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BMI FAQs

This page answers a wide range of questions about Body Mass Index (BMI):



What is BMI?

BMI is an acronym for Body Mass Index, and it is an indicator of the amount of fat which is stored in the human body. It provides a measure of the appropriateness of your weight to your height, based on health issues which are known to arise as a result of having a BMI which is too low or too high.

Who invented BMI?

The BMI formula and its name were invented by different people, living in different centuries. The BMI formula was developed in the 1830s by Adolphe Quetelet, a Belgian mathematician and social statistician. He used his skills in calculus and statistics to analyse the relationship between height and weight in populations, and formulated what was known for 140 years as the Quetelet Index. In 1972 the American biologist and physiologist Ancel Keys published a paper endorsing the validity of the index, and in the process he renamed it and popularised it as the Body Mass Index.

Why is BMI important?

BMI is important as it is able to predict, with considerable accuracy, mortality and morbidity rates in populations. What this means is that BMI is not only able to predict the rates at which people within a given population or group will die and suffer disease, but it is able to predict the incidence of the different types of diseases which occur in the population and which cause those deaths. However, as Adolphe Quetelet (see ‘Who invented BMI?’ above) and Ancel Keys stressed, BMI’s predictive abilities apply to populations only, and not to individuals within it.

How is BMI calculated?

BMI is calculated using only the variables of weight and height. Your BMI is equal to your weight in kilograms divided by your height in metres squared. You can also calculate your BMI using imperial units, in which case your BMI is equal to your weight in pounds divided by your height in inches squared, multiplied by 703. See the BMI page for these equations and for further information on the Body Mass Index. To calculate your own BMI, use our BMI calculator.

When should BMI be used?

The calculation of BMI can be used for males and non-pregnant females between the ages of 18 and 65. Although BMI is calculated for children in exactly the same way as for adults, the results are used in an entirely different way and for that reason a standard BMI calculator should not be used for those under the age of 18. Also, the physiology of elderly people differs from other adults (due to factors such as muscle wasting and height reduction), and consequently BMI calculations should not be made for that group. BMI calculations are also unsuitable for those who have developed significant amounts of muscle mass through physical exercise, as the results will tend to indicate higher levels of body fat than is actually the case.

Where can I calculate my BMI?

You can calculate your BMI right here at KiwiCover. Unlike most (if not all) other BMI calculators, our BMI calculator will also calculate the number of kilograms by which you need to reduce (or increase) your weight in order to place yourself just inside the healthy weight range. Also, the KiwiCover BMI calculator takes your gender and ethnicity into account, unlike many other BMI calculators which disregard these important evidence-based factors. To add even more value, our BMI calculator takes your waist metrics into account, and uses the indicators of waist circumference and waist-to-height ratio as important supplementary measures to BMI.

What BMI categories are there?

The five principal categories for BMI are detailed in the BMI Categories and Ranges section of the main BMI page. That section contains a table which provides an example of BMI categories and ranges based on New Zealand European males and females, but it is important to note that this is an example only. While category descriptions remain the same, BMI ranges tend to differ according to ethnicity, and those differences can be significant. That is why ethnicity is an important factor in the KiwiCover BMI calculator.

Does BMI measure body fat?

BMI does not directly measure body fat. BMI is a proxy measure of body fat based on the variables of weight and height, and as such it provides an estimation only. Because BMI does not provide a direct measure of body fat, its use is confined to certain people. For example, it should not be used for pregnant females, for people under the age of 18 or over the age of 65, or for those who have developed significant muscle mass through physical activity.

Is BMI officially recognised?

Yes, BMI is officially recognised by the World Health Organisation, and it is endorsed by a wide range of government agencies and medical organisations throughout the world. This official recognition has led to BMI being used extensively in both the public and private sectors in all developed countries.

Is BMI used by doctors?

Yes, BMI is used by doctors from general practice through to specialist level. Its use in the medical field is not limited to doctors, but extends to a wide range of health professionals including nurses, dieticians, nutritionists, epidemiologists and research scientists.

Is BMI used by insurers?

Yes, BMI is used by life and health insurers when assessing applications for life, health, trauma, income protection and disability insurance. On its own it can act as a trigger point for requesting medical examinations or tests, but it is more often used in combination with other health factors and particular health conditions.

Does body frame size affect BMI?

