I recently fermented a batch of beer (5 gallons) in a 7gal carboy, which left about 2 gallons of space above (assuming you filled it all the way to the top). I also have a 5gal carboy, but that would be 5 gallons if I filled it all the way to the top.

My question is, if I am going to brew 5 gallons of beer, how much head space should there be? Here are the options I see,

  1. Fill up the 5gal carboy, and leave a little bit of space in the top
  2. Buy a 6gal carboy, and fill it up to 5 gallons.
  3. Use the 7gal carboy, and fill it up to 5 gallons.
  4. Use the 6 or 7 gallon carboys, and fill it up over 5 gallons.

4 Answers 4


I usually do 4/5gl in a 6gl carboy, not as a particular matter of "best", but just of convenience and what I acquired over time.

You definitely want some headspace, if only to keep krausen under control and minimize the amount of blowoff you have to redirect somewhere. For more "productive" yeasts, I'll still use a blow-off tube, but if I under-collect and/or use a less krausen-y yeast, I often don't need to.

For primary, 5gl in a 7gl carboy would probably be just fine, too; you're easily going to fill and purge that much headspace with CO₂ in the course of a normal primary ferment. If you're doing any sort of secondary or aging is really when having too much headspace comes into play.

  • I think I read before that you want a very little amount of head space on secondary, is that right?
    – Metropolis
    Sep 15, 2015 at 19:55
  • That's correct. While in primary, you'll be displacing and venting any O₂ in headspace with CO₂, and for a while, the O₂ is desirable anyways. In secondary / after primary fermentation, you want to introduce as little O₂ as possible, so having as minimal headspace as possible is ideal. You can also vent with CO₂ and add a proper airlock to help limit O₂ ingress if you do have a large headspace.
    – jsled
    Sep 16, 2015 at 14:08

You don't want to fill a carboy all the way up to the top because the foam that is produced during the initial active phase of fermentation could get into the air trap reducing its effectiveness.

I have recently done a couple of batches with about 3.5 gallons of wort in a 5-gallon carboy and they have been some of the best beer I have tasted. So leaving a fair bit of room at the top of the carboy does not seem to have a detrimental effect on the beer. Initially the space is filled with air but that is useful to get the yeast to start reproducing and eating the sugars. Soon that space will get filled with carbon dioxide and then it'll be fine for the rest of the fermentation.

Another minor point is that when you rack the beer from the carboy into the keg, bottles or a secondary fermentation vessel there will be a circle of grunge left on the side of the carboy where the top of the wort was. It's easier to clean this with a large bottle brush if it is not so close to the top of the carboy.

So all in all I would go with option 3.

  • So what is a good amount of space to leave at the top? Is 5 gallons in a 7gal carboy to much?
    – Metropolis
    Sep 15, 2015 at 19:26
  • I think that would be just fine, i.e., not too much room. Have you tasted the batch yet? Sep 15, 2015 at 21:15
  • I tasted it a few times before bottling. It will be conditioning until next Saturday though. It tasted pretty good.
    – Metropolis
    Sep 16, 2015 at 14:43

I had an email discussion about this with John Palmer. I am paraphrasing here, but basically he said that in primary the head space isn't an issue at all (see jsled's explanation which is spot on). It's only in secondary where it is a problem (oxidation).

Of course he also said that while in the past many brewers espoused racking to secondary, he was of the opinion that most beers do NOT require/need it, and that the risk could outweigh the gain. Again, he said most, not all.

So, that was good enough for me. I normally make 3GL batches anyway, as I do have a couple of 3GL carboys. However, sometimes I ferment my brews in a 5 or 6GL carboy with no problem.


Real question is how big do you need to have X final beer.

If you keg it's generally 4.75-5gal you want at the end.

Depending on the style and fermentaion. I've found you can hit 5 gallons easily by brewing 6 pretty easily.

Carboys are at capacity at the shoulder, I use 6 or 6.5s primary, and 5 gal secondary usually. The idea is to allow headspace for kreuasen to expand and fall back in without exiting the primary carboy, this is not an issue in secondary and can be filled much higher.

You can eliminate O2 exposure completely in secondary buy filling secondary carboy with starsan and purging it out with c02 by using a sealed racking cane. The one I use has a stainless tube, orange seal, and sanitary filter. Once empty cover with foil, and use the foil to seal the hose when filling with your primary beer.

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My Dopplebock this morning. As you can see, I should have used 6.5' instead of 6g carboys here, and maybe some nofoam. I did lose a little volume, the Bubblers were 3inches deep to start.

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