The size of an 8-ounce glass depends on the style of the glass, but each glass can hold 8 fluid ounces, 1 cup, or 236.5 mL. If it says that it’s an 8-ounce glass, it’s a small drinking cup.
Depending on how it’s made and what it’s made of, the size of a glass may change. Most glasses made of glass are bigger and heavier than they need to be for their size, while most glasses made of plastic are smaller and lighter than they need to be for their size. Cocktails are usually served in glasses that hold 8 oz.