How to Calculate BMI (Body Mass Index)
Learn the standard medical formula for calculating Body Mass Index (BMI) using both metric and imperial measurements.
Biostatistics & Public Health Screening Researchers
What is BMI?
Body Mass Index (BMI) is a simple mathematical ratio of a person's weight to their height. While it does not directly measure body fat, it is widely used by medical professionals as a quick screening tool to categorize individuals into broad weight groups: underweight, normal weight, overweight, and obese.
The Metric BMI Formula (kg/m²)
If you measure your weight in kilograms and height in meters:
BMI = Weight (kg) / [Height (m)]²
Example: A person weighs 70 kg and is 1.75 meters tall.
- Height squared: 1.75 × 1.75 = 3.0625
- Divide weight by height squared: 70 / 3.0625 = 22.86
- BMI = 22.9 (Normal weight)
The Imperial BMI Formula (lbs/in²)
If you measure your weight in pounds and height in inches, you must multiply the result by a conversion factor of 703:
BMI = [Weight (lbs) / [Height (in)]²] × 703
Example: A person weighs 150 lbs and is 65 inches tall.
- Height squared: 65 × 65 = 4225
- Divide weight by height squared: 150 / 4225 = 0.0355
- Multiply by 703: 0.0355 × 703 = 24.96
- BMI = 25.0 (Borderline overweight)
Check Your BMI Instantly
To check your body mass index in metric or imperial units and view your healthy weight range, use our free BMI Calculator.
Frequently Asked Questions
Is BMI accurate for athletes?
No. BMI only accounts for total weight, not body composition. Athletes with high muscle mass may be classified as "overweight" or "obese" by BMI standards, even if their body fat percentage is very low.