Muscle is the tissue you least want to lose and the one most likely to go during rapid weight loss. This calculator estimates your lean body mass, your skeletal muscle mass, and how that compares to published reference ranges.
Estimates from published anthropometric equations, not a body composition scan. Skeletal muscle mass uses the Lee equation, which carries a standard error of roughly 2.8 kg. Treat the direction of travel over time as more meaningful than any single figure.
The calculator returns three related figures. They measure different things and are useful for different reasons.
Everything that is not fat: muscle, bone, organs, connective tissue and body water. It is the larger of the two numbers, and it is what the scale is really weighing once you subtract fat. If you enter a body fat percentage, lean mass is calculated directly from your own measurement rather than predicted from a population equation, which makes it considerably more reliable.
The subset of lean mass that is actual contractile muscle, the tissue you build with training and lose with inactivity or rapid weight loss. It is typically around a third to a half of lean body mass. This is the number that matters for strength, metabolic rate and how well you age.
Muscle mass divided by height squared, in the same way BMI adjusts weight for height. It lets you compare against reference ranges regardless of how tall you are. It is the standard screening measure for sarcopenia, the age-related loss of muscle.
These thresholds come from the work of Janssen and colleagues, who linked low muscle index to physical disability risk in a large adult population. They are screening bands, not a diagnosis.
Muscle mass in line with a healthy young adult reference population.
Sometimes described as class I sarcopenia. Associated with a measurable rise in functional decline over time.
Class II sarcopenia. Associated with a significantly increased risk of physical disability. Worth raising with a provider.
There is no single agreed cut-off for sarcopenia. Different research groups and clinical societies publish different thresholds depending on the population studied and the measurement method used, and a scan-based diagnosis will not always agree with an equation-based estimate. Read these bands as a prompt to ask a question, not as a verdict.
In any rapid weight loss, some of what you lose is lean tissue rather than fat. Studies of GLP-1 medications have reported that a substantial share of total weight lost can come from lean mass, particularly when intake drops sharply and nothing is done to protect muscle.
Appetite suppression is the mechanism that makes these medications work, and it is also what makes protein intake easy to undershoot without noticing.
Muscle is metabolically active tissue. Losing it lowers resting energy expenditure, which makes weight regain easier and the next attempt harder.
It also carries the functional cost: strength, balance, bone loading and the ability to stay independent later in life all track with muscle mass.
The two interventions with the strongest evidence are adequate protein intake and resistance training two to three times a week. Neither is optional if you want the weight you lose to be mostly fat.
Your provider can set a protein target for your body weight and treatment stage rather than a generic number.
The scale cannot tell you what you are losing. A falling weight with falling muscle mass is a different situation from a falling weight with stable muscle mass, and they call for different adjustments.
Track muscle mass alongside weight during treatment, and bring both to your follow-up appointments.
A supervised program sets a protein target, builds in resistance training and tracks body composition rather than just the scale, so that what you lose is what you meant to lose.
