So there are lots of threads about how much various diapers hold as well as some pretty thorough research based on pouring liquid into diapers a various rates, etc., which I suppose is the way the manufacturers estimate this.
It crossed my mind this morning that there is a way to gather this information in vivo, so to speak.
The next time you have a pretty full diaper, weigh yourself before and after changing it. The diaper, itself, contributes trivial weight, but replace it before the second weighing, if you want to be as precise as possible. The difference is the weight of the contents, all of which has nearly the same density as water. A pint is a pound, as they say, the world around (except in the pub). So a gallon (sorry, I come from a backward country) is eight pounds.
So here is the challenge. My morning diaper---triple diapered with cloth---weighed in at 6 1/2 pounds: about 3 1/4 quarts. (I should note that unless there is a leak or an early meeting, I usually wear the diaper through the morning. This one I changed at about 10:30am.)