The cup is a unit of measurement for volume, used in cooking to measure liquids (fluid measurement) and bulk foods such as granulated sugar (dry measurement). It is principally used in the United States and Liberia where it is a legally defined unit of measurement. Actualcups used in a household in any country may differ from the cup size used for recipes; standard measuring cups, often calibrated in fluid measure and weights of usual dry ingredients as well as in cups, are available.
As a result of the fact that the imperial cup is actually out of use and the other definitions differ little (±3%), the U.S. measuring cups and metric measuring cups may be used as equal in practice.
No matter what size cup is used, the ingredients of a recipe measured with the same size cup will have their volumes in the same proportion to one another. The relative amounts to ingredients measured differently (by weight, or by different measures of volume such asteaspoons, etc.) may be affected by the definitions used.