What the calculator is actually solving for
Body recomposition asks a different question than a weight-loss calculator: not “what should I weigh,” but “what would I weigh at my goal body fat if I kept every pound of muscle I have right now.” That’s the number this tool returns, and it’s usually a lot higher than people expect.
The math is a set of identities, nothing proprietary:
- Fat mass = weight × body-fat %
- Lean mass = weight − fat mass
- Goal weight = lean mass ÷ (1 − goal body-fat %)
- Fat to lose = current weight − goal weight
Work a case through. Someone at 200 lb and 30% body fat carries 60 lb of fat and 140 lb of lean mass. To reach 22% body fat while holding all 140 lb of lean, they’d weigh 140 ÷ 0.78 = 179.5 lb. Only 20.5 lb comes off the scale, but their body-fat percentage drops eight points and their fat mass falls by a third.
That gap between “20 pounds” and “a third of my body fat” is the whole reason to think in recomposition terms. A conventional goal-weight calculator would have told this person to aim for 160 lb, and hitting 160 while keeping 140 lb of lean mass would mean reaching 12.5% body fat: a completely different, much harder project than they thought they were signing up for.
Why the scale is the wrong instrument
Fat and muscle weigh the same per pound, so the scale can’t tell you which one you lost. Two people who each drop 20 lb can end up in opposite places: one loses 18 lb of fat and 2 lb of lean, the other loses 12 lb of fat and 8 lb of lean. Same number on the scale, very different body, and very different metabolic outcome, because lean mass is the largest determinant of how many calories you burn at rest.
That’s why recomposition uses three measurements instead of one:
- Body-fat percentage: the composition itself. DEXA is the accurate answer, bioimpedance is the cheap approximation, and the lean body mass calculator is a reasonable estimate if you have neither.
- Waist circumference: a good proxy for abdominal fat that moves early and costs nothing to track.
- Strength in the gym: if your lifts are holding or climbing during a deficit, your muscle almost certainly is too. It’s the most practical lean-mass monitor most people have.
The two levers that decide the outcome
Everything else is detail. Recomposition comes down to protein and resistance training, and the research is unusually consistent on both.
Protein: 1.6 to 2.2 g per kilogram of body weight per day, or about 0.7 to 1 g per pound. A randomized trial in young men on an aggressive deficit with intense training found the high-protein group gained lean mass while losing more fat than the lower-protein group, which merely maintained. Meta-analyses of protein and resistance training converge on roughly 1.6 g/kg as the point where added protein stops producing extra lean mass in normal conditions, and reviews of trained athletes in a deficit push the recommendation higher still, because the leaner and hungrier you are, the more protein does. The protein calculator works out your own range and the per-meal split.
Resistance training two to three times a week, with progression. Protein supplies the material; lifting supplies the signal that the muscle is still needed. Without the training stimulus, a calorie deficit strips lean mass regardless of how much protein you eat. The body has no reason to keep tissue it isn’t using. Cardio is good for you and does very little for this specific problem.
The third factor is the deficit itself. Too small and nothing happens; too big and lean mass goes with the fat. Which brings us to the timeline.
The timeline is not negotiable
This calculator estimates weeks from a fat-loss rate of 0.5% to 1% of body weight per week (for a 200 lb person, 1 to 2 lb a week). That’s the band where most people can lose fat without shedding muscle, and it’s the honest answer to “how long does this take.”
Twenty pounds of fat at that rate is 10 to 20 weeks. Not a month. If you have 50 lb to lose, you’re looking at most of a year, and the timeline gets slower as you go, because a fixed percentage of a shrinking body weight is fewer pounds each week.
The temptation is always to go faster, and going faster reliably costs lean mass, which lowers your resting metabolism, makes maintenance harder, and pushes you toward the regain that makes people distrust the whole exercise. Speed is the enemy of the goal here.
The leaner you already are, the slower you should go. At 30% body fat there’s plenty of fat to draw on and the body defends muscle reasonably well. At 15%, the same deficit takes a much larger share of the difference from lean tissue.
Recomposition on a GLP-1
GLP-1 medications change the arithmetic in one direction and not the other. They make the deficit effortless (appetite drops, the calories fall away without willpower), but they do nothing at all for the muscle side of the equation. If anything they make it harder, because a suppressed appetite is the exact condition under which protein intake collapses first.
Analyses of the DXA substudies in GLP-1 trials have found that a substantial share of the weight lost can be lean tissue when protein and training aren’t deliberately prioritized. That doesn’t make the drugs a bad idea; it makes the two levers above non-negotiable while you’re on one. Practically:
- Hit the protein number even on days nothing sounds good. Front-load it at breakfast, when appetite is usually highest and nausea lowest, and use a shake when solid food is a struggle.
- Keep lifting through the loss, even at reduced volume. Two sessions a week is enough to hold the signal.
- Expect the scale to move faster than the 0.5–1% band, and treat that as a reason to be more careful with protein and training, not as a sign it’s going well.
The full picture is in our guide to muscle loss on a GLP-1.
Limitations
The math is only as good as your body-fat input. If that number came from a bathroom scale’s bioimpedance reading, treat the output as directional: the goal-weight figure can move several pounds on a two-point error in body fat. Use the same measurement method every time so the trend is meaningful even if the absolute number isn’t.
The calculator also holds lean mass exactly constant, which is a simplification in both directions: beginners and people returning to training often gain lean mass during a deficit, and anyone losing weight fast without training will lose some. And goal body-fat percentages below roughly 14% for women and 6% for men are athlete territory: achievable for some, hard to hold, and not something to chase from a calculator.
This is general fitness and nutrition education, not a medical or nutrition prescription.