Giving you a strict number of exactly how many calories you need to eat to gain weight to be suitable for your desired weight class is a loaded question. First, you will need to find your RMR or resting metabolic rate. This is the number of calories you burn in a day regardless of what physical activities you participate in. You must consider activities like walks, strength training, or even fidgeting. Adding 300 - 500 calories to the amount of calories you burn in a day is most often ideal for increasing lean muscle mass.