BMI Calculator
Calculate your Body Mass Index with metric and imperial units.
This tool uses the standard BMI formula (weight/height²) recommended by the WHO. All calculations happen locally on your device.
Understanding Body Mass Index
BMI (Body Mass Index) is a simple screening measure calculated by dividing weight in kilograms by height in meters squared (kg/m²). While not a direct measure of body fat, BMI correlates with more direct measures such as skinfold thickness and underwater weighing. Categories: Underweight < 18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥ 30.
How BMI Calculation Works▾
Body Mass Index (BMI) is calculated by dividing weight in kilograms by the square of height in meters: BMI = weight(kg) / height(m)².
For imperial units, the formula adjusts: BMI = (weight(lbs) * 703) / height(inches)². The multiplication by 703 is the conversion factor that accounts for the unit difference between metric and imperial.
BMI Categories (WHO standard):
- Underweight: BMI < 18.5
- Normal weight: 18.5 ≤ BMI < 25.0
- Overweight: 25.0 ≤ BMI < 30.0
- Obese (Class I): 30.0 ≤ BMI < 35.0
- Obese (Class II): 35.0 ≤ BMI < 40.0
- Obese (Class III): BMI ≥ 40.0
The tool includes input validation to prevent unrealistic values (height must be between 50cm and 300cm, weight between 1kg and 500kg).
Step-by-Step Usage Guide▾
1. Select your preferred unit system: Metric (kg/cm) or Imperial (lbs/inches).
2. Enter your weight.
3. Enter your height.
4. Your BMI is calculated instantly with the category classification.
5. The color indicator shows your BMI category visually.
Limitations of BMI▾
BMI is a screening tool, not a diagnostic measure. It has several limitations:
Athletes: Muscular individuals may have a high BMI despite having low body fat, because muscle is denser than fat.
Elderly: BMI may underestimate body fat in older adults who have lost muscle mass.
Children: BMI-for-age percentiles should be used for children and teens, not standard adult categories.
Ethnicity: BMI cutoff points may vary for different ethnic groups. Asian populations may have higher health risks at lower BMI values.
Pregnancy: BMI should not be used during pregnancy.
For a comprehensive health assessment, consult a healthcare professional who can consider waist circumference, body fat percentage, and other clinical factors.
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Frequently Asked Questions
What is BMI?▾
BMI (Body Mass Index) is a measure of body fat based on height and weight. It is calculated as weight (kg) divided by height (m) squared.
What is a healthy BMI range?▾
A BMI between 18.5 and 24.9 is considered normal. Below 18.5 is underweight, 25-29.9 is overweight, and 30+ is obese.
Is BMI accurate for everyone?▾
BMI is a general screening tool. It may not be accurate for athletes, elderly, pregnant women, or children. Consult a healthcare professional for personalized advice.