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BMI

BMI means Body Mass Index. It is a measure of the proportion of body fat to total body weight, and it is used to define healthy and unhealthy weight ranges.

BMI Formula

BMI is calculated using your weight and your height. It is equal to your weight in kilograms divided by your height in metres squared. Alternatively, using imperial units, it is equal to your weight in pounds divided by your height in inches squared, multiplied by 703. These equations are as follows:

BMI Formula

For example, a person who weighs 75 kilograms and is 1.75 metres tall has a BMI equal to 24.5 (75/(1.75 x 1.75) = 24.5).

BMI Categories and Ranges

The value of the Body Mass Index is in its ability to predict mortality and morbidity rates in populations. Numerous studies have shown that increasing BMI values correlate with increasing risk of death and disease, particularly cardiovascular disease and certain types of cancer. For example, in a major US study1 it was found that the lowest mortality risk for men occurred at a BMI of 23.5 to 24.9, while in women the lowest mortality risk occurred at a BMI of 22.0 to 23.4. Mortality risk increased sharply in people with a BMI over 30.0.

While the World Health Organisation officially recognises just one set of BMI categories and ranges for males and females of whatever nationality, it is well established that mortality and morbidity risks vary according to gender and ethnicity. For example, lower ranges are more appropriate for Asian populations compared to Caucasians, while higher ranges are more appropriate for Pacific Islanders. The KiwiCover BMI Calculator uses factors which take account of gender and ethnicity.

Example BMI Categories and Ranges: New Zealand European
BMI Categories Male BMI Ranges Female BMI Ranges
Obese more than 30.0 more than 29.0
Overweight from 25.0 to 30.0 from 24.0 to 29.0
Healthy weight from 18.5 to 25.0 from 18.5 to 24.0
Underweight from 16.0 to 18.5 from 16.0 to 18.5
Severely underweight less than 16.0 less than 16.0

BMI Limitations

BMI is a proxy measurement for the ratio of body fat to total body mass, because it does not directly measure body fat. It makes an assumption about the degree of body fat based on weight and height, but it is not able to discriminate between lean and adipose (fat) tissue. For this reason it is not suitable for those who have developed significant muscle mass through training, such as professional athletes.

The exponent ‘2’ in the BMI formula is also somewhat arbitrary. In fact, the exponent would be ‘3’ if tall people were proportionately larger than short people in all dimensions, as in that case weight would be proportional to height cubed. However, tall people tend to have narrower frames in relation to their heights than do short people, so that the most accurate exponent for populations as a whole has been found to be around 2.5. The adoption of ‘2’ was for ease of calculation, but a consequence is that BMI can incorrectly place some tall people in the overweight category, and correspondingly it can incorrectly indicate a healthy weight category for some short people.

The above BMI categories and ranges are only suitable for people between the ages of 18 and 70. They are not appropriate for children, even though BMI for children is calculated in the same way. They are not appropriate for the elderly, as that group tends to be subject to some muscle wasting and reductions in height with ageing.

A body fat measure which is increasingly being used alongside BMI is the waist to height ratio. This provides an additional and valuable metric to BMI as it gives a measure of fat distribution in the body. This is important as it has been found that abdominal fat is more active metabolically than fat which is stored under the skin (subcutaneous fat), and this activity can be harmful to health.

Important Information

  • The information in this page is not intended to provide, and is not a substitute for, medical advice of the type provided by qualified medical professionals. For specific medical advice relating to your own health, see your doctor.

  • The KiwiCover BMI Calculator calculates your BMI based on your inputs, but it is not intended to provide an indication of your personal mortality or morbidity risks.

 
  1. Calle E., et al. Body Mass Index and Mortality in a Prospective Cohort of U.S. Adults. The New England Journal of Medicine, October 7, 1999; Volume 341, Number 15.