You don't necessarily have to use a calorie and weight calculator. But there is no harm is using one. If you are watching your weight or if you are on a diet, then you can use one of these calculators and monitor your progress. My daughter was on a diet sometime back and she used an online calorie calculator to keep track of her daily calorie intake. This allowed her to make sure that she never exceeded her daily limit. Set a realistic target such as 1300 calorie per day and stick to it. This is not as hard as it sounds.