How Is Body Mass Calculated?

Introduction

According to the Centers for Disease Control, more than 72 million Americans are considered overweight or obese. Even children are at risk, with an estimated 16 percent of children considered obese. With the word "obesity" being thrown around so often in today's society, it is important to understand how someone is calculated to be at an obese weight.

Body Mass Calculation

Obesity is measured using a scale called the body mass index (BMI). Your BMI takes into consideration your height and weight, and using a mathematical equation estimates your overall body fat. Although this is not the most accurate way to determine body fat composition, it is a cheap and effective way to get an estimate.
You can calculate your BMI using the following equation provided by the Centers for Disease Control:
BMI = (your weight (lb) / [your height (in)]2) x 703

Ranges

Once your BMI is calculated, you can use the BMI scale to see where you stand. A normal BMI score should range between 18.5 and 24.9. Anything less than 18.5 is considered underweight. A body mass index score between 25.0 and 29.9 is considered overweight (or sometimes called pre-obesity). Finally, a body mass index score of greater than 30 is considered obese.

For Children and Teenagers

When calculating the BMI for children and teenagers, BMI calculators include both age and gender into the calculation. The BMI is first calculated normally and then plotted on a BMI-for-age growth chart. From here you can obtain a percentile ranking for your child and compare them to the average height and weight for his age and gender. You can find the CDC's BMI calculator for adults, as well as a calculator for children and teenagers, in the additional resources section of this article.

Disadvantages

Because BMI is only based off of a mathematical equation, it does have its limitations and disadvantages. Firstly, it only estimates your body fat percentage, not your exact makeup. Next, it doesn't take into consideration body differences in bone size, muscle mass or adult gender differences. For example, women tend to have more body fat than men. Also, adult BMI scores also do not take into account age. As you age, your body tends to have more body fat than younger people. To combat some of these disadvantages, the Mayo Clinic suggests that you also take both waist circumference measurements and waist-to-hip ratio to give a better idea of health risks regarding your weight.