BMI Calculator
Free BMI calculator — enter your age, gender, height and weight to get your Body Mass Index, BMI Prime, Ponderal Index, and healthy weight range.
BMI Categories (WHO / US Standard)
| Category | BMI Range | Health Risk |
|---|---|---|
| Severely Underweight | < 16.0 | High |
| Underweight | 16.0 – 18.4 | Moderate |
| Normal Weight | 18.5 – 24.9 | Low |
| Overweight | 25.0 – 29.9 | Moderate |
| Obese Class I | 30.0 – 34.9 | High |
| Obese Class II | 35.0 – 39.9 | Very High |
| Obese Class III | ≥ 40.0 | Extremely High |
How BMI is calculated
Metric: BMI = weight (kg) ÷ height² (m²)
US / Imperial: BMI = 703 × weight (lbs) ÷ height² (in²)
BMI Prime = BMI ÷ 25 (ratio to upper limit of normal weight)
Ponderal Index = weight (kg) ÷ height³ (m³)
Comprehensive Guide to the BMI Calculator
The Body Mass Index (BMI) is a widely recognized medical and fitness metric used to categorize an individual's weight relative to their height. Developed in the 19th century by the Belgian statistician Adolphe Quetelet, BMI has become a standard diagnostic screening tool utilized by healthcare professionals, personal trainers, and wellness coaches worldwide.
Core Principles & Calculation Methodology
At the heart of the BMI calculation lies a straightforward mathematical ratio: Mass divided by Height squared. In the metric system, the formula is expressed as BMI = weight (kg) / height² (m²). For those utilizing imperial measurements: BMI = 703 × weight (lbs) / height² (in²). According to the WHO, the resulting index values are standardized into categories — below 18.5 is underweight; 18.5–24.9 is normal; 25–29.9 is overweight; and 30+ is obese (further subdivided into three classes).
Important Considerations & Best Practices
While the Body Mass Index is an excellent, cost-effective screening tool for the general population, it has important limitations. Because the calculation relies solely on height and weight, it does not distinguish between lean muscle tissue and body fat. Consequently, highly muscular individuals may receive an "overweight" classification despite having low body fat. BMI should be interpreted as a general starting point in a broader health assessment, not a definitive, standalone diagnostic metric.
Frequently Asked Questions
According to the World Health Organization (WHO), a healthy BMI for adults falls between 18.5 and 24.9. Values below 18.5 are underweight, 25–29.9 is overweight, and 30 or above is classified as obese. These thresholds are the same for both metric and imperial calculations.
The BMI formula is a simple mass-to-height ratio and does not analyze body composition. Dense muscle tissue weighs more than fat, so muscular athletes may be classified as overweight or obese despite having low body fat. For a more precise assessment, body fat percentage or waist circumference measurements are recommended.
For children and teenagers (ages 2–19), the same formula is used but results are interpreted using age- and sex-specific growth percentile charts from the CDC. A BMI at or above the 85th percentile is overweight, and at or above the 95th percentile is obese. Adult cutoff numbers do not apply to children.
No. BMI is a screening tool, not a clinical diagnosis. A full health evaluation by a licensed healthcare professional — including blood tests, body composition analysis, blood pressure, and family history review — is required to assess your actual health status. Always consult a doctor before making health decisions based on BMI alone.
Yes. You can switch between US (pounds and feet/inches), Metric (kilograms and centimeters), and Other units using the tabs at the top. The calculator automatically applies the correct formula for each unit system.