There’s no single formula you can plug your weight into and get an exact muscle mass number. Muscle mass requires either a body composition measurement from a device or an indirect estimate using your body fat percentage. The method you choose depends on how precise you need to be and what equipment you have access to.
Before diving into methods, it helps to know what you’re actually measuring. Skeletal muscle mass is the weight of all the muscles attached to your skeleton, the ones you control voluntarily. Lean body mass is a broader category that includes muscle but also water, organs, bones, and connective tissue. Water alone makes up more than 70% of lean body mass. Many home scales and even clinical tools report lean body mass, not skeletal muscle specifically, so the number you see may be higher than your actual muscle weight.
The Body Fat Subtraction Method
The simplest approach uses your body fat percentage to back into an estimate. If you know your total weight and your body fat percentage (from a scale, calipers, or a scan), you can calculate your lean body mass in two steps:
- Step 1: Multiply your total body weight by your body fat percentage (as a decimal). This gives you your fat mass. For example, a 180-pound person at 20% body fat carries 36 pounds of fat.
- Step 2: Subtract fat mass from total weight. That gives you lean body mass: 180 minus 36 equals 144 pounds.
This number includes organs, water, and bone, not just muscle. Skeletal muscle typically makes up roughly half of lean body mass in most adults. So a rough skeletal muscle estimate would be around 50% to 60% of that lean figure, though this varies by age, sex, and fitness level.
Average Muscle Mass by Age and Sex
A study of 468 adults measured skeletal muscle mass as a percentage of total body weight across age groups. These averages give you a useful benchmark to compare against.
For men: ages 18 to 35 averaged 40% to 44%, ages 36 to 55 averaged 36% to 40%, ages 56 to 75 averaged 32% to 35%, and ages 76 to 85 fell below 31%.
For women: ages 18 to 35 averaged 31% to 33%, ages 36 to 55 averaged 29% to 31%, ages 56 to 75 averaged 27% to 30%, and ages 76 to 85 fell below 26%.
If you’re a 160-pound woman aged 30, these ranges suggest your skeletal muscle mass is somewhere between about 50 and 53 pounds. These are population averages. Someone who strength trains regularly will sit higher; someone sedentary will sit lower.
Home Scales Using Bioelectrical Impedance
Smart scales and handheld devices estimate body composition by sending a weak electrical current through your body. Muscle contains more water than fat, so it conducts electricity more easily. The device measures resistance and plugs it into a prediction formula that factors in your height, weight, age, and sex.
The accuracy varies quite a bit by brand. When researchers compared three popular consumer BIA devices against DEXA scans (a clinical gold standard), the best-performing device landed within 10% error for 58% of measurements. The other two managed that accuracy only 30% to 35% of the time. That means your reading could be off by more than 10% on any given measurement, which for someone with 70 pounds of muscle could mean a 7-plus-pound error in either direction.
These scales are more useful for tracking trends over time than for giving you a single accurate number. Weigh yourself under the same conditions each time: same time of day, same hydration level, same device. The trend line matters more than any individual reading.
DEXA Scans
Dual-energy X-ray absorptiometry is widely considered the most accessible clinical method for measuring body composition. A DEXA scan breaks your body into three components: bone mineral, fat tissue, and lean soft tissue. It also provides regional data, so you can see how muscle is distributed across your arms, legs, and trunk.
One important caveat: DEXA doesn’t directly measure skeletal muscle. Muscle has roughly the same density as organs like the liver and other non-fat tissues, so the scan groups them together as “lean mass.” The skeletal muscle number on your DEXA report is an estimate derived from validated equations. Studies show this estimate correlates well with MRI and CT measurements, which can distinguish muscle from other tissues but are expensive and not practical for routine use.
DEXA scans typically cost $50 to $150 out of pocket at sports medicine clinics and universities. The scan takes about 10 minutes and involves minimal radiation exposure.
Skinfold Calipers
Calipers measure the thickness of a fold of skin and the fat underneath it at specific sites on your body. The most common single site is the triceps, measured at the midpoint of the upper arm between the shoulder and elbow. A trained technician pinches the skin, places the caliper jaws on the fold, and records the thickness in millimeters. More comprehensive protocols use four sites: biceps, triceps, the area just below the shoulder blade, and just above the hip.
These measurements feed into equations that estimate your overall body fat percentage. From there, you use the subtraction method described above to get lean body mass. Calipers are inexpensive and portable, but accuracy depends heavily on the skill of the person taking measurements. The same person should measure you each time to keep results consistent.
The Skeletal Muscle Index
Clinicians use a metric called the Skeletal Muscle Index to assess whether someone has dangerously low muscle mass, a condition called sarcopenia. The formula is straightforward: divide your skeletal muscle mass in kilograms by your height in meters squared.
SMI = skeletal muscle mass (kg) ÷ height (m²)
General clinical thresholds place low muscle mass below roughly 7.3 kg/m² for women and 9.5 kg/m² for men, though cut-offs vary depending on the population studied. This metric is most relevant for older adults or people with chronic illness where muscle loss affects function and health outcomes. If you’re tracking fitness progress in your 30s, the raw muscle mass number or the fat-free mass index is more practical.
Fat-Free Mass Index for Fitness Tracking
The Fat-Free Mass Index normalizes your lean mass for your height, similar to how BMI normalizes total weight. The formula is:
FFMI = lean body mass (kg) ÷ height (m²)
For a person who weighs 80 kg with 15% body fat and stands 1.78 m tall, lean mass would be 68 kg, and FFMI would be 68 divided by (1.78 × 1.78), or about 21.5. Standardized reference ranges aren’t universally established, but values around 18 to 20 are typical for average men, 20 to 22 suggest above-average muscularity, and values above 25 are rare without pharmaceutical assistance. Women’s values run several points lower across each range.
FFMI is useful when you want to compare muscularity between people of different heights or track your own progress over months and years of training.
Which Method to Choose
Your choice depends on your goal. If you want a quick baseline, a home BIA scale and the body fat subtraction method will get you in the ballpark. If you want clinical-grade data for a serious training program or health concern, a DEXA scan gives you the best combination of accuracy, detail, and affordability. If you just need to know whether you’re gaining or losing muscle over time, any consistent method works, as long as you repeat it under the same conditions and pay attention to the direction of change rather than the absolute number.