Funny story a little off topic. My wife is from Sweden, and she brought some measuring cups over when she moved here to the states. She brought a 1l cup that also had the pint and 2 pint markings on it. I've used these markings for years (seriously like 12-13) and never thought anything of it.
Well one day I decided to mark the gallons on my carboy using this perfect 2pt/1qt measuring cup. In 1/4 increments I marked the carboy glass with a sharpie all nice and neat and used clear packing tape to seal it so it wouldn't wash off over time with washing and such.
However I was noticing my 6 gallon carboy were coming out to about 5.25-5.5 gallons in total. It was strange at the time but I figured the "6 gallon" print on the box might be an engineering estimation by the manufacturer and maybe they shrink during the glass blowing or something.
The other week when I had a wine that I bottled, I had right at the 5 gallon mark. So I knew I had 25-26 bottles to full up. So I laid out 25, got them washed and sanitized. Filling them up I noticed that I was going to run over, not by a glass or two, but I ran over by FIVE BOTTLES worth! Seriously? 5?... Wtf?!?!
I grabbed my wifes measuring cup and our digital scale. Zeroed out the weight if the container and went to fill 1 liter of water which we all know to weigh 1kg. It checked out. So what was the issue? Apparently the other side was marked at the imperial pint/quart scale. I verified by cooling the weight of an imperial gallon og water and weighing it. Sure enough I had marked all 10 of my carboys using imperial gallons!!!! Ugh.
So anyway, this solved the mystery of why two gallons if prease cider came out to 1.5 gallons in the carboy and also why I was constantly running over my bottling estimation. Talk about a pain in the ass. Now every time I need a volume measurement I've got to multiply by 1.2 to get my us gallons.
Anyone that knows me knows that this is THE reason I'm using the metric system more and more when I go to home brewing. Comes in especially hand when figuring out parts per million.
Sorry for the winded post, but I find it pretty funny that an American like me got beat by not using the metric system