The grams of carbohydrate in a serving of oatmeal varies based on the type of oatmeal, any ingredients added and whether it includes milk or water. A single serve packet of plain, instant oatmeal prepared with water contains 19 grams of carbohydrate.
One-quarter cup of unprepared, steel-cut oats contains 23 grams of carbohydrate. Preparing oatmeal with 1 cup of milk rather than water adds 11.03 grams of carbohydrates for whole milk, 11.42 grams for reduced-fat milk, 12.18 grams for low-fat milk and 12.15 grams for skim milk. The addition of fruits and caloric sweeteners increases the carbohydrate content, as well.