How much of your weight loss
comes off as muscle
Most calculators ask what drug you take. This one starts with how much fat you have, because that is what decides the split, then applies what your lifting and your protein are worth on top.
Muscle Loss Calculator
How much of your loss comes off as lean mass
LEAN MASS YOU'D LIKELY LOSE
6.8–12lb
23% of a 40 lb loss over about 24 weeks. The rest, around 31 lb, comes off as fat.
LEAN MASS LOST, COMPARED
IF YOU DID NOTHING
17lb lost
NO LIFTING · REFERENCE PROTEIN
WITH YOUR PLAN
9.2lb lost
NO LIFTING · 165 G PROTEIN
Your plan projects 7.5 lb less lean mass lost: the same total weight off, a different mix of tissue.
RANGE WIDTH REFLECTS HOW YOUR FAT MASS WAS MEASURED: ±4KG ON THIS TAB, PLUS THE MODEL'S OWN ±20%.
Your body and your 40 lb goal, held fixed. Only the two things you control move.
| PROTEIN G/DAY ↑ | None | 1 | 2 | 3+ |
|---|---|---|---|---|
| 165 g | 9.2 | 7.3 | 6.2 | 5.4 |
| 144 g | 9.7 | 7.7 | 6.5 | 5.6 |
| 123 g | 12.0 | 9.4 | 7.9 | 6.9 |
| 101 g | 14.0 | 11.0 | 9.6 | 8.4 |
| 80 g | 17.0 | 13.0 | 11.0 | 9.9 |
PROTEIN AXIS RUNS FROM YOUR PER-SEX FLOOR TO YOUR BILD HEALTH TARGET: SIZED TO FAT-FREE MASS, NOT BODY WEIGHT. THE TRIALS BEHIND THE MODEL STOP PAYING AROUND 160 G, SO ROWS ABOVE THAT READ ALIKE. YOUR CURRENT SETTING IS OUTLINED.
Disclaimer: This calculator projects a range from published weight-loss trials. It is an estimate for education, not a prediction about you and not medical advice. It is not intended for people with conditions that affect body composition or protein needs. Talk to your provider before changing how you eat or train.
Updated August 28, 2026
IN SUMMARY
On a GLP-1, roughly a quarter to two-fifths of the weight you lose comes off as lean mass. But that span is an average across trials, and your own number depends far more on your body than on your prescription. The calculator above starts where the evidence starts: the more body fat you have when you begin, the smaller the share of each pound that has to come from muscle. It then applies the two things you control. Lifting three times a week cuts the lean share by about 43%, and twice a week gets you most of the way there. Getting protein from the reference intake up to your own fat-free-mass protein target cuts it by about 50% again. Losing faster pushes it the wrong way. The output is a range, not a number, and the range is honest about how well you have measured yourself.
What the calculator is doing
- 01
Your starting fat mass sets the baseline
The one input that moves the answer most. A body with more fat to spend covers more of the deficit with fat, so the same diet costs it less lean tissue.
- 02
Lifting and protein scale it down
Both are applied as proportions, not flat credits: the trials they come from report proportions, and a flat credit would mean something different to a lean body than to a heavy one. Three sessions a week is where the training effect was measured; fewer sessions earn a reduced credit, and once a week is an estimate below what the trials tested.
- 03
There is no drug term at all
Pooled trials put lean loss on incretin medications close to where lifestyle change alone puts it. The medication changes how much and how fast you lose, and the calculator already asks you both.
- 04
The tabs buy precision, not a different answer
Basic, Smart and Advanced run the identical rule. What changes is how well your fat mass is known, and the range you see narrows to match.
Three numbers the model is built on
Every coefficient in the calculator traces to a published arm. These are the three doing the most work.
Pooled across 20 randomised trials, lean mass was 17.5% of the weight lost when resistance training was added to lifestyle change, against 26.2% for lifestyle change alone. The trials pooled here are between-study, so read these as what each pooled group reported, not as a head-to-head.
Across a 40% energy deficit, both the doubled and tripled protein arms held on to proportionally more fat-free mass than the reference intake, and the tripled arm added nothing over the doubled one. The calculator's protein credit stops where that trial's benefit stopped.
