How to Measure Body Composition

body composition

Any old bathroom scale can tell you how much you weigh, but the number it yields means less for your health than you think. Rather, it’s your body composition, not your weight in pounds, that determines your risk for heart disease, diabetes, and other chronic illness. 

What is body composition?

Physicians use the term “body composition” to refer to the proportion of muscle, bone, fat, and water that make up your body. Whether you have big bones or water weight, your body composition is unique to you and reflects several factors, such as sex, race, and age. 

For instance, women naturally have a higher percentage of body fat than do men. Bone density tends to decrease with age, while race can impact muscle density. 

Because body composition fluctuates, especially when gaining or losing weight, it’s a good idea to regularly check it throughout your lifetime. The aging process also affects body composition, so an elderly person should track theirs for osteoporosis (loss of bone density) and sarcopenia (loss of muscle mass).

But perhaps the most critical element to watch is your body fat percentage. This one aspect of body composition can have a tremendous impact on your health.   

Is fat bad for you?

Fat gets a bad rap, but not all fat is detrimental, and some fat is even essential to the body’s function. It just depends on the type of fat. 

There are two kinds of fat: fat mass (stored mass) and non-fat mass (essential fat). Your body relies on essential fat to protect and serve organs such as your brain, heart, lungs, liver, spleen, intestines, and muscles. Tissues rich in lipids (fat) support your entire central nervous system and even show up in your bone marrow.

All other fat is stored as a reserve for your body, though a little excess is not without its benefits, too. Fatty tissue helps cushion joints and can promote a healthier body temperature. That’s why half of your stored fat accumulates near your organs to protect and insulate them. Otherwise, your body uses stored fat as a source of energy once it’s depleted your carbohydrates, which usually only happens after at least 20 minutes of cardio exercise.

In general, however, most of us carry around a bit too much excess fat, and many are “overfat.” According to this 2017 study, 76% of the world’s population is overfat—enough to create an obesity pandemic of enormous proportions.

Stored fat Is holding you back

Across demographics, obesity can lead to a myriad of chronic illnesses in both adults and children. Being overfat means you’re more likely to develop hypertension, high cholesterol, diabetes, heart disease, and other diseases of the lungs, gallbladder, and joints. Even a thin person could be at risk for some of these diseases if their body fat percentage is too high.

But if you’re also overweight, that extra fat can weigh you down and drain your energy by making your body work overtime whenever you move. The proportion of fat to muscle affects not just your figure but also your energy and metabolism.

Why is body composition important?

It’s easy to look at the negative side effects of excess fat and surmise, “I need to lose weight.” But the pathway to better health isn’t as simple as how much you do or don’t weigh. Sometimes the amount of weight you lose isn’t as important as the type of weight you lose: fat, muscle, or just water?

Therefore, tracking body fat percentage can be both more accurate and more encouraging than stepping on a scale. Have you ever kept a diet or exercise program but lost negligible weight? In that instance, your efforts to lose weight might have instead helped replace pervasive fat for lean muscle—a far better outcome than just a lower number. It’s not all about weight!

Finally, besides guiding a weight loss program, body fat percentage can also inform your doctor about your overall health. It’s a better rubric than how far you tip the scales. 

Body Fat Composition vs. BMI

Another rubric for measuring overall health is finding out where you lie on the BMI, or Body Mass Index. This global scale uses your height and weight to broadly categorize your health in relation to what’s considered “normal” or “healthy.”  

To calculate your BMI, all you have to do is plug your measurements into a BMI calculator, which will spit out a number within the following ranges:

UnderweightHealthy weightOverweightObese
Under 18.518.5–24.925–29.930 or more

BMI is easy to calculate and requires no special equipment, but it reveals far less information about your health than your body composition does. Your BMI score can’t tell you what proportion of fat to muscle to bone density you walk around in. It could instead cast the wrong verdict on where you stand.

For example, because muscle weighs more than fat, a healthy, muscular athlete might register as “overweight” on the BMI index. And someone who is skinny but has little muscle mass may walk away from their report thinking they’re the picture of health. Calculating your body fat composition is a more accurate method than the BMI, no matter your age or level of fitness.

Waist circumference measurement

Besides charting your BMI and measuring your body composition, your doctor may also take a measurement of your girth—the circumference of your waist and hips. Believe it or not, the shape that fat takes makes a difference on your overall health.

