What Is BMI?
Definition, Formula, and Limitations
What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a population screening metric that relates body weight to height. It provides a simple indication of weight status categories for adults and is widely used in healthcare and fitness settings.
BMI was developed in the 19th century by Belgian mathematician Adolphe Quetelet and has become a standard tool for assessing weight status in relation to height. While it's a useful screening tool, it has important limitations that should be understood.
BMI Formula and Calculation
Calculation Methods
Metric Formula:
BMI = weight (kg) ÷ [height (m)]²
Imperial Formula:
BMI = 703 × weight (lb) ÷ [height (in)]²
Example Calculation
Example: Person weighing 70kg, height 1.75m
BMI = 70 ÷ (1.75)² = 70 ÷ 3.06 = 22.9
BMI Categories and Interpretation
Underweight: BMI < 18.5
May indicate insufficient body weight for height. Consult healthcare provider if concerned.
Normal Weight: BMI 18.5-24.9
Generally considered healthy weight range for most adults.
Overweight: BMI 25-29.9
May indicate excess body weight. Consider lifestyle changes.
Obesity: BMI ≥ 30
Indicates significant excess body weight. Medical consultation recommended.
Important Limitations of BMI
Does Not Distinguish Fat from Muscle
BMI cannot differentiate between muscle mass and fat mass. Athletes with high muscle mass may have elevated BMI despite being very fit and healthy.
Does Not Reflect Fat Distribution
BMI doesn't indicate where fat is stored in the body. Central obesity (belly fat) is more concerning than peripheral fat distribution.
Population Variations
BMI cutoffs may differ across demographics, ethnicities, and age groups. Some populations may have different healthy weight ranges.
Age and Gender Considerations
BMI doesn't account for age-related changes in body composition or gender differences in fat distribution patterns.
When BMI is Most Useful
Better Assessment Methods
Body Fat Percentage
More accurate measure of body composition using methods like DEXA scans, bioelectrical impedance, or skinfold measurements.
Waist-to-Hip Ratio
Better indicator of health risk as it measures fat distribution, particularly central obesity which is linked to health problems.
Waist Circumference
Simple measurement that indicates abdominal fat and associated health risks, often more predictive than BMI alone.
This information is for educational purposes only and should not replace professional medical or fitness advice. BMI is a screening tool and has limitations. Individual health needs vary based on age, body composition, health conditions, and other factors. Consult with a healthcare provider for personalized health guidance, especially if you have concerns about your weight or health status.
Calculate Your BMI Now
Now that you understand BMI, use our free BMI calculator to determine your current BMI and learn about your weight status.