What happens when you build muscle
Training sets the stimulus, and the building happens during recovery. Heavy sets close to muscular failure signal to your body that the muscle it has is not enough. In the hours and days afterwards it builds more muscle protein than it breaks down, as long as energy and building material are available.
This happens in small steps. Visible differences appear over months, not weeks. Beginners gain much faster in their first year of training than advanced lifters, and the possible gain shrinks with every year. Knowing this lets you plan more calmly and keeps one weak week from throwing you off.
Why a small surplus is enough
Building muscle needs energy, but only a limited amount. Whatever goes beyond that need, your body stores as fat. A large surplus therefore barely speeds up muscle gain, but it lengthens the diet that follows.
In their review of the offseason for natural bodybuilders (2019), Iraki and colleagues recommend a gain of about 0.25 to 0.5 percent of body weight per week. Beginners can aim for the upper end, advanced lifters for the lower. At 80 kg that is 200 to 400 grams per week, which is less than the scale fluctuates from one day to the next.
You estimate the surplus that fits this pace from your maintenance needs. The bulk and cut calorie calculator converts the target pace into a daily calorie number. That is a starting value: whether it is right only becomes clear from the trend.
Measure the pace by the trend, not the daily value
Water, salt, carbohydrates and gut contents shift your daily weight by one to two kilos. A gain of 300 grams per week disappears completely in that noise. That is why the smoothed trend counts: it shows how much you are really gaining per week.
If the weekly rate of the trend is above your target for two to three weeks, take out a little energy; if it is below, add some. Small steps, for example 100 to 200 kcal per day, are enough, because every change only shows up cleanly in the trend after a few weeks. More on this in the guide to weight tracking.
Training: tension, volume, progress
The most important training factor is that you manage more over time: more repetitions with the same weight, or the same repetitions with more weight. Without this progress there is no reason for new muscle tissue, even in a calorie surplus.
For volume, it helps to look at the hard sets per muscle and week. A meta-analysis by Schoenfeld, Ogborn and Krieger (2017) found a dose-response relationship: more weekly sets went with more muscle gain, with the biggest gains from about ten sets per muscle and week. But more is only better as long as you recover from it. Details are in the training guide.
Protein and the rest of your nutrition
Protein supplies the building material. In a meta-analysis, Morton and colleagues (2018) found that the additional benefit of protein for muscle gain levels off at about 1.6 g per kg of body weight per day. If you want to be on the safe side, plan up to about 2.2 g; more does no harm to healthy people, but brings no measurable benefit either.
You split the rest of your calories between carbohydrates and fat, in a way you enjoy and that supports your training. Carbohydrates fill the glycogen stores, which are needed for many sets. A bulk is not a free pass to eat anything: if you live mostly on highly processed foods, you exceed the planned surplus faster than you think.
When the bulk ends
There is no fixed rule. Many people end the building phase when their body fat has risen so far that they no longer feel comfortable, or the following cut would get too long. Others plan fixed blocks of a few months and insert phases at maintenance in between.
Waist circumference, photos under the same conditions and an estimate of body fat percentage help with orientation. The FFMI (fat-free mass index, described by Kouri and colleagues 1995) relates your fat-free mass to your height and shows how your muscle mass develops over the years.