There are several ways to measure lean muscle mass, ranging from clinical imaging that costs hundreds of dollars to bathroom smart scales you can use daily. The tradeoff is always between accuracy and accessibility. No single method is perfect, and understanding what each one actually measures will help you pick the right approach and interpret your results correctly.
Before diving into methods, one distinction matters: most tools don’t measure skeletal muscle mass directly. They measure “lean mass” or “fat-free mass,” which includes your muscles but also your organs, bones, and water. When a DEXA scan or smart scale gives you a “muscle mass” number, it’s typically reporting lean mass with bone mineral content subtracted. True skeletal muscle is a subset of that figure.
DEXA Scans
Dual-energy X-ray absorptiometry, commonly called DEXA, is widely considered a gold standard for body composition measurement. A DEXA scan uses two low-dose X-ray beams to distinguish between fat, lean tissue, and bone mineral. The scan takes about 10 to 15 minutes, and you simply lie flat on a table while an arm passes over your body. The result is a detailed breakdown of fat mass, lean mass, and bone density for your whole body and individual regions (arms, legs, trunk), which makes it useful for spotting asymmetries.
DEXA is accurate, but not flawless. Short-term precision for repeated measurements runs around 1 to 2%, and changes in body fat can introduce errors in bone density readings of roughly 5 to 10%. For tracking lean mass over time, the practical takeaway is that changes smaller than about 2 to 3% may fall within the scan’s margin of error. A single DEXA scan typically costs $75 to $200 at imaging centers or university labs, and most insurance plans don’t cover it for body composition alone.
One important caveat: a European working group on sarcopenia endorsed DEXA as a gold standard for muscle mass measurement, but independent researchers have pointed out that different DEXA manufacturers use different algorithms to calculate soft tissue composition. That means your results from one machine may not match results from a different brand. If you plan to track changes over time, use the same machine each time.
Bioelectrical Impedance Analysis (BIA)
BIA works by sending a weak electrical current through your body and measuring how quickly it travels. Muscle tissue, which contains a lot of water, conducts electricity faster than fat. The device uses that resistance measurement along with your height, weight, age, and sex to estimate your body composition.
BIA comes in many forms: medical-grade devices with hand and foot electrodes, handheld units at gyms, and consumer smart scales. The clinical versions with multiple electrode points are reasonably accurate for group-level research, but individual readings can swing significantly based on your hydration status. Drinking water shortly before a test can cause the device to overestimate your fat mass and underestimate your lean mass. Research has shown that even acute water consumption in healthy people causes statistically significant changes in BIA body composition results. For the most consistent readings, you’d ideally fast for at least 12 hours, avoid exercise the morning of testing, and empty your bladder beforehand.
The bigger issue is that BIA is estimating lean mass indirectly through total body water. Anything that shifts your fluid balance, from a salty meal to menstruation to a hard workout, changes the reading. This doesn’t make BIA useless, but it means any single measurement is a rough estimate. Its real value is in tracking trends over weeks and months when you test under the same conditions each time.
Smart Scales at Home
Consumer smart scales use the same BIA principle as clinical devices, but with only foot-to-foot electrode contact, which limits accuracy further. A study comparing three commercially available smart scales against DEXA found that while weight measurement was reliable (off by only about 0.3 kg on average), body composition readings were not. Muscular mass errors ranged from roughly 2 to 8 kilograms compared to DEXA values, with the scales consistently underestimating fat and producing unreliable lean mass figures. The errors were worse in people with higher BMI or greater body fat.
The researchers concluded that smart scales should not replace DEXA for clinical body composition assessment. That said, if you use the same scale at the same time of day under the same conditions (first thing in the morning, before eating or drinking, after using the bathroom), the trend line over months can still tell you whether you’re gaining or losing lean mass. Just don’t take any single number as gospel.
Hydrostatic Weighing
Hydrostatic weighing, sometimes called underwater weighing, measures your body density by comparing your weight on land to your weight while fully submerged in water. Because muscle is denser than fat, a person with more lean mass will weigh relatively more underwater. Your body density is then converted to a body fat percentage, and lean mass is calculated as the remainder.
The process requires you to sit on an underwater scale, exhale as completely as possible, and hold still while submerged. Most protocols call for five separate submersions to get a reliable average. It’s accurate and has been a research standard for decades, but it’s uncomfortable and impractical for routine tracking. You also need access to a specialized facility, typically a university exercise science lab.
