Perhaps viking is right and that the IKEA glasses are measured in British fluid ounces? From this page: 8 fluid ounces (UK) = 7.68608 fluid ounces (US).
In any case, if one has to drink an exact amount of a certain liquid every day, I would think that it would be better to use a measuring cup than to depend on the inexact proportions of a drinking glass.
Well, I live in Europe, and here we usually have a marking on the glass an inch or two below the brim, and the capacity of the glass is measured up to that mark. In fact if you order a beer in a German bar or restaurant, and the head starts below that mark, you have a legitimate complaint, and you can demand a new beer. Also, the capacity of our glasses are always given in milliliters, or centiliters, none of this confusion about fluid ounces and weight ounces.
To summarize, if you buy a 400 ml glass, it will actually hold close to 500ml if filled to the brim. The 400ml is up to a certain marking on the glass itself. The glasses are also regularly inspected to make sure that the marking is actually correct, so that customers will not get ripped off.
I lived in Germany for a while and I do indeed expect glasses to work the way Mycroft Holmes described.
The glasses in question have horizontal steps on the outside as a design element. They are molded, rather than hand blown glass; each glass is the same as the next. It would be easy to pour a glass each day filling it to say about a centimeter from the top, and use that as a measure. These have horizontal design elements that would give one a second line to double check that one had filled the glass to a specific level. I discovered that the capacity was incorrectly marked I was checking where was a good fill level.
What is so magical about measuring cups anyway? It is not as if glasses routinely change capacity dramatically. Why dirty a second dish unecessarily? Yes, there might be slight sacrifice in precision, but as long as it was close and there error was not always on one side, the average amount would be close enough. I have smaller glasses with horizontal design elements that I drink juice out of. I performed a similar test on them, checking to see what level of fill gave the correct measure. Besides, Measuring directly in the container the beverage is served means less chance for spills.
If a glass is marketed in the US as an 8oz glass, it should hold at LEAST 8 US fluid ounces of liquid. It should not be measured in British ounces, nor should it be based on weight, we’re in the US, and it’s a conveyance for liquids.
If I took a measured 8oz container of liquid, like a small OJ or milk, I should be able to pour it into my 8oz glass all at once, without spilling it all over the place. If the darned things hold just over 7oz, label them 7oz!
Just another argument for why the US should go metric.