For instance, having too much fat around the abdomen and waist—what doctors call an “apple shape”—may put you at a higher risk for developing diabetes and heart disease. Men tend to become apple-shaped as they accumulate extra fat, while women are more likely to gain excess fat in the thighs and rear and, by consequence, look pear-shaped.  

To measure your waist circumference, stand up and wrap a tape measure around your middle, just above your hip bones. Keep the tape horizontal and snug, but not too tight, as you breathe out and find your number. If your waistline is more than 35 inches (in non-pregnant women) or 40 inches (in men), you may need to slim down to stay healthy. 

How to measure Body Fat Percentage

Anyone can measure their BMI or waist circumference at home, but if you want to find out your body composition, you’ll probably need to book an appointment. 

There’s more than one way to measure fat, but perhaps the most basic way is to measure skinfolds using calipers. The fat present in your belly, thigh, hip, or arm serves as a good indication of how much extra fat your body has stored, and with the right technique your physician can get within a 3% margin of error.

For an experience with more tech—and less pinching—you could sit in a BodPod. This egg-like capsule is technically an air-displacement plethismographer, which calculates your body fat by measuring changes in pressure and density. 

The same principle of displacement guides underwater weighing, also called hydrostatic weighing. This method requires you to fully submerge in water while being weighed. Because fat is buoyant, your underwater weight should reflect your body’s mass sans fat.

How to measure Body Composition

If you’re looking for more information than just a body fat percentage, the options grow more complex and expensive. An MRI scan can indicate the actual location of your fat density and also reveal your muscle mass percentage. Bioimpedance Analysis (BIA) uses a low-grade electric current to distinguish your essential body fat from stored fat. Finally, to check on bone mineral content as well as fat percentage, you could hop on a table for a DEXA scan using dual-energy x-ray absorptiometry.

As intimidating as these tests sound, they’ve been engineered to be as comfortable and non-invasive as possible. For example, a DEXA scan is just 6 minutes long and requires little prep. The BodPod gives you immediate results and is relatively inexpensive, while the skinfold technique can be done in a doctor’s office. Some bathroom scales now come with a BIA setting, although the results might be a broad estimate at best. If you’re interested in learning your body fat percentage, ask your physician which method will promise the most accurate results for you.  

What is an Ideal Body Fat Percentage? 

Body fat is important, even essential, to overall health. The goal isn’t 0%—too little fat can lead to skin rashes, bone structure issues, an increased risk of infection, and poor exercise performance.

In general, women should aim for 21–32% total body fat, with 10–13% of it from essential fat. Men, on the other hand, should maintain 8–24% total body fat, 2–5% of which is essential.

Ultimately, a healthy body fat percentage will reflect not just your sex but also your age. At 20 years old, you can expect to gain at least 1–3% body fat per decade until you hit 60. At that point, your body fat (and your bone mass) will decline gradually.

Here’s a general breakdown of ideal body fat composition, by sex and age:

Ideal Body Fat Composition


How to improve body composition

The best way to improve your body composition–the proportion of muscles, bone, fat, and water in your body–is to try to lower your overall body fat percentage. Easier said than done, I know.

Think of it this way: in order to burn your body’s stored fat, you’ll need to introduce cardio into your routine. That’s 25-30 minutes of heart-rate-elevating exercise, 4-5 days per week.

To reduce the formation of excess fat in your body, you need to stop eating it! Swap out the bad kind (trans fats, e.g.) for the beneficial kind (polyunsaturated, e.g.), and limit your intake of sugar and simple carbohydrates (which the body converts to fat anyway).  

Finally, make sure you strength-train so that you can build lean muscle and tip your body composition back in your favor. You don’t have to buy a cumbersome exercise machine or join a costly gym to make progress. An inexpensive set of resistance bands, like the ones I use in the physical therapy clinic, can facilitate a full-body workout and set you in the right direction. 

Tim Fraticelli, DPT | Physical Therapist

Tim Fraticelli is a Physical Therapist, Certified Financial Planner™ and founder of He loves to teach PTs and OTs ways to save time and money in and out of the clinic, especially when it comes to documentation or continuing education. Follow him on YouTube for weekly videos on ways to improve your financial health.