body mass index (BMI)

An individual’s weight in kilograms divided by the square of the height in meters. Also called the Quetelet index. Abbr. BMI. The index is often used to describe how much fat an individual is carrying, but it can be misleading if the person is a heavily muscled athlete.

The U.S. National Heart, Lung and Blood Institute uses the body mass index to classify obesity:¹

  Obesity Class BMI
Underweight   less than 18.5
Normal   18.5–24.9
Overweight   25.0–29.9
Obesity I 30.0–34.9
II 35.0–39.9
Extreme obesity III 40 and above

The BMI is not the only obesity-related measure that indicates increased risk of disease. Another, independent, factor is waistline, since those who deposit fat around the hips (the pear-shaped) are at less risk than those who deposit it around the waist (the apple-shaped). Men with a waist larger than 40 inches, and women with waists over 35 inches, are at greater risk than individuals with the same BMI but waists below those limits.

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1. National Institutes of Health. National Heart, Lung and Blood Institute. et al.
The Practical Guide. Identification, Evaluation, and Treatment of Overweight and Obesity in Adults.
Washington, DC: NHLBI, 2000.

As a pdf file

Connecting obesity with health problems

Risk of hearing loss

A study of 68,421 women found that “Compared with women with BMI <25 kg/m², the multivariate-adjusted relative risk (RR) for women with BMI > 40 was 1.25 (95% confidence interval [CI], 1.14-1.37).” The hearing losses were self-reported.

Sharon G. Curhan, Roland Eavey, Molin Wang, Meir J. Stampfer and Gary C. Curhan.
Body Mass Index, Waist Circumference, Physical Activity, and Risk of Hearing Loss in Women.
The American Journal of Medicine, vol. 126, issue 12, pages 1142.e1-1142.e8, December 2013.

Variations on the BMI

Over the years, many suggestions for improving the BMI's diagnostic power have been made. Adolphe Quetelet himself suggested that, instead of a power of 2, 3 should be used for babies (roly-poly), 2 for teenagers (in his day, skinny), and 2.5 for adults.

In 2013 the distinguished mathematician Nick Trefethen suggested a power of 2.5, and multiplying the weight by 1.3. See Trefethen or the online calculator.


Wikipedia has an excellent entry on the BMI.

Managing Overweight and Obesity in Adults. Systematic Evidence Review From the Obesity Expert Panel, 2013.

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