A pound of body fat contain approximately 3450 - 3700 calories. If a person consumes an extra 500 calories per day, the following calculation will show in how many days will he/she gain one pound of fat. but remember so far, the scientists cannot determined exactly how much calorie make one pound of fat. therefore, we will take the approximate value and calculate. assume, 3500 cal. = 1 pound of fat number of days required to gain one pound if taken 500 calorie per day = 3500/500 = 7 days.