Carbohydrates in Potatoes. A 100 g serving of baked sweet potato, or approximately 1/2 cup, contains 21 g of carbohydrates. Of those, 3 g are fiber and 6 g are sugar – the rest of the carbohydrates come from starches. A 100 g serving of baked white potato also contains 21 g of carbohydrates, with 2 g of fiber and 1 g of sugar. Although not all starches are broken down at the same rate, the types found in potatoes break down and hit the bloodstream quickly.