My problem is that I have a 15L carboy a beer kit (Cooper's Irish Stout) that's supposed to be for 23L.

I vaguely recall reading somewhere that I should leave about 20% headspace (so I'm only brewing 12L). I'll be fermenting with Safale S-04.

Getting a larger fermenter is out of the question right now since I don't have the space (or approval of my better half).

My guess is that I shouldn't attempt to use the entire can and just make it up to 12L. I don't know if I can store the unused extract for a month before brewing it a 2nd batch. So I'm thinking of making up 12L of something that's a little more concentrated than the manufacturer's recommendation.

Has anyone done something like this? How concentrated can I make it and still have it palatable? I usually like big, heavy tasting beers.

  • I also found some posts on other forums where others have mentioned that it was possible to mix the 23L kit to 18L instead. On an unrelated note, I found this: coopers.com.au/coopers-forum/topic/10378 which seems to indicate that I can store the leftover extract in the fridge/freezer for a few weeks. So I might go with this route and ferment kit in 2 batches. Commented Jul 20, 2014 at 11:40

4 Answers 4


It's ok to have less than 20% headspace, you just risk a blowout, meaning it could overflow and make a mess. To counter this problem use a blowoff tube and bucket: blowoff tube and bucket


Having 20% headspace is a good idea, but not necessary as HourOfTheBeast pointed out.

It's perfectly fine to make a concentrated wort. I'd recommend pitching twice the amount of yeast to ensure fermentation goes well. The main concern here is making more alcohol by volume than the yeast can handle. S-04 should handle 10% and you'll probably only be making about 8.5%.

Irish stouts are generally roasty, dry affairs. The coffee flavours in your beer will be more pronounced and there may not be a lot of sweetness to balance.

Something you could consider is fermenting the concentrated wort and diluting at bottling time. If you use a 19L bottling bucket you could take your 12L of fermented beer and add ~6.5L of boiled water (along with your priming sugar) to make the full 19L of finished beer. Should come out a little more balanced.


Simple rule: 23L for a 30L vessel. Scale appropriately.

This allows enough room for the head / krausen to form and not overflow (most of the time).

  • By scale I mean scale the recipe. If you have a 15L vessel just halve the coopers kit ingredients. Commented Jul 23, 2014 at 5:15

Some kit brewers (including me when I was kit brewing) successfully use the "twocan" method. Two cans, water, and yeast. No sugar, dextrose, corn syrup, light dry malt, etc. As an experiment, I would use the full can, top up with hot/cold water to 13 litres, pitch yeast when wort is at the correct temperature, seal and wait. My experience with several other Cooper's kits suggests you will be pleasantly surprised.

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