BMI Calculator
Formula
Metric: BMI = weight (kg) / height (m)²
Imperial: BMI = (weight (lbs) / height (in)²) × 703
Instructions
Measure with consistent tools
Use a calibrated scale for weight and a stadiometer or wall ruler for height. Enter the values using the same unit system.
Toggle between metric and imperial units
Switch the calculator to kilograms/centimeters or pounds/inches as needed—your entries will be recalculated instantly.
Interpret the BMI category
Compare the BMI result with standard categories to screen for underweight, healthy weight, overweight, or obesity.
BMI Formula
BMI = Weight ÷ Height²
- Metric: BMI = weight (kg) ÷ [height (m)]²
- Imperial: BMI = weight (lb) ÷ [height (in)]² × 703
- Interpret results alongside other assessments such as waist measurements and body fat tests.
Understanding BMI
Body Mass Index (BMI) screens for weight categories that may lead to health problems. It does not directly measure body fat, but it correlates with more sophisticated assessments and is widely used in clinical practice.
Underweight
BMI < 18.5
Normal weight
18.5 ≤ BMI < 25
Overweight
25 ≤ BMI < 30
Obesity
BMI ≥ 30
Reminder: BMI is a screening tool. Muscular individuals or older adults may require additional measurements for an accurate picture of health.
When to Use This Calculator
- General health screening and wellness checkups
- Weight-management planning with a clinician or coach
- Population studies and fitness assessments
- Tracking progress during a structured diet program
- Educational comparisons alongside body-fat or waist measures
Common Mistakes to Avoid
- Treating BMI as a direct measure of body fat (muscle can raise BMI without excess fat)
- Using adult categories for children (use BMI-for-age percentiles instead)
- Comparing BMI across populations without accounting for age and ethnicity
- Ignoring waist circumference when cardiometabolic risk is a concern
Frequently Asked Questions
Is BMI accurate for athletes?
BMI may overestimate body fat for people with high muscle mass. Pair BMI with waist circumference, body composition, or guidance from a clinician.
What BMI range is considered healthy?
For many adults, 18.5-24.9 is classified as normal weight on standard charts. Age, ethnicity, and muscle mass can change how you interpret the number.
How often should I check my BMI?
Quarterly checks work for general tracking. Check more often if you are actively changing weight or following a medical plan.
Does BMI work for children?
Children and teens need BMI-for-age percentiles. Use pediatric growth charts or a specialized calculator for ages 2-19.