The Formula to Calculate Fat Mass & Lean Muscle Mass

The most accurate way to measure muscle mass is to first figure out your fat mass. There are only a few testing options that are truly accurate, but they tend to be costly and hard to access. There are less accurate formulas that might be a little off, but they can still give you an idea of your muscle-to-fat ratio.

Measuring your muscle mass can show you how much progress you're making in the gym. (Image: PeopleImages/E+/GettyImages)

Determine Your Muscle Mass Percentage

Instead of trying to figure out how much muscle mass you have, which can be difficult, it's best to focus on lean body mass or fat-free mass. The two are very similar.

Lean body mass is the weight of the bones, organs, skin, muscles and water in your body. Around 2 to 3 percent of that amount comes from fat in men, and 5 to 8 percent in women. On the other hand, measuring fat-free mass means taking out all fat completely and only measuring what's left.

Finding your true muscle mass is extremely difficult because you then have to separate the weight of muscle from the weight of organs, bones, water and skin. Keep in mind that the weight of your bones doesn't change very rapidly.

The weight of your skin and organs doesn't change much either. Therefore, you can measure the change in muscle mass by measuring your fat-free mass at two separate points.

Take Body Fat Measurements

The first and least accurate method of measurement is called BMI, which stands for body mass index. It's a number that is simple to figure out, which is why it's used so often in fast-paced environments like a doctor's office. All you need to do is divide your body weight in kilograms by your height in meters squared, according to the Centers for Disease Control.

While it's useful for some things, BMI isn't made for measuring a muscle-to-fat ratio. You'll need a slightly more specific measurement for that.

One of the most common methods is the skinfold test. Using large calipers, you pinch fat in three different sites on the body and plug the results into an equation that gives you an estimate of your body fat.

The three sites to measure for men, according to an article from ACE Fitness, are:

  • Chest
  • Abdomen
  • Thigh

For women, the sites are:

  • Tricep
  • Suprailiac (right above the hip joint)
  • Thigh

There's also a seven-site version of the test, but an October 2015 study published in Clinical Physiology and Functional Imaging shows that the two tests are equal in terms of accuracy.

You can find an online calculator for the three-site skinfold test on the ACE Fitness website. All you have to do is input your age, weight and gender, and then it'll indicate your body fat and lean mass by weight.

Consider Bioelectrical Impedance

Bioelectrical impedance analysis uses the natural elements of your body to determine your body fat levels. This option is usually more expensive than doing skinfold measurements, but it's more accurate — although it's not the most accurate test.

For this test, you either stand on a scale that has one metal plate under each foot, or grab a handheld device that has metal plates under the hands. A mild electrical signal, which you won't even feel, is sent through your body at lightning speed, and the machine reads what comes out.

Water in your body conducts electricity, explains a June 2014 review published in Sensors. However, fat doesn't conduct electricity. The machine receives a dampened electrical signal and measures how much it changed as it went through your body.

Bioelectrical impedance is more accurate than taking skinfold measurements, according to the research paper in Sensors, because it doesn't rely on a human being. The device does all the work. That takes away the chance of inaccurate measurement because of a human mistake.

However, it's far from perfect. For example, the amount of water you drink in the morning can throw off the results. It's recommended that you fast for eight hours before a bioelectrical impedance test, according to the review in Sensors, and empty your bladder beforehand. Even your posture can throw off the measurement, so make sure you perform the test in the same position each time.

There are two separate formulas that the numbers can be run through — one for males and the other for females. The formulas also take into account age, height, weight and even race. All factors need to be taken into account to give you a more accurate reading of body fat.

Use the DEXA Machine

DEXA is considered the gold standard of lean body mass measuring. While it's probably the most accurate way to measure muscle mass, it's generally costly to use. The machines are large and expensive, which is why you'll only find them in hospitals and clinics.

The DEXA machine is commonly used as a tool to measure bone density, making it a valuable tool in screening for osteoporosis, according to an article from the University of California, San Francisco Department of Radiology & Biomedical Imaging (UCSF). The article explains that DEXA has the highest accuracy of body fat measuring tools.

The DEXA scan can measure body fat, bone and lean tissue in specific areas of your body. Not only can you measure total-body lean mass, you can see if you're gaining muscle in a certain area. This is useful for injured athletes who want to see if they're regaining muscle mass in the spot where they are hurt.

In addition to being one of the best assessment tools for bone mass and body fat, DEXA is one of the most accurate ways to measure muscle mass. According to a January 2018 review published in the Journal of Cachexia, Sarcopenia and Muscle, DEXA scans are the best tool to evaluate a patient for sarcopenia, a muscle-wasting condition associated with aging.

DEXA seems to be the most accurate tool because it doesn't depend on predetermined formulas — like bioelectrical impedance does — to figure out how much muscle or fat you have. Plus, it can measure by body part.

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