BMI Calculator - Body Mass Index Calculator
Calculate your Body Mass Index (BMI) for adults, children, and infants with WHO and CDC standard guidelines
Adult BMI Calculator
Quick Facts
- BMI is a screening tool, not a diagnostic tool
- Same BMI formula for males and females
- Children use age-specific percentiles
- Infants use weight-for-length, not BMI
- Consult healthcare provider for personalized assessment
Understanding BMI (Body Mass Index)
What is BMI?
Body Mass Index (BMI) is a simple calculation using a person's height and weight. The formula is: BMI = kg/m² where kg is a person's weight in kilograms and m² is their height in metres squared.
How is BMI Calculated?
BMI is calculated the same way for both adults and children. The calculation is based on the formula:
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = [weight (lb) ÷ height (in)²] × 703
BMI Categories for Adults
According to the World Health Organization (WHO) and Centers for Disease Control (CDC), adult BMI categories are:
| BMI Range (kg/m²) | Category | Health Risk |
|---|---|---|
| < 18.5 | Underweight | Malnutrition risk, weak immunity |
| 18.5 - 24.9 | Normal Weight | Low risk |
| 25.0 - 29.9 | Overweight | Moderate risk |
| 30.0 - 34.9 | Obese (Class I) | High risk |
| 35.0 - 39.9 | Obese (Class II) | Very high risk |
| ≥ 40.0 | Obese (Class III) | Extremely high risk |
Does BMI Differ for Males and Females?
The BMI calculation formula is identical for both males and females. However, at the same BMI, women typically have about 10% more body fat than men. Despite this difference in body composition, the WHO and CDC use the same BMI categories for both genders.
BMI for Children and Teens (2-20 years)
For children and teenagers, BMI is age and gender-specific and uses percentile rankings from CDC growth charts:
- Underweight: < 5th percentile
- Healthy Weight: 5th to < 85th percentile
- Overweight: 85th to < 95th percentile
- Obesity: ≥ 95th percentile
Infants (0-2 years)
BMI is NOT recommended for infants and toddlers under 2 years. Instead, healthcare providers use:
- Weight-for-length charts (birth to 36 months)
- Length-for-age charts
- Head circumference-for-age charts
Limitations of BMI
BMI is a useful screening tool but has several limitations:
- Does NOT measure body composition (muscle vs. fat)
- Does NOT account for bone density
- May misclassify athletes with high muscle mass
- Does NOT differentiate between subcutaneous and visceral fat
- May vary by ethnicity and age
When to Consult a Healthcare Provider
You should consult a healthcare provider if:
- Your BMI is outside the healthy range
- You're concerned about your weight or your child's growth
- You're planning to start a weight loss program
- You have weight-related health conditions
Privacy First
All BMI calculations are performed entirely in your browser. No data is sent to our servers or stored anywhere. Your health information remains completely private.
References & Sources
- World Health Organization (WHO) - Obesity and overweight fact sheet. WHO Website
- CDC - Adult BMI Calculator - Centers for Disease Control and Prevention. CDC Website
- CDC - Child and Teen BMI Calculator - BMI-for-age percentiles. CDC Website
- CDC Growth Charts - Clinical growth charts for children. CDC Growth Charts
- NIH BMI Calculator - National Heart, Lung, and Blood Institute. NIH Website
- Mayo Clinic - BMI calculator and healthy weight information. Mayo Clinic
- Cleveland Clinic - Body Mass Index (BMI) comprehensive guide. Cleveland Clinic
- WHO BMI Data - Global Health Observatory data repository. WHO Data
- CDC Practitioners Guide - BMI for practitioners PDF. CDC PDF Guide
Medical Disclaimer: This BMI calculator is for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.