To calculate weight loss or weight gain (by counting calories) you need to know how many calories you eat and how many calories you burn. Since the latter is ne

To calculate weight loss or weight gain (by counting calories) you need to know how many calories you eat and how many calories you burn. Since the latter is nearly impossible to guess, it's usually estimated with the first one and weight loss during the first month. This automatically adjusts to any error in the calorie estimation. Lets say someone eats 2000 kcal a day, but the "real" number is actually 1900 kcal. Lets also assume that in fact his weight maintenance level is 1900 kcal. After a month the person has neither lost nor gained any weight. This person will then incorrectly assume that his true maintenance level is actually 2000 kcal, and reduce his estimated calories to 1900 kcal, thus starting the weight loss process. Yes, it would be nice to get perfect estimates, but it doesn't matter much in the weight loss or weight gain process. In addition, most people don't have a deficit or surplus of a mere 100 kcal, it can be 500 to 1000 kcal, which makes the error in "true" calories even less important.