For the first three months most bypass post-ops manage about 500 calories, that can vary by 200 calories either way depending on how fast the person heals and if they stick to their doctors dietary instructions. It is almost impossible to over-eat early on but you can make unhealthy choices of the little meals you do it. The goal is once healed and eating normally (usually about 6 months) to maintain 800-1200 calories per day depending on gender (men can eat more calories) and activity level (active people can lose & maintain weight loss on more calories then non-active people). However, it should be noted that it is not important after a bypass to calorie count, balanced nutrition and healthy activity are the most important things to consider. Only if weight slows or stops for long periods should you evalute your calories. Even then eliminating bad behaviour that has crept in like snacking or making unhealthy meal choices too often usually should fix the problem.