Worked example: 80 kg, maintaining on 2,759 kcal
| Rate | Surplus | Daily target | Per week | % of bodyweight |
|---|---|---|---|---|
| Lean | 150 kcal | 2,909 | 0.14 kg | 0.17% |
| Moderate | 300 kcal | 3,059 | 0.27 kg | 0.34% |
| Aggressive | 500 kcal | 3,259 | 0.45 kg | 0.57% |
Notice how small these are next to the cutting numbers. A 500 kcal deficit is a normal, defensible cut. A 500 kcal surplus is the aggressive option here, and for most people past their first year of training it is mostly fat.
Why the surplus is small and the deficit is not
The two sides are not symmetrical, and this is the single most useful thing on the page. Fat loss is limited by how much stored energy your body can release per day, which for most people is a lot. Muscle gain is limited by how fast the body can actually build tissue, which is very slow — on the order of 0.25 to 0.5 kg a month for a trained lifter, and less as training age rises.
Calories above what that process can use do not become extra muscle. They become fat. So the ceiling on a useful surplus is set by biology, not by appetite, and eating 800 over maintenance produces roughly the same muscle as eating 300 over, plus several extra kilograms you will later spend months removing.
What the weekly gain is made of
A rough and honest split, and it moves against you the longer you have trained:
| Training age | Plausible muscle gain | Practical surplus |
|---|---|---|
| First year | 0.5–1.0 kg a month | 300–500 kcal |
| Second to third year | 0.25–0.5 kg a month | 200–300 kcal |
| Beyond that | Under 0.25 kg a month | 100–200 kcal |
These are ranges from training literature and coaching practice, not measurements of you. They are here to set the scale of the answer, because someone eating for a kilogram of muscle a month in year five is eating for a kilogram of something, and it will not be muscle.
Methodology
Target = maintenance + surplus. The three surplus sizes are 150, 300 and 500 kcal a day. Weekly weight change converts at 7,700 kcal per kilogram — the energy density of body fat, the same figure the cutting calculator uses in the other direction — and is expressed both in absolute terms and as a share of bodyweight, because 0.45 kg a week means something very different at 55 kg and at 110 kg.
Unlike the cutting side, no clamp is applied. On a cut the rate is capped at 1% of bodyweight a week because an uncapped deficit on a small person stops being a diet; on a bulk the ceiling is already so much lower that a cap would never bind.
Limitations
The 7,700 kcal per kilogram conversion is the energy density of fat, and a bulk is not building only fat. Lean tissue is roughly 70–75% water, so a kilogram of it stores far less energy than a kilogram of fat. This means the weekly figures above are, if anything, conservative during a period when you are actually gaining muscle: the scale may move faster than the arithmetic predicts. We use the fat figure anyway because it is the only defensible single number and because inventing a blended one would imply a precision that does not exist.
Maintenance itself is uncertain by roughly ±10% before you have collected two weeks of your own weight data. That error is larger than the difference between the lean and moderate surplus, which is why the honest instruction is: pick one, hold it for two weeks, weigh daily, average, and adjust from the trend rather than from this page.
Maintenance also rises as you get heavier, so a surplus that worked in week one is a smaller surplus by week ten. Recalculate every 3–4 kg.
Nothing here models training. A surplus without progressive resistance training reliably produces fat gain and very little else — the calories are permission for the training to work, not a substitute for it.
The floor applies on this side too, even though the arithmetic runs upwards. If the maintenance figure you entered is itself below roughly 1,500 kcal a day for a man or 1,200 for a woman, the problem is not the surplus — it is that the number underneath it is too low for a general-purpose calculator to be working from. Recheck it on the maintenance calculator, and if it is genuinely that low, speak to a registered dietitian or your doctor before adding anything to it.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you. If you have any condition affected by what you eat, or you are deliberately gaining weight after illness or under-eating, that conversation belongs with a doctor or a dietitian rather than with a web page.