obesity). |
Mathematical Formulas
Metric: BMI = weight (kg) / [height (m)]²
Example: 70 kg / (1.75 m × 1.75 m) = 70 / 3.0625 = 22.86 kg/m²
Imperial: BMI = (weight (lbs) / [height (in)]²) × 703
Example: (154 lbs / [69 in]²) × 703 = (154 / 4761) × 703 = 22.74 kg/m²
Important Clinical Limitations of BMI
While BMI is an effective statistical screening tool for large populations, it has notable physiological limitations when applied to individuals:
- Muscle Mass vs. Fat: Muscle tissue is substantially denser than adipose tissue. Athletes, bodybuilders, and heavy laborers may register as "overweight" or "obese" despite having very low body fat percentages.
- Fat Distribution: BMI does not measure visceral fat (abdominal fat surrounding organs), which is a key predictor of cardiovascular risk. Waist circumference and waist-to-height ratio are valuable complementary metrics.
- Age and Bone Density: Older adults naturally lose bone density and muscle mass (sarcopenia), meaning an older person may have higher body fat than a younger person with the same BMI.
- Ethnicity Variations: Research indicates that cardiovascular and diabetes risks occur at lower BMI thresholds (≥ 23 kg/m² for overweight) in many Asian populations.