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How BMI Works: The Complete Guide to Body Mass Index

QuickWebCalcs
2 min read

Body Mass Index, or BMI, is a widely used metric to categorize a person's weight relative to their height. But how does it actually work?

The Formula

The math behind BMI is relatively simple.

For the metric system, it is calculated as: Weight (kg) / [Height (m)]²

For the imperial system, the formula includes a conversion factor: [Weight (lbs) / Height (in)²] × 703

You can easily calculate your own using our free BMI Calculator.

The Categories

Once you have your number, the World Health Organization classifies it into the following categories:

  • Underweight: Less than 18.5
  • Normal weight: 18.5 to 24.9
  • Overweight: 25 to 29.9
  • Obese: 30 or greater

Limitations of BMI

While BMI is a fantastic, quick screening tool, it has limitations. It does not distinguish between fat mass and muscle mass. Therefore, a highly muscular athlete might be classified as "Overweight" or even "Obese" despite having very low body fat.

For a more accurate representation of body fat, many fitness professionals recommend using the U.S. Navy Method, which you can calculate using our Body Fat Calculator.

Conclusion

BMI is a starting point, not the whole story. It provides a quick health snapshot but should always be combined with other metrics like waist circumference and regular medical checkups.