A five ounce paper kitchen cup holds exactly five ounces, when filled to its brim. I know, because I checked it myself with my handy kitchen measuring cup. This means that your average trip to the sink to pull a one-time-use cup will result in your obtaining about four ounces, because no sane person would fill the cup to the top and try to hoist it without risking any spillage.
But then, I’m probably not a sane person for checking the measurement of a paper cup,
We assign measurements to everything, then complain about how they actually work in the real world. For example, snacks such as potato chips note that the contents settle in the bag. Have the manufacturers thought about making a smaller package? Probably not, as people would complain about the shrinkage.
We’re okay with preventing spillage, but not shrinkage. Smaller packages are better for the environment, however. The containers we take for granted end up in landfills and ocean floors, but possibly an ideal world would enable us to send packaging back to its origin and used again. In the days of glass bottles, they could be recycled, and activistors (actors who champion causes) such as Jason Mamoa work hard to bring options such as aluminum canned water into play.
Meanwhile, I put that paper cup back in its place on the kitchen counter to use again. It will last a while there, being waxed. I also started using a kit with reusable utensils, which in the past month has kept about 90 plastic forks and spoons out of the trash. It’s small, but I think it helps.