Something is off in my markings not sure which. A store bought gallons of water is more than 4 quarts in my pryex measuring cup, A store bought gallon is more than NBs botting bucket gallon. Gallon is over by a cup in each.This sounds stupid but could there be more than a gallon of water in a gallon of water? Not sure which to trust, granted its a difference of a cup but im anal about that stuff
I used a Pyrex measuring cup to mark off my carboy in 1 gal. increments. I wouldn’t trust just a gallon jug. A cup seems like a lot but a plastic jug could easily have volume variations - mostly due to filling inconsistencies I would think. I’d go by the measuring cup.
bottling buckets/ale pails are notoriously ‘off’,…didn’t realize that about pyrex and store-bought though. Personally, I would trust the store bought, as the quality-control folks @ Poland Spring/other water-slingin’ giants are likely more accountable than hash marks on your Pyrex 
Alternative points of view - that clears things up! :mrgreen:
They are just allowing for your boil off… :o
hah it all make sense now
hah it all make sense now[/quote]
LOL
Since both the bucket and the pyrex are “off” by the same amount, I’d lean toward the water jug being the culprit. I think the pyrex and bottling bucket are both accurate at a gallon.
If you had three thermometers and two were reading the same temp, I think the odd-man-out is the one that’s incorrect.
Since a pyrex measuring cup is purpose built to be an accurate measuring device, I’d suspect they have more to lose by being inaccurate. I’d trust the measuring device over a jug of water.
While I would agree that it makes sense that the pyrex measuring cup and bucket are probably accurate, or closer to accurate than the jug since they apparently match, that would mean that you got the 1 in a million bucket that actually has accurate volume markings…that’s kind of like spotting bigfoot in your backyard.
The only way to know for sure is to weigh out a gallon of water, at 60° a gallon weighs 8.34#.
1 cup out of a gallon is only a 6% difference, which really isn’t that big an error when there are humans involved. Your pyrex cup is probably more accurate than that, but since you have to fill it repeatedly to make a gallon you have lots of opportunity for human error.
The water jug can PROBABLY hold better tolerance than 6%, but maybe they don’t bother. Also, since people are unlikely to complain about getting more than they paid for, they may deliberately err on the positive side so that even worse case filling tolerance gives you at least a gallon. That’s what I’d do anyway.
I would trust the measuring cup, provided you are being very careful and know your measurements are accurate. If it happens to jive with the bucket, then congrats you got lucky.
