The principle behind Weight Watchers is that each food has a certain number of points based on calories, fat levels, carbs, etc. You are only allowed a certain number of points per day, depending on your weight, body mass index (BMI), and other factors. This calculator can be used to help you add up how many points you have already eaten that day, and see how many points you have left remaining by subtracting your total eaten from the total amount you are allowed per day. I am a mathematics teacher, so adding and subtracting is something that I can do in my head. I do not need the calculator for it. However, I really like using the calculator to keep the running total of how much I have eaten throughout the day so that I do not have to think back and remember what I have eaten and how many points it is. After each time I eat, I just add that number on the calculator right then and leave the total up until the next time I eat when I add that number to it. This way I do not have to constantly be trying to think back and remember everything I ate and how many points it was.