[tsimionescu]

That's the old, less accurate way. By that measure, fiber tends to have quite a lot of calories, as it's a carbohydrate, but that is now known to be wrong - fiber passes through our digestive systems almost completely undigested, and yields almost 0 calories.

Today, calorie counts are usually obtained by checking the protein, fat, sugar and fiber contents, and using known values for calorie/g of each. Those well-known values are ideally obtained either through human calorimetric studies, or through burning protein, fat, sugar directly.