Muscle mass calculator.
Estimate your skeletal muscle mass (SMM) - the muscle attached to your skeleton that you train and move with - from your sex, age, height, and weight. This muscle mass calculator uses the Lee (2000) equation, cross-validated against MRI, and returns your estimated muscle in both kilograms and pounds plus your SMM as a percentage of body weight.
Your skeletal muscle mass (SMM)
77 lb
about 34.7 kg · % of body weight
Skeletal muscle mass
the muscle itself, not lean mass
77 lb
SMM share
% of total body weight
%
Typical for your sex
population average (Janssen 2000)
38.4%
An estimate, not a scan. A smart scale or DXA measures muscle differently, so numbers will not match exactly. Recheck the same way each time and watch the trend.
the short answer
Skeletal muscle mass is estimated with the Lee (2000) equation: SMM(kg) = 0.244 x weight(kg) + 7.80 x height(m) - 0.098 x age + 6.6 x sex - 3.3, where sex is 1 for men and 0 for women. For example, a 30-year-old man at 80 kg and 1.80 m has about 33.9 kg (75 lb) of skeletal muscle, roughly 42% of his body weight. Skeletal muscle mass is the muscle itself, which makes it a smaller and more specific number than lean body mass.
What skeletal muscle mass is and how it is calculated.
Skeletal muscle mass (SMM) is the total weight of the voluntary muscle attached to your skeleton - the tissue you contract to move, lift, and hold posture. It is the muscle you are actually training for, which makes it the number most lifters and coaches care about. You cannot weigh it directly on a bathroom scale, so it is estimated from measurements that track with it: sex, age, height, and body weight.
This calculator uses the Lee et al. (2000) anthropometric equation, which was developed and cross-validated against whole-body MRI in 244 adults across a wide range of body sizes. It runs on metric units internally:
sex = 1 for men, 0 for women
If you enter imperial units, the tool converts pounds to kilograms and feet or inches to meters before the math, then reports your skeletal muscle in both pounds and kilograms, along with SMM as a percentage of your total weight.
One honest simplification: the full published model also includes a race or ethnicity term (0 for White or Hispanic, minus 1.2 for Asian, plus 1.4 for African American). This tool implements the practical height, weight, sex, and age version and uses the White or Hispanic reference, so for other groups it can be off by roughly a kilogram or so in either direction. It also cannot see how muscular you actually are the way a scan can, so treat the figure as a well-anchored estimate rather than a precise measurement.
Skeletal muscle mass vs lean body mass vs FFMI.
These three terms get used interchangeably, but they measure different things and it is worth keeping them straight. Skeletal muscle mass is only the voluntary muscle attached to your skeleton. Lean body mass is everything that is not stored fat, so it is a larger figure that also includes bone, organs, connective tissue, and body water - your skeletal muscle is a big chunk of it, but not the whole thing. Estimate that wider number with the lean body mass calculator.
FFMI, the fat-free mass index, is a different idea again: it takes your lean mass and scales it by height, so it describes how muscular you are relative to your frame rather than giving a raw weight. Two people can carry the same skeletal muscle mass but land at very different FFMIs if they are different heights. Turn your lean mass into that muscularity score with the FFMI calculator, and get an accurate body fat reading first with the body fat calculator.
In short: skeletal muscle mass is the most specific of the three, lean body mass is the broadest, and FFMI is the height-adjusted comparison. Coaches often glance at all three because each answers a different question - how much muscle, how much non-fat weight, and how muscular for the frame.
What counts as a normal skeletal muscle mass percentage.
The clearest sourced benchmark comes from the Janssen et al. (2000) whole-body MRI study of 468 adults aged 18 to 88. It reported skeletal muscle as a share of total body weight, so you can compare your SMM percentage against a real population rather than an invented norm. These are averages, not goals - a lean, well-trained person can sit comfortably above them.
| Group | Skeletal muscle as % of body weight |
|---|---|
| Adult men (average) | about 38.4% |
| Adult women (average) | about 30.6% |
| General adult band | roughly 30% to 40% |
Source: Janssen I, Heymsfield SB, Wang Z, Ross R. Skeletal muscle mass and distribution in 468 men and women aged 18 to 88 yr. J Appl Physiol, 2000. Skeletal muscle mass declines with age in the same study, most noticeably in the lower body, which is one reason strength and muscle become worth defending as you get older. Persistent low muscle in older adults is a medical question, not something to self-diagnose here - if that is a concern, talk to a qualified professional.
