Fats in Butter. Butter is about 80% fat, and the rest is mostly water. Basically, it is the fatty portion of milk that has been isolated from the protein and carbs. Butter is one of the most complex of all dietary fats, containing more than 400 different fatty acids. It is very high in saturated (about 70%) fatty acids, and contains a fair amount of monounsaturated fatty acids (about 25%).