BMI Formula
Body Mass Index formula for metric and imperial units.
Learn how to calculate BMI with worked examples and interpretation tables.
The Formula
BMI = weight (kg) / height² (m²)
BMI = 703 × weight (lb) / height² (in²)
BMI = 703 × weight (lb) / height² (in²)
Body Mass Index (BMI) is a simple screening measure that estimates body fat based on weight and height. It was devised in 1832 by the Belgian mathematician Adolphe Quetelet.
BMI does not measure body fat directly. It does not account for muscle mass, bone density, or fat distribution.
Variables
| Symbol | Meaning |
|---|---|
| BMI | Body Mass Index (kg/m²) |
| weight | Body weight in kilograms (or pounds for imperial) |
| height | Height in meters (or inches for imperial) |
| 703 | Conversion factor for imperial units |
BMI Categories (Adults)
| 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 180 lb and is 5'10" (70 inches) tall. What is their BMI?
BMI = 703 × weight / height²
BMI = 703 × 180 / (70)²
BMI = 126,540 / 4,900
BMI = 25.8 (Overweight)
Limitations
- Athletes with high muscle mass may have a high BMI but low body fat
- Elderly people may have a normal BMI but higher body fat due to muscle loss
- BMI does not distinguish between fat stored around the waist (visceral fat) and fat elsewhere
- Children and teens require age-specific BMI percentile charts
When to Use It
- Quick screening tool for general weight status
- Population-level health studies and statistics
- Starting point for health assessments (not a diagnostic tool on its own)