You have to burn 3500 calories to lose one pound of weight. This is why you should use a calorie counter regularly. How to lose 20 pounds? You have to have a calorie deficit* of 70,000 calories to lose 20 pounds (*burn 70,000 more calories than the number of calories you eat).1  Enter your age and gender. 2  Enter your height and weight.ou have to have a calorie deficit* of 70,000 calories to lose 20 pounds (*burn 70,000 more calories than the number of calories you eat).