Fitness Articles

Reference Guide to Body Composition

An In-Depth Look at Body Fat, BMI and More


Body Mass Index

What it is: Body Mass Index (BMI) is a quick and easy way to determine, in general terms, if one’s weight is appropriate for one’s height. It has recently been used to quantify an individual's obesity level.

What it measures: Like it says above, BMI helps determine if a person’s weight is within an appropriate range for their height and frame size. It does NOT measure one’s body fat level.

How it works: The equation for BMI is weight (in kilograms) divided by height squared (in meters). To convert pounds to kilograms, divide by 2.2. To convert inches to meters, multiply by .0254. A “healthy” BMI ranges from 18.5 to 24.9.

Where to find it: BMI is commonly used in doctor’s offices, in gyms, and in many weight loss programs. You can use our BMI calculator to find out where you stand.

Accuracy: Since only an individual's height and weight are used, BMI does not provide a differentiation of fat and lean muscle weight. For most adults, however, there is a clear correlation between higher BMI and negative health consequences.

Limitations: BMI is an average that is based on population studies. Because it does not differentiate between fat and nonfat weight, it may overestimate body fat in athletes and others who have a muscular build. In the same way, it may underestimate body fat in older persons and others who have lost muscle mass.

Girth Measurements

What they are: The use of girth and length measurements is a quick, easy and inexpensive method to estimate body composition or describe body proportions. These measures are based on the assumption that body fat tends to be distributed at various sites on the body, such as the waist, neck and thighs, so that is where measurements are often taken. (Muscle tissue, on the other hand, is usually located in places such as the biceps, forearm and calves, which tend to store very little fat.)

What they measure: Some girth measurements use the circumference of various sites on the body to estimate one’s true body fat percentage. Other girth measurements (such as the waist-to-hip ratio) estimate one’s health risk based on these measures.

How they work: Using a cloth tape, girth and length measurements are taken from specific points on the body, such as those described above. The waist-to-hip ratio is one of the most commonly used values to reflect the degree of abdominal obesity.
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About The Author

Nicole Nichols Nicole Nichols
A certified personal trainer and fitness instructor with a bachelor's degree in health education, Nicole loves living a healthy and fit lifestyle and helping others do the same. Nicole was formerly SparkPeople's fitness expert and editor-in-chief, known on the site as "Coach Nicole." Make sure to explore more of her articles and blog posts.

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