PET Carboys have incorrect volume markings

Just wanted to pass along a 'heads up' to those of you who may be using the PET carboys sold at places like MoreBeer (not Better Bottle - they're made by The Vintage Shop). First off, I love the things and I have 5 of them. But the volume markings on them are wrong.

I got a 1000ml graduated cylinder for Christmas to help me calibrate all of my containers. What I found is that 5 gallons of water into my 6 gallon carboy actually fills to about 1 inch above the 5 gallon mark. There's a 6 gallon mark midway up the shoulder but I couldn't even fit 6 gallons into the thing. I think I had ~200ml left over after filling to the very top.

The bummer is that my efficiency apparently hasn't been as good as I had thought. But, I suppose it's better to be accurate.

I have to ask. Are you filling the 1000ml cylinder 4 times and calling it a gallon. Cause its not. 3785ml = 1 gallon. Over 5 gallons your over filling by 1075 ml.

Volume is just a very inaccurate measure of water in the first place, depending on the temperature of your water you could probably hit an inch high or an inch low in that container, maybe even a larger range than that. Weight is a much more accurate measurement.

But yes, it is very helpful to have a quick rough estimate of how much wort you have, I like to mark all my carboys with electrical tape at different volumes of 70° F water.

Not that I doubt you, given that we all know that the plastic brewing buckets can be way off, but are you sure you're doing the conversion from Liters (1000ml=1L) to US Gallons correctly? Yes, the markings are nominal, and really can't be trusted . . . but if you're going to correct them, make sure you do the conversions properly.

Also, how far down are you carrying the measurement? Are you using the standard 3.8L to the gallon, or are you going so far as to note 1Gal = 3785ml?

Are you sure you measured correctly? 1 liter isn't exactly one quart. Did you measure the cylinder to be accurate? 1000 ml (1 liter) of water should weigh 1 kg, and one liter is .26 gallons. So one gallon is 3785 ml. Just double checking.

Edit: I type too slow...

Alright, I went back and actually did the math on my estimate before, I was off a bit, between 4°C and 80°C, 5 gallons of water will very by about 500cm^3. So, depending on the dimensions of the vessel, you'd probably only see about 2cm of variation. The comment about the plastic not really being completely rigid and expanding when it gets filled is a probably a better source of error.

Since 1 gallon = 4 quarts and 1 quart = 946.35ml, I filled my 1000ml cylinder up to the 940ml mark and used my 10ml pipette to measure out the remaining 6.35ml to get 1 quart. I measured this out 8 times to calibrate my general use 2 gallon container, marking it as I added each quart, and then I used that container to measure gallons into my carboy. I didn't measure the water temp but it was pretty much room temp.

Also, the gallon markings on the 6.5 gallon ale pail I use for bottling were pretty much dead on. I've known for a while that my ale pail and my PET carboys didn't agree, which is why I asked for the 1000ml cylinder for Christmas, but I just assumed my PET carboy was the one that was accurate. I guess not.

EDIT: To clarify, I originally became suspicious when instead of topping up to the 5-gal mark on my PET carboy like I normally do, I for some reason topped up to the 5-gal mark on my bottling bucket and then transferred the wort into my PET carboy. When I did that, I noticed the volume topped at ~1" above the 5-gal mark, which is nearly 0.5 gallons over.