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How to Calculate your Body Mass Index (BMI)

Updated on September 12, 2014

Your Body Mass Index (BMI) is a number computed based on your weight and height. Invented in the early 1800's, the BMI is a mathematical tool used by doctors to assess the health risks of a particular person of interest. Today, you can use the information to determine where you stand within the commonly accepted range of healthy and unhealthy weights. Typically the higher your BMI is, the higher your risk is for being diagnosed with some of the various obesity related diseases plaguing the world. These diseases include, but are not limited to, diabetes, hypertension, and heart disease.

BMI Formulas

The formula to determine your body mass index is pretty simple. Your BMI is equal to your mass divided by the square of your height. Of course the units of which you measure your height and weight does matter. If you are measuring your height in meters and your mass in kilograms, the formula is simply:

If you are measuring your height in inches and your weight in pounds, the formula becomes:

If you are weary of using formulas you can use this neat online calculator or the chart below instead.

This chart is based on the latest categories from the World Health Organization
This chart is based on the latest categories from the World Health Organization | Source

BMI Categories

The BMI has been divided into 4 categories (some countries use 6 categories) based on numerous studies of risk factors for obesity related diseases.

Below 18.5
18.5 - 24.9
25.0 - 29.9
30.0 and Above

Note that what is considered normal, underweight, or obese may differ from country to country. The above table is for the USA. When determining the BMI for children (younger than 18) this table should not be used. Typically a doctor will compare the BMI number with those of other children in the same age group. A child may be considered underweight or obese if they rank below the 5th percentile or above the 95th percentile.

Limitations of the Body Mass Index

Many of you out there would probably agree that the number computed from these formulas does not adequately represent everyone in every case. For example, athletes could be considered obese based on the BMI even though they may actually have a really low body fat percentage. The BMI also has problems when it comes to assessing the health risks of the elderly because these individuals have typically lost considerable amounts of muscle mass. Caution should be used when the BMI calculation is applied in either of these cases. In addition to this, the BMI does not account for the many different body shapes and bone structures that people in this world have. Because of these limitations, the BMI should always be used in conjunction with other methods of health risk assessment such as the waist and neck circumference or body fat percentage.

Does BMI Matter?

The really is no easy answer to this questions. Many doctors today still use the BMI as an indicator of someone's overall health. However, as we have seen, the BMI does not tell the whole story. There are certainly many other things, such as lung capacity and body fat percentage, that are also just as important. So if your BMI says that you are obese I wouldn't worry about it so much. Again, the BMI is an indicator and not the holy grail of body numbers. After all, the human body is complicated and our species has a wide variety of traits. No single number or formula can tell you everything that you need to know about your health.


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    • profile image

      personal training Norwich 4 years ago

      How can you calculate your body mass index? My brother keeps complaining about her weight. I know that he is just right but I need a way of proving it. And I herd about body mass index can anyone tell me how it works??

    • CWanamaker profile image

      CWanamaker 6 years ago from Arizona

      @kerlynb - I am glad you found this useful. This hub is also useful for me. I learned a few new thinks while researching the content for this.

      @abhipsitabose - Thanks and will do!

    • abhipsitabose profile image

      abhipsitabose 6 years ago from India

      useful do check out my hub .....this is for the underweight..very helpful

    • kerlynb profile image

      kerlynb 6 years ago from Philippines, Southeast Asia, Earth ^_^

      Voted your hub useful. I do check my BMI on the net (there are lots of websites that offer free, quick, and accurate calculations)every now and then. I'm actually quite conscious about my health. While some people do not like BMI as a gauge, I personally find it very good in making my self strive for a better shape. When I see my BMI heading towards 20s, my alarm bell rings :)