A cupful is the amount of something that fills one cup. When a recipe calls for a cupful of flour, you fill a measuring cup to the top with flour. If you drink a cupful of water, you drink however much water your cup holds.

The word comes up most often in cooking and baking, where precise amounts matter. A cupful of sugar is different from a cupful of milk, even though both fill the same cup, because sugar is solid and milk is liquid. In American recipes, a standard measuring cup holds eight fluid ounces, but when someone says “hand me a cupful of berries,” they usually just mean a regular cup filled with berries, not an exact measurement.

You might notice that cupful forms its plural in an unusual way: the plural is cupfuls, not “cupsful.” You'd say “add three cupfuls of water,” because you're counting the number of cup-filling portions, not the number of cups themselves. This follows the same pattern as words like handful (handfuls) or spoonful (spoonfuls).