What this estimates, and what it doesn’t
This reads four things that predict muscle loss (your rate of loss, your protein intake, your resistance training, and your age) and tells you which one is the problem. It does not measure your muscle. Nothing typed into a web form can. Lean mass is measured by DEXA or by a reasonable BIA machine, and if the answer genuinely matters to you, that scan is worth more than every calculator on this site combined.
What an estimate is good for is triage. Most people asking “am I losing muscle?” have one obviously weak input and three fine ones, and they don’t know which is which. That’s a question a few numbers can answer.
Rate of loss is the number to watch
The most useful output here is your loss expressed as a percentage of body weight per week, because that’s the form the guidance actually takes.
The commonly cited target is 1–2 pounds a week, which for most adults is somewhere around 0.5–1% of body weight. Past roughly 1% per week, the share of the loss coming from lean tissue tends to climb.
The percentage framing matters because pounds lie at the extremes. Two pounds a week at 320 lb is 0.6%, comfortably fine. Two pounds a week at 140 lb is 1.4%, which is a different situation entirely, at the same number on the scale.
| Your pace | What it means |
|---|---|
| Under 0.5%/week | Slow and protective. Fine indefinitely. |
| 0.5–1%/week | The usual target range. |
| 1–1.5%/week | Fast. Protein and lifting stop being optional. |
| Over 1.5%/week | Faster than most guidance supports. Worth raising with your prescriber. |
On a GLP-1, pace often isn’t a choice you consciously made: appetite drops sharply, intake collapses, and the rate follows. That’s precisely why it’s worth checking rather than assuming.
Why GLP-1s make this a live question
Every method of losing weight costs some lean tissue. Body-composition work on GLP-1 weight loss puts the lean-mass share of the total at roughly 25–40%, and studies range wider in both directions. That’s broadly what shows up after bariatric surgery and in ordinary calorie restriction too. It isn’t unique to these drugs.
What is somewhat particular to GLP-1s is the mechanism. The medication works by suppressing appetite, and protein is usually the first thing to fall short when you’re not hungry. It’s the most filling macronutrient and the least appealing when nothing sounds good. So the drug creates fast loss and a protein shortfall at the same time, the two ingredients that decide how much muscle goes with the fat.
That’s the problem the three levers below exist to solve, and none of them require stopping the medication.
Lever one: protein
The single most reliable dietary defense. In a deficit, roughly 1.6–2.0 g per kilogram of body weight per day is the range research supports for preserving lean mass (for a 180 lb person, about 130–165 grams).
Most people reading this are well short. The estimator compares what you entered against the 1.6 g/kg floor and tells you the gap in grams, which is more actionable than a ratio. Practical version: eat protein first at every meal, front-load breakfast when appetite is highest and nausea lowest, and use a shake on the days solid food is a struggle. That last one isn’t cheating; it’s how people actually hit the number on a low-appetite day.
The protein calculator gives you the full range and a per-meal target.
Lever two: resistance training
Protein supplies the material. Lifting supplies the reason.
Muscle is metabolically expensive, and a body in a deficit sheds tissue it isn’t using. Regular resistance work is the signal that says this is being used, keep it. The research on preserving lean mass during GLP-1 treatment consistently points the same way: higher protein plus resistance training beats either one alone, and beats diet alone by a wide margin.
Two sessions a week is the usual threshold, and the bar for what counts is lower than people assume. Bands, bodyweight work, and a couple of dumbbells at home all qualify. Walking and cardio, for this specific purpose, do not. They’re good for plenty of other things and they are not a muscle-retention signal.
More on the mechanism in our guide to GLP-1s and muscle loss.
Lever three: pace
The slowest lever to pull and sometimes the most effective. If you’re losing over 1% of your body weight per week and you’ve already fixed protein and training, the pace itself is what’s left.
Pace isn’t fully in your hands on a GLP-1, but it isn’t fully out of them either. Eating enough (not just enough protein, but enough total food) is the lever most people haven’t considered, because appetite suppression makes chronic under-eating easy to do accidentally. If pace stays high after that, it’s a reasonable thing to raise with your prescriber; dose and titration speed are adjustable.
Knowing your starting point helps here too. The lean body mass calculator estimates how much of your current weight is lean tissue, which puts the pounds you’re losing in context.
Age and sex
Both are in the inputs because both change the stakes rather than the mechanics.
Lean mass declines on its own from roughly the sixth decade onward, and the decline accelerates for women after menopause. The same rate of weight loss costs more muscle at 65 than at 35, and there’s less in reserve to start with. That doesn’t argue against losing weight. The health case for it is generally stronger with age, not weaker. It argues for the protein and the lifting being non-negotiable rather than nice-to-have.
What the read-out is worth
A heuristic, honestly labeled. The thresholds come from published guidance; the way they’re weighted and banded into low, moderate, and high is our construction, built to be useful rather than validated in a trial. Treat a “high” as a prompt to fix the named input, not a diagnosis, and treat a “low” as a reason to keep doing what you’re doing, not proof that nothing is happening.
If you want to actually know, get scanned. Everything above is education, not medical or nutrition advice.