How much of a surplus will cause weight gain?Watch
To give you an idea, you require approximately a 3,500 calorie surplus to gain a pound of fat, or a 7,700 calorie surplus to gain a kilo of fat. So as an example, if you have a daily surplus of 100 calories, you will gain a maximum of a kilo of fat in 77 days. This will be quite hard to notice as it will be largely hidden by daily fluctuations in weight. Also, I say a maximum of a kilo, because if you are weight training, a lot of this will be going to other things, such as recovery and muscle protein synthesis (as long as you eat enough protein), so you will gain fat even slower than that. Of course if your surplus is larger, you will gain fat faster and it will be more noticeable.