Understanding BMI


Body Mass Index (BMI) is a commonly used index of relative weight. BMI is calculated as weight (kilograms) divided by height (meters) squared. The index is often used to assess obesity in individuals. It is easy to measure and calculate; no sophisticated equipment is required.

 


Go straight to the BMI calculator

By convention, an individual with a BMI of 30 or more is considered obese. This applies to both men and women. For comparison, this would relate to a direct measure of approximately 25% body fat in men and 30% body fat in women.

 

The situation is more complex in children, where the BMI threshold of obesity varies with age and sex. The charts below show how the BMI obesity threshold changes with age for boys and girls. Threshold BMI levels initially equal about 20, approaching the obesity level of 30 found in adulthood. Also included in the two charts are BMI levels at which a child is considered overweight, showing a trend similar to that of the obese value.
Obesity and BMI in boys
Obesity and BMI in girls

How accurate is BMI in predicting fat, and what are its limitations? This is important because body composition rather than body mass is the more significant predictor of health risk. That is, knowing the level of body fat is more important than simply knowing body weight. Studies have found that BMI is not always accurate in predicting an individual's levels of body fat.

"Significant numbers of people with a BMI below 30 kg/m2 are also obese and thus misclassified by BMI" 1

"Measurement of body fat is a more appropriate way to assess obesity in people with a BMI below 30 kg/m2" 1

In one study, of the subjects with a BMI below 30 (not obese according to BMI), about one-third of the men and almost one-half of the women were obese according to their measured levels of body fat. Some individuals who were found to be obese judging by percent fat had a BMI as low as 20. The conclusion is that BMI assessment produces significant numbers of false negatives - seemingly non-obese individuals by BMI who nevertheless qualify as obese by percent fat.

Individuals with a BMI greater than 30 are almost invariably obese, based on other standards, but significant numbers of individuals with a BMI less than 30 are misclassified.


Race also has an effect on BMI prediction accuracy. Body composition differs among various races at a given BMI. For example, body fat in certain asian ethnic groups was under-predicted by BMI, sex, and age, when an equation derived from a caucasian population was used. BMI/%Fat ratios were also different among the asian ethnic sub-groups.
2

The following tools are provided to help calculate and assess BMI. The form on the left is suitable for children, and provides reference information for comparing the calculated BMI to the normal population. Adults can go to the wt|ht chart to review their predicted obesity levels.

 

BMI Form for Kids

The BMI form previously seen here is no longer in use. Please follow the link below to go to the new BMI calculator.

Interactive BMI Calculator

You may be prompted to install a program called Java, which is required to run the calculator.

Chart for Adults

Adult Wt|Ht and Obesity
Click to see chart

 

 

References used on this page:

  1. Frankenfield et al, Limits of body mass index to detect obesity and predict body composition. Nutrition 2001;17:26-30.
  2. Deurenberg-Yap et al, The paradox of low body mass index and high body fat percentage among chinese, malays,and indians in Singapore. Int J Obes Relat Metab Disord 2000; 24(8):1011-1017.



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