How to Calculate BMI
Body Mass Index (BMI) is a number calculated from your weight and height that serves as a rough indicator of whether your weight is in a healthy range for your height. It is widely used by clinicians as a screening tool, though it has well-known limitations — particularly for athletes and older adults. The formula itself requires only two measurements and can be computed in seconds.
Last updated: March 31, 2026
The Formula
Metric: BMI = weight (kg) ÷ height (m)² Imperial: BMI = 703 × weight (lbs) ÷ height (inches)²
Variable Definitions
| Symbol | Name | Description |
|---|---|---|
| weight | Body Weight | Your weight in kilograms (metric) or pounds (imperial) |
| height | Height | Your height in metres (metric) or inches (imperial). Not centimetres. |
| 703 | Imperial Conversion Factor | Converts the result from kg/m² to the equivalent BMI value using lbs and inches |
Step-by-Step Example
A person weighs 78 kg and is 1.80 m tall. Calculate their BMI and determine their category.
Given
Solution
- 1Square the height:
1.80 × 1.80 = 3.24 m² - 2Divide weight by height squared:
78 ÷ 3.24 = 24.07 - 3Identify the BMI category:
24.07 falls in the range 18.5–24.9 → Normal weight
BMI is 24.1 — classified as Normal weight (range 18.5–24.9).
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Common Mistakes to Avoid
Using centimetres instead of metres — 180 cm must be written as 1.80 m before squaring.
Using the imperial formula (× 703) with metric inputs, or vice versa.
Treating BMI as a direct measure of body fat — it is a proxy that cannot distinguish muscle from fat.
Applying adult BMI categories to children — for those under 18, BMI must be compared to age- and sex-specific growth charts.