Health · Quick Answer

What is BMI and how is it calculated?

Body Mass Index (BMI) = weight (kg) / height (m)². In pounds and inches: BMI = (weight × 703) / height². The WHO defines normal weight as 18.5-24.9, overweight as 25-29.9, and obese as 30+.

The formula

Metric: BMI = weight (kg) / height (m)²

Imperial: BMI = (weight in lbs × 703) / (height in inches)²

WHO categories

Under 18.5: underweight
18.5, 24.9: healthy weight
25.0, 29.9: overweight
30.0, 34.9: obese (class I)
35.0, 39.9: obese (class II)
40.0+: severely obese (class III)

Limitations

BMI is a population-level screening tool, not a diagnostic. It doesn't distinguish between fat and lean mass, so very muscular people can be classified as overweight. For individual health assessment, body-fat percentage and waist-to-hip ratio are more informative.

Run the numbers

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