Lean-Mass
Preservation Planner
Rapid weight loss burns muscle as well as fat. Enter your rate of loss, protein, and training to estimate how much of what you lose is lean mass — and see how much muscle a better plan would save.
Your plan
Estimated lean-mass share of loss
≈ 3 of every 10 lb you lose is lean mass, not fat
Your three levers
How this works
Methodology reviewed July 2026The planner starts from a well-replicated observation: in typical weight loss without countermeasures, somewhere around a quarter of the weight lost is fat-free (lean) mass rather than fat. It uses 25% as a baseline and then adjusts it up or down according to the three levers with the strongest evidence for changing that split — the rate of loss, protein intake relative to body weight, and resistance training frequency. A pace above roughly 1% of body weight per week pushes more of the loss toward lean tissue; protein at or above about 1.6 g/kg and two or more resistance sessions a week pull it back toward fat. The adjusted figure is capped to a plausible 8–45% range. These adjustment sizes are a transparent editorial synthesis of the trial literature, not a single published equation, and are meant to rank the impact of your choices rather than to predict an exact body-composition result.
lean_share = 25%
+ rate_adjustment (slower is better: −5 to +12 pts)
+ protein_adjustment (≥1.6 g/kg best: −10 to +10 pts)
+ training_adjustment (≥3 sessions best: −10 to +8 pts)
lean_share = clamp(lean_share, 8%, 45%)
weekly_lean_loss = weekly_loss × lean_share
weekly_fat_loss = weekly_loss × (1 − lean_share)
Current weight · Weekly weight loss · Daily protein (g) · Resistance training days per week
Estimated lean-mass share of loss · Weekly fat vs lean split · Protein target · Muscle-sparing rate cap · Best-case optimized share
- Baseline lean-mass loss of about 25% reflects average dieting without high protein or resistance training; individuals vary widely.
- Protein is compared against body weight in kilograms, using roughly 1.6 g/kg as the muscle-protective threshold and 2.2 g/kg as a practical upper target.
- Resistance training means genuine progressive strength work, not incidental activity.
- A muscle-sparing rate of loss is treated as roughly 0.5–1.0% of body weight per week.
- This is a ranking heuristic with hand-chosen adjustment sizes, not a validated predictive model; the exact percentage should not be taken literally.
- It cannot account for age, sex, starting body-fat level, training experience, sleep, or medication effects, all of which influence muscle retention.
- GLP-1 medications reduce appetite and can make adequate protein intake harder to hit, which the model does not detect — it only uses the protein number you enter.
- Only direct body-composition measurement (DEXA, BIA, underwater weighing) reveals your actual fat-versus-lean split.
- Rapid weight loss and very low protein are the conditions most associated with muscle loss; if this tool flags them, treat that as a prompt to discuss your plan with a clinician or dietitian, not as a diagnosis.
- Protein targets should be individualized in the presence of kidney disease or other conditions — do not adopt a high-protein target on the basis of this tool alone.
- This planner is educational and is not medical or nutritional advice.
This calculator is for educational purposes only and is not medical advice, diagnosis, or treatment. Results are estimates based on published formulas and population averages — your individual values may differ. Nothing here is calculated on a server: everything runs in your browser and no data is stored or sent anywhere. Always consult a qualified clinician before making health, medication, or training decisions.