Your maintenance calories, the surplus on top, the macros to hit, and how fast the scale should move.
Your bulking numbers
Expected weight gain
Macros at the midpoint
Maintenance comes from the Mifflin-St Jeor equation (Mifflin et al., 1990) multiplied by your activity factor. Protein is set by the selector, fat is held at 25% of total calories, and carbohydrate takes the remainder. The gain figure is an estimate, not a prediction: it converts the surplus at roughly 7,700 kcal per kilogram, and real early-phase gain runs higher because glycogen and water arrive first.
To gain weight you eat above maintenance, and the only real decision is by how much. A 5–10% surplus adds muscle slowly with little fat; 20% adds weight faster and more of it is fat. For a 75 kg lightly active man of 28, maintenance works out at about 2,375 kcal. Those three settings give roughly 2,494–2,613 kcal a day, 2,613–2,732, and 2,850. The gap between the gentlest and the harshest is 356 kcal, which over twelve weeks is close to four kilograms of difference in total mass gained.
Three steps, all visible so you can check them.
Weekly gain then divides the daily surplus by roughly 7,700 kcal per kilogram and multiplies by seven. A 297 kcal surplus works out near 0.27 kg a week. That conversion assumes the tissue you add behaves like stored energy at a fixed cost, which is why the output is shown as a range and why the first fortnight will beat it.
| Setting | Surplus | Typical weekly gain, 75 kg | Fits |
|---|---|---|---|
| Lean bulk | 5–10% | 0.11–0.22 kg | Already lean, want to stay visibly lean, training age above two years |
| Standard bulk | 10–15% | 0.22–0.32 kg | Most people in a planned gaining phase of 12–20 weeks |
| Aggressive bulk | 20% | About 0.43 kg | Underweight, coming back from a long break, or a short deliberate mass phase |
A rule that saves people months: a beginner can put on muscle fast enough to justify the top of that table, and someone five years into training cannot. If you have been lifting seriously for years, the extra calories in an aggressive bulk mostly become fat you will spend a later phase removing. The mini bulk and mini cut guide covers running these phases in shorter cycles.
Percentage splits break during a bulk. Fix protein at 30% of calories and every time you raise your intake you also raise your protein target, even though your body mass has not changed. Setting it per kilogram of body weight keeps the number tied to the thing it depends on: an 80 kg lifter at 1.8 g/kg needs 144 g whether they are eating 2,600 or 3,100 kcal.
The defaults here are the calculator's choices, not a clinical recommendation: protein from your selection, fat pinned at 25% of total calories, carbohydrate taking whatever is left. Fat has a floor worth respecting; carbohydrate is the lever, because it is the macro that fuels the training the whole phase depends on. Swap the split if your coach or your dietitian gave you a different one.
It cannot tell you how much of the gain is muscle. Nothing on a website can; that needs body-composition measurement over months. It cannot see your training programme, and a surplus without progressive overload adds fat with impressive consistency.
The equation itself has a known error band. Mifflin-St Jeor lands within 10% of measured resting metabolic rate for about 82% of adults (Frankenfield et al., 2005). For the other 18%, the miss can exceed 300 kcal before the activity multiplier adds error of its own, and the multiplier is the part you pick about yourself. Treat the output as a starting point and correct it against three or four weeks of weight data. It is also not appropriate for under-18s, for pregnancy, or alongside a medical condition that affects energy balance; ask a doctor or a registered dietitian in those cases.
Nowhere. Every calculation on this page runs in your browser in JavaScript. Nothing you type is sent to a server, stored, or logged, and there is no account and no backend behind this tool. Reload the page and the numbers are gone.
Between 5% and 20% over maintenance, decided by how lean you are and how long you have trained. In absolute terms that is roughly 120 to 500 kcal a day for most adults. Larger surpluses do not build muscle faster past a point; they add fat faster.
For a 2,400 kcal maintenance it is a 21% surplus, at the aggressive end. For a 3,300 kcal maintenance it is 15%, a standard bulk. The percentage is what matters, which is why this calculator works in percentages rather than a fixed number.
The calculator prints the range implied by your own surplus. As a shape: 0.1–0.2 kg a week on a lean bulk, 0.2–0.3 on a standard one. Expect the first two weeks to overshoot, because glycogen binds water and the scale reports it as gain.
Untrained people and those returning after a long break sometimes do. If you have trained consistently for years, plan for one or the other and use short phases instead.
Different equations. Harris-Benedict and Katch-McArdle return different resting figures from the same inputs, and Katch-McArdle needs a body-fat percentage. This page uses Mifflin-St Jeor because Frankenfield's 2005 review rated it the most accurate across non-obese and obese adults.
No. The activity multiplier already contains them. Adding a session again is the most common double-count in calorie tracking, and during a bulk it hides itself as fat gain you cannot explain.
Delete the two references above and step 1 has no justification and the error band in "Where this calculator stops" disappears. They are load-bearing, not decoration.