Health & Fitness
6 min read
•Updated 2026-09-17
How BMI is Calculated: Metric & Imperial Formulas, WHO Categories & Limitations
A complete clinical and mathematical overview of Quetelet’s Body Mass Index metric and its clinical interpretation.
By AnyCalcHub Health & Wellness Desk
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1. The BMI Mathematical Formulas
Body Mass Index (BMI) was created by Belgian mathematician Adolphe Quetelet in the 1830s. It normalizes body mass across different body heights.
Mathematical Formula
Metric: BMI = Weight (kg) / (Height (m))^2 Imperial: BMI = (Weight (lbs) × 703) / (Height (inches))^2
Step-by-Step Worked Example
Weight72 kg
Height1.75 meters (175 cm)
Step 1:Square the height: 1.75 × 1.75 = 3.0625
Step 2:Divide weight by height squared: 72 ÷ 3.0625 = 23.51
Final Result: BMI is 23.5 (Normal Health Weight Category)
2. WHO Health Weight Classifications
Standard international classification ranges established by the World Health Organization:
- Underweight: < 18.5
- Normal Weight: 18.5 – 24.9
- Overweight: 25.0 – 29.9
- Obesity Class I: 30.0 – 34.9
- Obesity Class II: 35.0 – 39.9
- Obesity Class III: ≥ 40.0
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Frequently Asked Questions
Common questions and mathematical nuances for this calculation.
What are the main limitations of BMI?
BMI does not differentiate between lean muscle mass, bone density, and adipose fat tissue. Athletes and bodybuilders often register as overweight or obese despite possessing low body fat percentages.
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