Why coaches track muscle, and how to track change well.
Muscle is the point of most training, so it is the thing worth measuring. In a fat-loss phase the win condition is not just a smaller scale number - it is fat coming off while skeletal muscle holds. That is what keeps a client looking athletic instead of soft at a lower weight, and it depends on enough protein and hard resistance training inside the calorie deficit. In a gaining phase the same number confirms a calorie surplus is turning into muscle and not mostly fat, and muscle also anchors protein targets, which are often set per pound of lean tissue with the protein calculator.
Here is the honest part about tracking. Any predictive equation, smart scale, or DXA scan measures muscle a little differently, so their absolute numbers will not agree. What is reliable is the direction of change when you measure the same way each time. So do not chase one number - triangulate. Watch four things together over three to six weeks: scale weight, tape measurements at a few sites, progress photos in the same light and pose, and strength on your main lifts. When those move in the right direction together, real muscle is changing, whatever any single device reads.
The friction for a coach is doing this for every client, every week, as weights and measurements shift. A coaching platform stores each client's check-ins, recomputes their body composition as new numbers land, and charts the trend - so your time goes into adjusting the plan based on what the trend shows, not re-running a calculator one client at a time.
Track muscle and body composition for every client - in one place.
Coachway lets you log each client's weight, measurements, and photos, then watch muscle and body composition trend over time - alongside their meal plan and training in a branded client app. It is the operating system for online fitness and nutrition coaches, so you stop stitching together calculators, spreadsheets, and chat for your whole client list.
Muscle mass calculator FAQ.
What is a muscle mass calculator?
A muscle mass calculator estimates your skeletal muscle mass (SMM) - the muscle you move and train with, attached to your skeleton - from your sex, age, height, and weight. This one uses the Lee et al. (2000) anthropometric equation, which was built and cross-validated against whole-body MRI. It returns your estimated skeletal muscle in kilograms and pounds, plus that muscle as a percentage of your body weight.
How is skeletal muscle mass calculated?
This tool uses the Lee (2000) equation: SMM(kg) = 0.244 x weight(kg) + 7.80 x height(m) - 0.098 x age + 6.6 x sex - 3.3, where sex is 1 for men and 0 for women. Enter imperial units and the tool converts pounds to kilograms and feet or inches to meters before the math. The published model also has a race term; this practical version uses the White or Hispanic reference, which is stated on the page.
What is the difference between skeletal muscle mass and lean body mass?
Skeletal muscle mass is only the voluntary muscle attached to your skeleton. Lean body mass is a bigger number: it is everything that is not stored fat, so it also includes bone, organs, connective tissue, and body water. Your skeletal muscle is a large part of your lean mass but not all of it - which is why the two numbers are not the same. Estimate the wider figure with the lean body mass calculator.
What is a good skeletal muscle mass percentage?
In the Janssen et al. (2000) MRI study, skeletal muscle averaged about 38.4 percent of body weight in men and about 30.6 percent in women, with a general adult band of roughly 30 to 40 percent. Higher is usually better for strength and metabolism, and muscle tends to decline with age. Treat these as population averages, not targets - a lean, muscular person can sit well above them.
How accurate is this muscle mass calculator?
The Lee (2000) equation predicts total-body skeletal muscle with an R-squared near 0.86 and a standard error of about 2.8 kg against MRI, so a single estimate can be off by a few kilograms. It cannot see how muscular you actually are the way a scan can. Use it for a starting figure and, more importantly, to track change over time - the trend in your own number matters far more than one absolute reading.
How do I build more skeletal muscle mass?
Build muscle with progressive resistance training two to four times a week, enough protein (roughly 0.7 to 1 gram per pound of body weight per day), and at least maintenance calories or a small surplus. Recheck this calculator every few weeks alongside a tape measure, progress photos, and your working weights in the gym. If those are all trending up together, the gain is real muscle.
This calculator is general information, not medical advice. Predictive equations estimate skeletal muscle mass and cannot replace a DXA scan or clinical assessment - if you have a medical condition or are concerned about muscle loss with age, consult a qualified professional.
Keep going: estimate your broader non-fat weight with the lean body mass calculator, score how muscular you are for your height with the FFMI calculator, and get a body fat reading with the body fat calculator. Browse every free tool in the calculator library.
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