In adults aged 65 and over on the same diet programme, lean mass fell 2% in the resistance group and 5% in the aerobic-only group. An older cohort than a typical GLP-1 user, and worth knowing when reading the size of that gap.
Why starting fat mass decides the split
Forbes established the relationship in 2000, and it is the least controversial thing on this page: the more fat you carry at the start, the smaller the share of a loss that comes off as lean tissue. The relationship is curved, not straight. Going from 12 kg of fat mass to 25 kg changes the projected lean share far more than going from 45 kg to 58 kg does.
This is why a calculator that leads with your medication is answering the wrong question: the drug moves how much and how fast you lose, which is what the tracker records week to week. Two people on the same dose, losing the same forty pounds, get genuinely different splits if one of them started forty pounds heavier. Forbes also gives the calculator its rate term: as the deficit deepens, lean tissue makes up an increasing fraction of what comes off. That work predates these medications entirely, which is part of why it transfers: it is a statement about deficits, not about drugs.
Where the model is soft
The backbone is not Forbes' own published constant. His equation was built for infinitesimal weight changes, and Hall showed in 2007 that at large losses (bariatric magnitudes, which GLP-1 users now reach) the original consistently under-predicts fat-free-mass loss. Our constant is refit to correct for that. It is ours, not his.
The rate term is the weakest thing here and is set deliberately low. The direction is well supported, but our calibration arms cannot confirm the size: how fast each study lost weight is tangled up with who was in it and how fat they were. Two of the arms point the other way outright. So the rate input moves the answer, but not much, and it will stay that way until there are arms that can settle it.
Two more limits worth naming. The lifting and protein effects come almost entirely from people not on these medications (athletes, dieters, older adults), and carrying them across is a reasonable assumption rather than a proven one. And the whole thing is fitted to ten study averages, which is a small number to fit four parameters to. The band the calculator prints is sized with that in mind.
Why your first DEXA will look worse than this
DEXA counts glycogen and the water held with it as lean mass. The first four to six weeks of a deficit are a published, quantified phase of their own, with rapid composition changes that are not muscle leaving. A scan taken inside that window will read low, and reading it as muscle loss is the most common way people scare themselves early.
The fix is boring: treat the first scan as a starting line, not a result, and compare scans a couple of months apart. That is also how Bild Health charts it: fat and lean held apart as separate lines, with scans landing as anchor points against a body-composition scale's cadence rather than as verdicts on their own.
What each one is worth
Four inputs move the number and one does not. The effects multiply rather than add, so the same credit is worth more absolute lean mass to a leaner body, which is the backbone doing its job.
| Input | Effect on the lean share | Where it comes from |
|---|---|---|
| Starting fat mass | The largest driver, and a curve rather than a slope | Forbes 2000, refit for large losses per Hall 2007 |
| Lifting, 3+ sessions a week | Cuts it by about 43% | Villareal 2017's resistance arm; pooled training arms |
| Lifting, 2 sessions a week | Cuts it by about 34% | Interpolated: no arm we fitted separates two from three |
| Lifting, 1 session a week | Cuts it by about 22% | Extrapolated below the trials' floor: our weakest number |
| Protein, up to double the reference intake | Cuts it a further 50%, then stops | Pasiakos 2013's three arms across a 40% deficit |
| Rate of loss | A small push upward above 0.75% of body weight a week | Direction from Forbes; size deliberately conservative |
| Your medication | No term at all | Pooled incretin lean shares sit near lifestyle change |
Outputs are projections from published trial averages, not predictions about an individual. The training effect was measured on protocols running three sessions a week. The two-session row is our own interpolation inside the range those trials support; the one-session row sits below it entirely and is the least certain number on this page.
Estimating the split versus measuring it
The calculator is a model of people like you. The app is a record of you. Bild Health reads fat and lean mass from a body-composition scale through Apple Health and anchors them with imported DEXA scans, so the number you are working against is measured rather than assumed.
