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Understanding Weight: BMI & Body Fat

The number you see when you step on the scale doesn't tell the whole story about how healthy (or unhealthy) you may be. (Image credit: Shutterstock/Photo Africa)

Weight is usually one of the first topics that come up in discussions about personal health. But the number you see when you step on the scale doesn't tell the whole story about how healthy (or unhealthy) you may be. 

Those looking for a clearer picture of what their weight really means in relation to their health may want to take a look at metrics other than weight. One of these metrics is the body mass index, or BMI, an estimation of body fat that can be calculated using a person's height and weight. Health-conscious individuals might also want to know more about their body composition, which refers to the proportion of fat tissue you have, relative to lean tissue (muscles, organs, etc.).

Health care professionals use BMI, as well as advanced body composition tools, including special X-ray machines, to assess their patients' risk factors for certain weight-related health conditions. And anyone can use readily available tools — such as online BMI calculators, skinfold calipers and bioelectrical impedance devices — to better assess their health and physical fitness from the comfort of their own home.

What is BMI? 

The body mass index is an estimation of a person's body fat, according to the National Institutes of Health. To calculate BMI, divide your weight (in pounds) by your height (in inches) squared, and then multiply that number by a conversion factor of 703. This same calculation can be used when measuring weight in kilograms and height in meters, but no conversion factor is needed. 

For most individuals, a BMI between 18.5 and 24.9 means that their weight is normal, or "acceptable," according to the Centers for Disease Control and Prevention (CDC). A BMI between 25.0 and 29.9 means a person is overweight, and those with a BMI over 30.0 are considered obese. 

Those whose BMI puts them in the "overweight" category should consider losing weight if they also have two or more health risk factors, such as smoking, inactivity or high blood pressure, according to Katherine Tallmadge, a registered dietitian and author of the book "Diet Simple: 195 Mental Tricks, Substitutions, Habits & Inspirations” (Lifeline Press, 2011). 

"The evidence is solid that the risk for various cardiovascular [diseases], cancer, arthritis and other disease starts to rise at a BMI of 25," Tallmadge told Live Science, "This is why experts try to encourage people to be at a BMI between 18.5 and 24.9. Men tend to be at the higher end of the scale and women toward the middle and lower end, but your desired weight may vary depending on your personal [and] health goals."

Body fat

Knowing your BMI can give you an idea of how healthy your weight is. However, this metric isn't a reliable tool for everyone. Athletes and people with a muscular physique may have a BMI that would classify them as overweight or even obese, even though they are clearly fit. This is because a BMI doesn't distinguish between what percentage of a person's total weight is composed of muscle and what percentage is fat, according to the CDC.  

BMI is also not always a reliable metric for those who fall into the "normal" weight category. It's possible to have a normal weight status but still have an unhealthy amount of fat compared to lean muscle, according to New York University's Langone Medical Center, which lists several methods for finding out how much of your total body weight is made up of fatty tissue — a metric known as body fat percentage. These include:

  • Dual energy X-ray absorptiometry (DEXA) scan: Used in research institutions and health care facilities to measure bone density, DEXA scans calculate body fat by subtracting the weight of lean soft tissue in the body from a person's total weight. 
  • Hydrostatic weighing: Commonly known as underwater weighing, this method involves submerging a person in water to calculate the mass per unit volume of his or her body. Like DEXA scanners, the tanks used for hydrostatic weighing are usually found at universities and health care facilities and are not always available to the public. However, some fitness and health care facilities do offer this type of body composition testing for a fee.  
  • Air displacement plethysmography (ADP): Similar to underwater weighing, this method uses displaced air (rather than displaced water) to calculate a person's mass per unit volume. One popular version of this test is the Bod Pod, an ADP machine that can be found at many fitness and research facilities. 
  • Bioelectrical impedance analysis (BIA): One of the most accessible ways to measure your body fat percentage at home is with a BIA device, many of which are sold as scales or handheld devices. Such devices send a tiny electrical current through your body (so tiny you don't even notice it). By measuring the impedance, or opposition, to this current through your body, the devices can estimate your total body water. This measurement can then be used to estimate both lean mass and body fat percentage.
  • Skinfold calipers: This low-tech solution to measuring body fat involves using a handheld instrument known as a caliper to measure the thickness of folds of skin on different parts of the body. These measurements can then be plugged into a variety of equations to calculate body fat. 

Each of these methods for measuring body fat has its pros and cons, but the only options for home use are bioelectrical impedance analysis and skinfold calipers. The measurements you get with a BIA device can fluctuate dramatically depending on how much water is in your body. For this reason, you should use the device under the same conditions every time you want to measure your body fat (e.g., in the morning, after having one glass of water but nothing to eat). Skinfold calipers may provide more accurate body fat readings at home, but you'll need to practice to become proficient in how to use them, according to the DailyBurn, a health and fitness website. 

Regardless of how or if you choose to measure your body fat, you should always speak with your doctor before undertaking a weight loss plan, according to Tallmadge. He or she will be able to give you more information on how to safely lose weight and how to maintain the proper diet and exercise routine to keep the weight off, she said. 

Follow Elizabeth Palermo @techEpalermo. Follow Live Science @livescience, Facebook & Google+.

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Elizabeth Palermo
Elizabeth Palermo
Elizabeth is a Live Science associate editor who writes about science and technology. She graduated with a bachelor of arts degree from George Washington University. Elizabeth has traveled throughout the Americas, studying political systems and indigenous cultures and teaching English to students of all ages.