A Low carb Calculator to find unlisted carbs based on a food products nurtrition label. This calculator will tell you if there are any uncounted Carbohydrate grams on the food label. It works by using the formula Calories = (Fat X 9) + (Carbohydrate X 4) + (Protein X 4) . Note that this formula is not perfectly exact, but an approximation. The program takes the difference between the stated calories and actual calories and divides by 4 to yield grams, and then adds that to the Carbohydrate grams listed on the label.