- Project a range from published trial averages
- Show what changing your protein or your training is worth
- Guess your fat mass from height, weight and sex (±4 kg)
- Your actual fat and lean mass, tracked as separate lines
- A real rate of loss from your own weight trend, not a slider
- DEXA scans imported as anchor points for the scale between them
Where these numbers come from
- 01
Body fat content influences the body composition response to nutrition and exercise
Annals of the New York Academy of Sciences · 2000 · Forbes GB. Ann N Y Acad Sci. 2000;904:359–365
- 02
Body fat and fat-free mass inter-relationships: Forbes's theory revisited
British Journal of Nutrition · 2007 · Hall KD. Br J Nutr. 2007;97(6):1059–1063
- 03
The FASEB Journal · 2013 · Pasiakos SM, et al. FASEB J. 2013;27(9):3837–3847
- 04
Aerobic or Resistance Exercise, or Both, in Dieting Obese Older Adults
New England Journal of Medicine · 2017 · Villareal DT, et al. N Engl J Med. 2017;376(20):1943–1955
- 05
Diabetes, Obesity and Metabolism · 2026 · Eisa N, Barood O. Diabetes Obes Metab. 2026;28(6):4818–4827. doi:10.1111/dom.70666
The muscle loss calculator, answered
Why do leaner people lose more muscle?
Because your body spends what it has most of. Forbes showed an inverse curvilinear relationship between how much fat you start with and the share of a loss that comes off as lean tissue: the more fat you carry, the more of the deficit fat covers. Someone starting at 45 kg of fat mass has a much deeper reserve to draw on than someone starting at 12 kg, so the same diet costs them less muscle. This is the single biggest driver in the calculator, and it is why two people on the same medication and the same plan get different answers.
How accurate is this?
It is a projection with an honest range, not a measurement. The model was fitted against ten published study-arm averages and leaving each one out in turn moves the prediction by about a fifth. On the Basic tab the bigger error is often not the model at all: it is the guess at your fat mass, which carries about ±4 kg. That is why the answer is shown as a band and why the band narrows on the other two tabs.
Do GLP-1 medications make you lose more muscle?
Not by themselves, on the evidence we have. In a meta-analysis of 20 trials, lean mass was 26.2% of the weight lost with lifestyle change alone and sat in a similar place across incretin medications. The difference between medication and lifestyle was not statistically significant. What changes on a GLP-1 is the inputs: more total loss, often faster, and protein missed on the days appetite disappears. That is why this calculator has no drug term. It asks what you weigh, how fast it is coming off, and what you are doing about it.
Why does more protein stop helping at some point?
Because a trial went looking for the extra benefit and did not find it. Pasiakos ran three arms through a 40% deficit: the reference intake, double it, and triple it. Both higher arms held on to proportionally more fat-free mass than the reference, and triple did nothing that double had not already done. So the calculator ramps its protein credit up to the doubled intake and then stops. Rewarding you past that point would encode a benefit the evidence does not support.
My DEXA scan says I lost more lean mass than this predicted.
Check when you took it. DEXA counts glycogen and the water bound to it as lean mass, and the first four to six weeks of a deficit are a distinct phase with their own rapid composition changes. A scan taken inside that window reads low for reasons that are not muscle. Compare scans taken a couple of months apart instead, and treat the first one as your starting line rather than as a result.
Is lifting or protein the bigger lever?
Lifting, slightly, but they are close and they stack. In the model, three resistance sessions a week cut the lean share by about 43%, two sessions by about 34%, and one by about 22%. Going from the reference protein intake to double it cuts it by about half again. Neither is a substitute for the other, and the honest framing is that the pair together moves the number far more than either alone. Both are targets to work toward, not prescriptions.
Related pages
- How to Keep MuscleOzempic Muscle Loss: How to Keep Muscle on a GLP-1
- Protein CalculatorProtein Calculator for GLP-1
- DEXA vs. Smart ScaleAre Body Fat Scales Accurate? DEXA vs. Smart Scale
- Body Fat CalculatorBody Fat Calculator: A Healthy Target by Age
- For MenGLP-1 Muscle Loss in Men: Keeping Muscle After 40
- For WomenGLP-1 Muscle Loss in Women: Menopause, Muscle and Bone
The calculator estimates the split. The app measures it.
Bild Health keeps fat mass, lean mass, protein, training and your dose history in one record — the lean number measured from your scale, anchored by imported DEXA scans.
Lose the fat. Keep the muscle.