Related topics: •  Weight Control

Finding out your body mass index (BMI) is a quick way to figure out if your weight is healthy for your height. Your BMI value is more useful for predicting your health risks than your weight alone, but it's even more useful to treat it as one of many factors that influences your health, including total body fat, waist circumference and waist-to-hip ratio.

Body mass index is calculated by this formula, developed by the Belgian statistician Adolphe Quetelet in the nineteenth century:

Weight in kilograms / (Height in meters)² = BMI

Since Americans measure weight and height in pounds and feet instead of kilograms and meters, the converted formula looks like this:

Weight in pounds / (Height in inches) ² x 703 = BMI

If you don't feel like pulling out a calculator, you can use our BMI calculator to figure out your BMI.

Quetelet decided that if the result of the calculation was greater than 30, it signaled obesity. This is still a good rule of thumb, but over the years nutritionists have developed more refined ways to interpret BMI values. For example, different BMI values can mean you are underweight, ideal weight, slightly overweight or obese, and these BMI ranges are slightly different for men and women.

Range for Women, for Men

Underweight Less than 19.1 for women, Less than 20.7 for men

Ideal weight 19.1 to 25.8 for women, 20.7 to 26.4 for men

Marginally overweight 25.8 to 27.3 for women, 26.4 to 27.8 for men

Overweight 27.3 to 32.2 for women, 27.8 to 31.1 for men

Very overweight or obese 32.3 to 44.8 for women, 31.1 to 45.4 for men

Extremely obese More than 44.8 for women, More than 45.4 for men

(Source: Understanding Nutrition by Whitney and Rolfes)

In June, 1998, the National Heart, Lung, and Blood Institute and the National Institute of Diabetes and Digestive and Kidney Diseases issued controversial new guidelines for clinicians to identify overweight people. These guidelines use the same ranges of numbers for men and women, and place many more people in the "overweight" category than previous estimates.

Underweight Less than 18.5

Normal 18.5 to 24.9

Overweight 25.0 to 29.9

Obesity (I) 30.0 to 34.9

Obesity (II) 35.0 to 39.9

Extreme Obesity More than 40

(Source: National Heart, Lung, and Blood Institute, June 1998)

Using BMI to predict overweight still has its limitations, though. It doesn't take frame size into account, so people with stockier builds may be considered overweight even if they don't have a lot of body fat.

Tests that physically measure body fat and distribution are better than BMI at telling if you are overweight. A skinfold measure, for example, uses an instrument called a caliper to measure the thickness of the fat layer on your arm or stomach. A bioelectrical impedance test shows how easily electricity travels through your body (fatty tissue slows down the current) to estimate what percentage of the body is fat. These tests involve a physical measurement by a trained technician at a gym or doctor's office.

Also, BMI is not a good predictor of overweight for these groups of people:

•Children and teens, because BMI ranges are based on adult heights

•Competitive athletes and bodybuilders, because heavier muscle weight may skew the results

•Pregnant or nursing women, because they need more fat reserves than usual

•People over 65, because even BMI values of 29 do not appear to be unhealthy at this age, and may even be a useful energy reserve in case of illness

References

Last updated August 24, 2009
Copyright © 1999 Consumer Health Interactive

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