If you purchased a sandwich from a restaurant or fast food chain, the nutritional information is likely available on the company's website. If you prepared your own sandwich, websites and mobile apps such as Calorie Count, MyFitnessPal and Livestrong allow you to compute the number of calories.
Popular websites and mobile apps allow you to "build" your own sandwich in order to determine exactly how many calories it contains. In addition to calorie counts, these trackers display other nutritional information, such as grams of fat, grams of sugar and sodium. The trackers also determine how many calories you should consume in a day, and how the calories in the sandwich factor into your caloric intake goal.