How much extra should you eat when bulking?

Short answer

Only a little. About 0.55% of body weight a week (about 0.4 kg at 70 kg) is enough. Eating about 600 kcal more per day than people who ate freely made fat rise 5 times as much, with no difference in lean mass. On the other hand, being about 500 kcal short per day stopped lean mass from growing.

How much extra should you eat when bulking?
'The more you eat → the more muscle': is it true?
A common belief is that the more you eat while building muscle, the better. We checked studies on how muscle and fat each changed depending on how many extra calories people ate.
What about about 600 kcal extra a day?
Fat rose 5 times as much, with no difference in lean mass
39 people added 4 strength sessions a week for 8-12 weeks. The group given a meal plan ate 3,585 kcal a day; the group eating freely ate 2,964 kcal. Body weight rose 3.9% vs 1.5% and fat 15% vs 3%, but lean mass (muscle and everything except fat) showed no difference.
Comparing maintenance, a bit more and a lot more?
The extra food went to fat
17 people lifted for 8 weeks while eating 169, 489 or 719 kcal more per day than usual. Muscle thickness and the heaviest squat showed no difference among the three; only bench press rose more in the group that ate the most. The faster someone gained weight, the thicker the fat under the skin became.
The other way round, does eating less stop muscle gain?
About 500 kcal short a day, and it did not grow
An analysis of trials where people lifted while short on calories found that lean mass grew less in the calorie-short groups. At about 500 kcal short per day it did not grow at all. Strength rose with no difference even when calories were short.
What about people who ate less than the target?
Same lean mass, half the fat
23 people trained for 12 weeks with a target of 300-500 kcal more per day than usual. The group that ate within an 8-hour window ate less than the target, yet lean mass gains of 1.34 kg vs 1.38 kg showed no difference. Fat rose 2.00 kg vs 4.36 kg.
In body weight terms, how fast should you gain?
About 0.55% of body weight a week
In a study where 21 people trained for 6 weeks while drinking an extra high-calorie shake, gaining about 0.55% of body weight a week meant that, on average, all the weight gained was lean mass. At 70 kg (154 lb) that is about 0.4 kg a week, or about 1.5 kg in 4 weeks.
Summary: eating a little more is enough
The extra went to fat
Eating about 600 kcal more per day did not add more lean mass, only more fat. On the other hand, being about 500 kcal short per day stopped lean mass from growing. Eating a little more than usual while keeping weight gain under about 0.55% a week (about 0.4 kg at 70 kg) is enough.