Yes, body frame size does affect a person’s measured Body Mass Index. People with very large body frames can record BMI measurements which indicate higher levels of body fat than is actually the case, and conversely, people with very small body frames can record BMI measurements which indicate lower levels of body fat than is actually the case. For this reason people in either of these categories ought to employ a degree of caution when interpreting their BMI results.

Does muscle mass affect BMI?

Muscle mass in itself does not affect BMI, but very high and very low levels of muscle mass in relation to people of the same gender and ethnicity can produce misleading BMI results. For example, body builders can develop substantial muscle mass and have low levels of stored fat, leading to BMI results which may indicate that they are overweight when that is not the case. Also, elderly people can experience muscle wasting but maintain high levels of stored fat, leading to BMI results which incorrectly indicate that they are in the healthy weight range. For this reason people in either of these groups should not rely on BMI calculations.

Is gender a factor in BMI?

Yes, gender is a factor in BMI, even though the World Health Organisation (for simplicity) uses the same BMI categories and ranges for both males and females. However, it is well documented that women tend to carry higher levels of stored fat than men1,2. For women with a BMI in the healthy weight range, fat comprises around 15% to 20% of total body weight, whereas the corresponding percentages in men are 10% to 15%. The reason why women tend to carry more fat than men is the biological expectation of pregnancy, and the imperative to nourish the feotus.

Is ethnicity a factor in BMI?

Yes, ethnicity is a factor in BMI, even though the World Health Organisation uses the same BMI categories and ranges for people of all ethnicities (as it does with gender). However, numerous studies have shown that body fat composition for people of the same BMI differs across ethnic groups3,4. In one study, for a BMI of 30 in New Zealand Europeans (equivalent to 43% body fat), the corresponding BMIs for New Zealand Maori, Pacific and Indian Asian women were found to be 34, 36 and 26 respectively5. The study found that the use of universal BMI ranges may not be appropriate for a comparison of the prevalence of obesity among differing ethnic groups.

How accurate is BMI?

In itself, BMI is entirely accurate, but the question probably relates to how accurate BMI is in classifying people as having either a healthy weight or an unhealthy weight. When used for populations, BMI has been found to be very accurate in predicting the incidence of mortality and morbidity due to specific medical conditions, and this is the use for which it was originally designed. It is less accurate in relation to individuals for the reason that it does not directly measure body fat. While for many people BMI is a reliable indicator of healthy or unhealthy levels of body fat, for some it can give incorrect indications. These can include, for example, people who are either very tall or very short, or people who are heavily muscled.

Can BMI tell me how much weight to lose?

Yes, BMI can indicate how much weight you ought to lose, and correspondingly (for those who are underweight) it can indicate how much weight should be gained. While the variables in the BMI equations can be re-ordered to calculate this, a much simpler way is to use the KiwiCover BMI calculator. For people who are not in the healthy weight range for their gender and ethnicity, the calculator automatically calculates the number of kilograms by which their weight should be either reduced or increased, in order to achieve a weight which is just inside the healthy weight range.

Are there any other measures I could use?

Yes, there are other measures you could use in addition to BMI, and these relate to waist metrics. While BMI can indicate healthy and unhealthy levels of body fat, it is not able to measure how fat is distributed in the body, and this is an important factor. For example, two people of the same gender and ethnicity, and who have an identical BMI, could have very different distributions of body fat. One could, for example, be centrally obese while the other is not. Excess amounts of abdominal (or visceral) fat can be much more harmful than other types of stored fat.

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. National Heart, Lung and Blood Institute, Clinical Guidelines on the Identification, Evaluation and Treatment of Overweight and Obesity in Adults: The Evidence Report. NIH Publication No. 98-4083, September 1998, National Institutes of Health.
  2. Deurenberg, P., et al. Body Mass Index as a Measure of Body Fatness: Age and Sex Specific Prediction Formulas. The British Journal of Nutrition (1991), 65, 105-114.
  3. Luke, A. Ethnicity and the BMI–body fat relationship. British Journal of Nutrition (2009), 102, 485-487.
  4. Rahman, M., et al. Accuracy of Current Body Mass Index Obesity Classification for White, Black, and Hispanic Reproductive-Age Women. Obstetrics & Gynecology, 982, Vol. 115, No. 5, May 2010.
  5. Rush, E.C., et al. BMI, Fat and Muscle Differences in Urban Women of Five Ethnicities from Two Countries. International Journal of Obesity (2007), 31, 1232-1239.