BMI Formula Explained: What It Is and How to Calculate It
Understand the science behind Body Mass Index (BMI) and learn how to calculate it manually or with our tool.
Body Mass Index (BMI) is a widely used screening tool for weight categories. While it doesn't measure body fat directly, it correlates with various metabolic and disease outcomes. But how is it actually calculated?
The Metric Formula
Most of the world uses the metric system for BMI. The formula is simple:
$$ BMI = \frac{weight (kg)}{height (m)^2} $$
Example
- Weight: 70 kg
- Height: 1.75 m
$$ BMI = \frac{70}{1.75 \times 1.75} = \frac{70}{3.0625} \approx 22.86 $$
A BMI of 22.86 falls within the Normal Weight category (18.5 – 24.9).
The Imperial Formula
If you use pounds and inches, the formula is slightly different:
$$ BMI = \frac{weight (lb)}{height (in)^2} \times 703 $$
Example
- Weight: 154 lb
- Height: 5'9" (69 inches)
$$ BMI = \frac{154}{69 \times 69} \times 703 = \frac{154}{4761} \times 703 \approx 0.0323 \times 703 \approx 22.7 $$
Understanding the Categories
Once you have your number, where do you fit?
- Underweight: < 18.5
- Normal weight: 18.5 – 24.9
- Overweight: 25 – 29.9
- Obesity: 30 or greater
Limitations of BMI
Remember, BMI is a simple calculation. It does not account for:
- Muscle mass (athletes may test as "overweight")
- Bone density
- Overall body composition
Check your BMI: Want to skip the math? Enter your details in our BMI Calculator for an instant result.
Conclusion
Monitoring your BMI is a good starting point for a healthy lifestyle. Combine it with other metrics like body fat percentage for a complete picture of your health.