The question people ask is how big a surplus to run. The question that determines the answer is how much muscle you can actually build in a week, because the surplus above that number becomes fat by definition.
Plausible muscle gain, trained lifter: ~0.1–0.2 kg/week
Energy cost of that tissue: ~150–300 kcal/day
Surplus above it: stored as fat, roughly 1:1 by energy
Which means a 700 kcal surplus does not build more muscle than a 250 kcal one. It builds the same muscle and adds roughly 0.4 kg/week of fat you will spend eight weeks removing.
The rate, by training age
Beginner: 0.5% of body weight per week is achievable and mostly lean, because capacity to build muscle is high.
Intermediate: 0.25 to 0.5%. Most people reading this.
Advanced: 0.25% or slower, and honestly measured over months rather than weeks.
At 80 kg, 0.25 to 0.5% is 0.2 to 0.4 kg a week. That is slow enough that you cannot distinguish it from noise on a daily scale, which is why the weigh-in protocol matters more during a bulk than during a cut.
Why the surplus is the wrong lever to obsess over
You cannot measure a 250 kcal surplus with a tracker carrying a 12% error band. On 2,800 kcal that is ±336 kcal of uncertainty — larger than the surplus itself.
So the practical approach is not to hit a calculated surplus precisely. It is to set a target, log accurately, and let the weight trend tell you whether the surplus is real. If you are gaining at 0.6 kg a week you are over; at 0.05 you are under. The scale adjudicates, not the calculator — but only if the intake number is stable enough for the comparison to mean anything.
The part people skip
Deciding in advance how long the bulk runs and what body-fat level ends it.
Without that, a bulk becomes indefinite, and an indefinite bulk becomes a longer diet than the one you were avoiding.
Questions we get asked
How fast should I gain weight while bulking?
Roughly 0.25 to 0.5% of body weight per week for most trained lifters — about 0.2 to 0.4 kg a week at 80 kg. A beginner can productively gain faster because their capacity to build muscle is higher; an advanced lifter should gain slower because it is lower and the surplus goes somewhere. Faster gain is not more muscle, it is the same muscle plus more fat.
How big should my calorie surplus be?
Around 200 to 300 kcal above maintenance covers the useful range for most trained lifters. The arithmetic is that a kilogram of tissue is roughly 7,700 kcal, so a 250 kcal surplus supports roughly 0.23 kg a week of gain — which is at the top of the rate most people can partition favourably.
Is a dirty bulk ever worth it?
Only if your constraint is genuinely that you cannot eat enough, which is real for some very lean, very active people. For everyone else the extra fat gained has to be dieted off later, and every week spent dieting is a week not spent gaining. The arithmetic rarely favours it.
Caleb Ostrowski
Editor · Stronger by Math
Coaches lifters and writes the arithmetic down. Nine years of client logs, most of them unglamorous. Buys every app on this site at retail and cancels most of them. No affiliate links anywhere on this desk.
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