BMI Formula (Body Mass Index)
Reference for the BMI formula: weight in kg divided by height in meters squared.
Covers metric and imperial forms with WHO category thresholds and examples.
The Formula
Metric
Imperial
BMI is a simple screening tool that estimates body fat based on weight and height.
It is widely used as a quick health indicator, though it does not measure body fat directly.
Variables
| Symbol | Meaning |
|---|---|
| BMI | Body Mass Index (kg/m²) |
| weight | Body weight in kilograms (kg) or pounds (lbs) |
| height | Height in metres (m) or inches (in) |
| 703 | Conversion factor for imperial units |
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 - 24.9 | Normal weight |
| 25.0 - 29.9 | Overweight |
| 30.0 and above | Obese |
Example 1 (Metric)
A person weighs 75 kg and is 1.78 m tall. What is their BMI?
BMI = weight / height²
BMI = 75 / (1.78)²
BMI = 75 / 3.1684
BMI ≈ 23.7 — Normal weight
Example 2 (Imperial)
A person weighs 185 lbs and is 5'10" (70 inches) tall. What is their BMI?
BMI = (weight × 703) / height²
BMI = (185 × 703) / (70)²
BMI = 130,055 / 4,900
BMI ≈ 26.5 — Overweight
When to Use It
Use the BMI formula when you need a:
- Quick general screening of weight status
- Starting point for health assessments
- Population-level health comparisons
BMI has limitations. It does not distinguish between muscle and fat.
Athletes or muscular individuals may have a high BMI but low body fat.
For a more complete picture, consider body fat percentage and waist-to-hip ratio.
This formula is for general educational purposes only. It is not medical advice. Consult a healthcare professional for personal health assessments.
Key Notes
- Formula: BMI = weight (kg) / height (m)²: In imperial units: BMI = [weight (lb) / height (in)²] × 703. WHO categories: Underweight <18.5; Normal weight 18.5–24.9; Overweight 25–29.9; Obese Class I 30–34.9; Class II 35–39.9; Class III ≥40.
- BMI does not distinguish fat from muscle: A muscular athlete with low body fat can have a BMI in the "overweight" range. A sedentary person with normal BMI may carry excess fat. BMI is a screening tool, not a direct measure of body composition or health.
- Different cutoffs for Asian populations: Multiple studies show increased metabolic and cardiovascular risk at lower BMI values in East Asian populations. Proposed thresholds: overweight ≥23; obese ≥27.5. The WHO standard thresholds were developed primarily from European data.
- Waist circumference complements BMI: Central obesity (excess abdominal fat) carries higher cardiovascular risk than the same BMI with fat distributed elsewhere. Waist >102 cm (men) or >88 cm (women) is considered high risk regardless of BMI.
- Applications: BMI is used in epidemiological research (population-level health tracking), clinical screening (triggering further assessment), insurance underwriting, public health policy, and as an entry criterion for weight management programs and bariatric surgery evaluation.