Your muscle-gain projection, in numbers
A training block changes more than the scale. This page models what a surplus plus resistance training does to your lean mass, body fat and circumference measurements over the horizon you choose.
Your details
Everything is calculated in your browser. Nothing is uploaded and no account is needed.
What the model actually changes
Muscle gain is modelled as a controlled surplus: your calorie target sits above maintenance, and the resulting weight gain is split between lean tissue and fat. BodyTrajectory assigns roughly 70% of the gain to lean mass by default, then caps the body-fat rise at about one percentage point — if an all-out surplus would push body fat higher than that, the excess gain is booked as lean mass instead.
That cap matters, because it is what keeps the waist from widening while the shoulders grow. The projected circumference for the waist is clamped so it never exceeds your current value, while shoulder, chest and arm are widened in proportion to the change. A lean bulk should not read as a waist-expansion plan.
Why the number on the scale is the least useful one
During a recomposition phase, weight is a poor signal: you can add a kilo of lean tissue while losing a kilo of fat and see a flat scale. That is why this page reports lean mass and each circumference alongside weight, and why the trend curves are more informative than any single reading.
Circumference figures are estimates from an anthropometric regression based on your height, weight and body composition. They are not tape measurements. Treat them as a direction of travel with a stated error band, not as a prediction you can hold us to within a centimetre.
Where the numbers come from
Every number on this page comes from published sports-science formulas (Mifflin-St Jeor for basal metabolic rate, standard activity multipliers for total daily expenditure, and anthropometric regressions for circumferences). It is a calculation, not an AI-generated image and not a photograph.
Predictions are estimates for motivation, not medical, nutritional, or fitness advice. Circumference values come from anthropometric regressions based on your height, weight and body composition — not measurements.
Common questions
- How accurate is a muscle-gain projection?
- It is an estimate from population-level formulas, not an individual measurement. Training experience, programming, sleep, protein intake and genetics all move the real outcome, and none of those can be derived from height, weight and age. Use it to compare directions and magnitudes, not to predict a precise figure.
- Why does the projection cap body-fat gain?
- Because an uncapped surplus would produce a projection where the waist keeps growing while losing definition — which is not what a lean bulk is for. Capping the rise keeps the projection internally consistent with the goal you picked.
- Does this need my photo?
- No. Photo collection was removed. The projection only needs height, weight, age, sex and activity level, and the body outlines on your report are calculated shapes drawn from your measurements — not a photograph and not a prediction of how you will look.
- What horizon should I pick?
- Muscle accrues slowly, so short horizons mostly show measurement noise. Longer horizons give a more meaningful trend, but the assumptions drift the further out you go, which is why the model caps at 52 weeks.
Gym Transformation by horizon
Other scenarios
- Weight LossA deficit does not just remove weight — it changes the ratio between fat and lean tissue.
- Stop Working OutDetraining is the scenario people underestimate, because the scale often stays put while the body composition changes underneath it.
- Yoga BodyA yoga practice usually does not change your weight much.
- Postpartum RecoveryPostpartum recovery is not a fat-loss race.