How do I Measure Body Fat?

S. Gadd

Body fat can be simply defined as the percentage of fat that the body contains. While a certain amount of body fat is essential for normal body function, too much body fat leads to increased risk of diseases such as type 2 diabetes and heart problems. There are many different ways to measure body fat, ranging from fairly high tech to very simple.

Body fat calculation is one way to determine ideal weight.
Body fat calculation is one way to determine ideal weight.

The body mass index (BMI) is a better measure of body fat than a standard bathroom scale because it takes into consideration the weight to height ratio. A BMI of less than 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and over 30 is obese. One potential problem with BMI is that it tends to overestimate the body fat of people with a lot of muscle mass, such as Olympic athletes, and tends to underestimate the body fat of people who have lost muscle mass, such as the elderly. Despite these exceptions, however, BMI is considered an accurate assessment of body fat in the majority of people.

In men, a waist circumference greater than 40 inches is associated with a higher risk of developing obesity-related diseases.
In men, a waist circumference greater than 40 inches is associated with a higher risk of developing obesity-related diseases.

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Waist circumference is a method to measure body fat by determining a person’s abdominal obesity. In this simple method, a tape measure is wrapped around the waist at the level of the navel and the circumference is recorded. A circumference greater than 40 inches in men or 35 inches in women is associated with a higher risk of developing obesity-related diseases.

BMI, or the body mass index, is considered an accurate way to asses body fat in most people.
BMI, or the body mass index, is considered an accurate way to asses body fat in most people.

Body fat calipers are a popular method of measuring percent body fat and are available at most gyms or health clubs. These calipers measure the amount of subcutaneous fat, or fat underneath the skin, in various locations throughout the body, including the back of the arms and the abdomen. Basically, the skin is pinched and the jaws of the caliper measure the thickness of the fat layer. These values are then put into an equation to determine the overall percentage of body fat.

An illustration of human skin, with the subcutaneous fat in beige.
An illustration of human skin, with the subcutaneous fat in beige.

The body fat scale uses the method of bioelectrical impedance analysis (BIA) to measure body fat. A small and safe amount of electrical current is passed through the body and the amount of resistance to the current is measured. Due to the fact that the signal travels quickly through muscle but slower through fat tissue, the measure of resistance reflects the percentage of body fat.

A healthy BMI for most men is between 19 and 24.9.
A healthy BMI for most men is between 19 and 24.9.

Hydrostatic weighing, or underwater weighing, is based on the concept that fat is lighter than water, so someone with higher body fat will weigh less underwater than someone with lower body fat. This process involves sitting on a scale inside a tank of water and blowing all of the air out of the lungs for an underwater weight measurement. Percent body fat is calculated by comparing the underwater weight to the weight outside of the water.

Measuring the circumference of the waist is one way to estimate body fat.
Measuring the circumference of the waist is one way to estimate body fat.

The most accurate way to measure body fat is a dual energy X-ray absorptiometry (DEXA) scan. This is a low-radiation, full-body X-ray often used to screen for osteoporosis. A DEXA scan can give an extremely accurate assessment of body composition by comparing the measurements of fat mass, lean mass, and bone mass.

Women who have higher than 30 percent body fat are considered obese.
Women who have higher than 30 percent body fat are considered obese.

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