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How do I Calculate BMI?

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  • Written By: Mary Elizabeth
  • Edited By: Bronwyn Harris
  • Last Modified Date: 21 October 2018
  • Copyright Protected:
    2003-2018
    Conjecture Corporation
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BMI is the abbreviation for Body Mass Index and refers to the ratio of a person’s weight to his or her height squared. For most people, the BMI reliably indicates body fat, and this information is an aid in screening for categories of weight that can be precursors of future health problems. You can calculate BMI for yourself by using a formula or use an online calculator. Calculations of BMI, whether done yourself or using online calculators, can provide only general information, and do not address specific cases. It is recommended by experts that they be used in consultation with a healthcare professional.

It is important to note that a person can have a BMI that is not normal but still be healthy. Likewise, a person can have a normal BMI and be unhealthy. It is possible for the BMI calculation to overestimate body fat when people are physically fit and underestimate it when people have a low amount of muscle mass.

If you are going to calculate BMI, you need to know your height and weight. A height taken at your doctor’s office can be used. Otherwise, height should be measured standing in bare feet on a solid floor rather than a rug or carpet. Use a pencil placed flat atop your head to make a tiny mark on the wall, or better yet, have a friend do it. Then measure from the floor to the mark.

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A weight from a scale that you know to be accurate is ideal, but you may just be using your bathroom scale. If you are not sure it is correct, you can still calculate BMI, but don’t stake too much on the results. Also be aware that some digital scales only show even digits for tenths of pounds. This means that a reading of, say, 146.6 lbs (66.5 kg) could mean anything from 146.5–146.7 lbs (66.45 kg–66.54).

Many BMI calculators are available online for both the Imperial system and the metric system. The analysis for children ages 2 to 18 is different than that for adults ages 19 to 65, so it is important that an appropriate calculator be used. Using Imperial measurements, you are asked to provide height in feet and inches and weight in pounds, often rounding to the nearest tenth. For metric measurements, you provide height in centimeters or meters and centimeters and weight in kilograms, often to the nearest tenth. For children, sex and birth date are also requested, rounding may be different, and you may be required to enter the date on which the measurements were taken.

After you calculate BMI, you may receive either a number or a classification. A simple classification scheme may have four BMI categories: Underweight at less than 18.5; Normal at 18.5 to 24.9; Overweight at 25 to 29.9; and Obese at 30 or higher. Some schemes divide the Obese category into Obese Class 1 at 30 to 34.99; Obese Class 2 at 35 to 39.99; and Morbid Obesity at 40 or greater.

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