How Our BMI Calculator Works: Formula, Accuracy, and Standards
Technical explanation of the BMI calculation formula, WHO classification standards, unit conversions, and accuracy considerations.
The BMI formula
Our BMI Calculator uses the standard World Health Organization formula: BMI = weight (kg) / height (m)^2. For imperial units, we use the adapted formula: BMI = 703 x weight (lb) / height (in)^2. The constant 703 converts imperial units to match the metric formula output.
WHO classification standards
| BMI Range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 - 34.9 | Obese Class I |
| 35.0 - 39.9 | Obese Class II |
| 40.0+ | Obese Class III |
Unit conversion
For metric inputs, height in centimeters is converted to meters by dividing by 100. For imperial inputs, we use the standard formula with pounds and inches. Both formulas produce identical BMI values for equivalent measurements.
Accuracy considerations
The calculation itself is mathematically exact. The limitations of BMI as a health metric (inability to distinguish fat from muscle, ethnic variations, etc.) are discussed in our BMI vs Body Fat Percentage comparison. The calculator displays results to one decimal place, which is the standard precision for clinical use.
Privacy
All calculation happens in your browser using JavaScript. Your weight and height inputs are never transmitted to any server, logged, or stored. The tool works offline once the page is loaded.
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