Body composition calculator
Skeletal muscle mass calculator
Estimate how much of your body is actual muscle — not lean mass, which counts bone and organs too. Enter height, weight, age and sex. Free, instant, no sign-up.
An offset from the source study's own cohorts. It reflects who that study measured, not biology — leave it on Reference unless you are reproducing a published figure.
Muscle is built slowly and lost quickly — the coach keeps the training and the protein honest.
What is skeletal muscle mass?
Skeletal muscle mass is the weight of the muscle you can actually train — biceps, quads, glutes and the rest. It is not the same as lean body mass, which every other calculator reports: lean mass is everything that is not fat, so it also counts bone, organs, blood and body water. Skeletal muscle is a subset of it, usually a bit over half. Confusing the two makes people think they carry far more muscle than they do.
How this calculator works
Lee et al. (2000) fitted total-body skeletal muscle mass against MRI in 244 adults: SMM = 0.244 × weight in kg + 7.80 × height in metres − 0.098 × age + 6.6 for men + a population term − 3.3. It explained 86% of the variance with a standard error of 2.8 kg, which is why this page reports a range rather than a single figure. The population term is an offset the authors reported for two of their own cohorts; it defaults to zero here.
Reading the number honestly
The equation knows your height, weight, age and sex — and nothing else. It cannot tell a lifter from a sedentary person of identical dimensions, so two people with the same inputs get the same answer no matter how differently they train. That makes it useful for tracking yourself over time, where the error largely cancels, and close to useless for comparing yourself to someone else. A DEXA or MRI scan is the real measurement.
Why there is no sarcopenia rating here
You will find calculators that take a muscle mass index and label it sarcopenia class I or II. Those cutoffs come from Janssen et al. and were derived using a bioimpedance equation, not this one. Run against this equation's output they misclassify ordinary healthy adults — an 80 kg, 180 cm man of 30 lands under the class I threshold. Mixing a cutoff from one measurement method with an estimate from another is not a screening test, so we do not print one. If you are worried about muscle loss, that is a conversation with a doctor.