6 gallon glass carboy safe capacity.

How much head space should one leave in a glass carboy? I will be using an airlock and not a blow off hose. Is 4.5 gallons safe or should i drop it down to 4 gallons?

Cheers!

I think it would depend on how wild of a fermenter the yeast is but I would probably keep the wort an inch or two below the curve of the neck of the carboy. I've fermented an irish stout in a carboy and I ended up losing alot out the blow off tube and since then I have used a bucket fermenter.

I'll be using US-05.

I just had a foam volcano in my 6.5 gallon carboy, which was filled to exactly 5.25 gallons. The previous batch was almost at an identical level, but gave no issues. I did a starter this time with a different yeast, and it was nuts.

I didn't own a blowoff tibe, and had to do an improvised one with my siphon hose and the nipple of the airlock. I think I'd have had to do it with a 4.5 gallon batch... this thing was insane (it blew the airlock several feet away).

Get yourself some antifoam drops (Fermcap S etc.) and add that to your fermenter. I ferment in 5 gal carboys, no blowoff, and like to fill them up to 4.8 gal. I do make a lot of lagers and for the ales I am usually on the low end of the fermentation temperatures so they don't foam as much.

If you are fermenting on the warmer side, or pitching onto a yeast cake, then you'll need more head space.

I use fermcap and usually put 5.5 gallons into a 6.5 gallon fermenter. For mead I have even filled it to 6.25 gallons with no problems.