Air Displacement (Bod Pod)
The Bod Pod uses the same density principle as underwater weighing but replaces water with air. You sit inside an egg-shaped chamber wearing a swimsuit and swim cap while the device measures how much air your body displaces. Each volume measurement takes about 50 seconds, followed by a brief breathing test to account for the air in your lungs. The whole session lasts around five minutes.
Bod Pod results correlate well with hydrostatic weighing, though studies in children have shown that the two methods can diverge slightly depending on sex. It’s far more comfortable than being dunked underwater, and it’s available at many university labs and some sports medicine clinics for $30 to $75 per session.
Skinfold Calipers
Skinfold testing uses a handheld caliper to pinch and measure the thickness of fat beneath the skin at specific body sites. These measurements are plugged into prediction equations to estimate body fat percentage, from which lean mass is derived. The most widely used equations are the Jackson-Pollock formulas, which take skinfold measurements from three or seven sites along with your age and sex to estimate body density. That density figure is then converted to body fat percentage using a standard formula.
The accuracy depends heavily on who’s doing the measuring. A skilled practitioner with years of experience can get results within 3 to 4% of DEXA-level readings. An inexperienced person using calipers at home will produce much less reliable numbers. Calipers also become less accurate at higher body fat levels, where it’s harder to separate the fat layer from underlying muscle. A basic set of skinfold calipers costs $10 to $30, making this the cheapest option after a tape measure, but self-measurement is difficult at sites like the subscapular (below the shoulder blade) or suprailiac (above the hip bone).
Ultrasound
Portable ultrasound is a newer option for measuring muscle thickness directly at specific body sites. Unlike the methods above, which estimate total lean mass for your whole body, ultrasound lets you measure the actual thickness of individual muscles in millimeters. Research comparing low-cost A-mode ultrasound devices to higher-resolution B-mode imaging found differences of less than 0.22 mm between the two, with correlation coefficients above 0.99. That’s excellent agreement.
Ultrasound is particularly useful for tracking muscle growth in specific areas, like whether your quadriceps are actually getting thicker from a training program. Some personal trainers and physical therapists now use portable ultrasound units for this purpose. It won’t give you a total body lean mass number, but it provides direct evidence of muscle tissue change that other methods can only infer.
MRI and CT Scans
MRI and CT imaging can directly visualize and quantify skeletal muscle tissue, making them the most precise tools available. A whole-body MRI can distinguish muscle from fat, organs, and bone with a level of detail no other method matches. CT scans are commonly used in research to measure muscle cross-sectional area at specific landmarks, such as a single slice at the third lumbar vertebra, which correlates strongly with total body muscle mass.
These are primarily research tools. A whole-body MRI for body composition purposes costs $500 to $1,000 or more and isn’t offered as a routine service at most imaging centers. CT carries meaningful radiation exposure and isn’t appropriate for elective body composition testing. For most people, DEXA offers the best balance of accuracy and accessibility.
How to Interpret Your Results
Once you have a lean mass number, the most useful way to contextualize it is through the fat-free mass index (FFMI). FFMI works like BMI but for lean tissue: divide your fat-free mass in kilograms by your height in meters squared. Population data from the U.S. National Health and Nutrition Examination Survey provides reference values for adults aged 25 to 69. Normal FFMI ranges are 17.5 to 19.7 kg/m² for men and 15.1 to 16.6 kg/m² for women. Values below 17.4 in men or 15.0 in women are considered low and may indicate insufficient muscle mass. The 50th percentile sits at about 19.2 for men and 16.0 for women.
FFMI is more informative than raw lean mass because it accounts for height. A 6’2″ person will naturally carry more muscle than a 5’6″ person, and FFMI lets you compare across body sizes. If you’re tracking progress from a strength training program, watching your FFMI trend upward over months is more meaningful than comparing your lean mass to someone else’s.
Getting the Most Accurate Reading
Whichever method you choose, consistency in testing conditions matters more than the method itself. For BIA and smart scales, test first thing in the morning after using the bathroom, before eating or drinking, and not after exercise. For DEXA, the same time of day and similar hydration state will reduce scan-to-scan variability. Avoid testing the day after an unusually hard workout, a very salty meal, or significant alcohol intake, all of which shift your fluid balance.
If your goal is tracking change over time, pick one method and stick with it. Comparing a DEXA number to a BIA number from three months later tells you almost nothing, since each method has its own systematic biases. But two DEXA scans six months apart on the same machine, or weekly BIA readings on the same scale under the same conditions, will reliably show you which direction you’re heading.