What is Body Mass Index (BMI)?
Body Mass Index, commonly known as BMI, is a numerical value calculated from a person's weight and height. First developed by Belgian mathematician Adolphe Quetelet in the 1830s, BMI was originally called the Quetelet Index. It gained widespread clinical adoption in the 1970s when American physiologist Ancel Keys studied it in large population groups and found it a reasonably accurate proxy for body fat levels in most adults.
The formula is straightforward: BMI = weight (kg) ÷ height² (m²). In imperial units: BMI = 703 × weight (lbs) ÷ height² (inches²). The result is a dimensionless number that corresponds to weight categories established by the World Health Organization (WHO).
WHO BMI Categories Explained
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Moderate health risk |
| 18.5 – 24.9 | Normal Weight | Low health risk |
| 25.0 – 29.9 | Overweight | Moderate health risk |
| 30.0 – 34.9 | Obese Class I | High health risk |
| 35.0 – 39.9 | Obese Class II | Very high health risk |
| 40.0 and above | Obese Class III | Extremely high risk |
Benefits of Knowing Your BMI
While BMI is not a perfect measure of health, it offers several practical benefits:
- Accessible baseline: It requires only weight and height — measurements anyone can take at home.
- Population-level insight: Doctors use it as a quick triage tool to identify patients who may benefit from further assessment.
- Trend tracking: Tracking BMI over months can reveal weight changes even before they become clinically apparent.
- Insurance and medical forms: Many health forms require BMI as a standard health metric.
- Goal setting: Helps provide a target healthy weight range to work toward with diet and exercise.
Limitations of BMI: What It Doesn't Measure
BMI has well-documented limitations that clinicians acknowledge. It does not distinguish between muscle mass and fat mass, which is why athletes often fall into the "Overweight" category despite having low body fat. It also doesn't account for fat distribution — visceral fat (around organs) is more dangerous than subcutaneous fat, but BMI cannot differentiate between them.
BMI thresholds were also developed primarily from data on European populations. Research shows that people of Asian descent may face metabolic risks at lower BMI values, which is why the WHO has proposed adjusted thresholds for Asian populations (e.g., overweight at 23.0 instead of 25.0).
Other factors BMI ignores include age (older adults typically have more fat at the same BMI), sex (women naturally have higher fat-to-muscle ratios), bone density, and hydration levels.
How to Use This BMI Calculator
Choose Your Unit System
Select "Metric" if your scale and measuring tape use kilograms and centimeters. Select "Imperial" if you use pounds and inches.
Enter Your Weight
Input your current weight. For best accuracy, weigh yourself in the morning before eating, in minimal clothing.
Enter Your Height
Enter your height in centimeters (metric) or total inches (imperial). Note: 5'10" equals 70 inches.
Review Your Results
The calculator returns your BMI value, weight category, a visual progress bar, and contextual advice. Use this as a starting point, not a definitive health judgment.