The total capacity of the vessel is just a hair shy of 7 gals. So the answer to this one really depends on wether you are making an aggressive / high gravity big beer or a more mild-mannered beer - as that really dictates how much head room your beer will need. And if you are planning to dump in 5.5 or 6 gals of beer AND you plan on having a big aggressive beer in there, then you would probably want to run a 1/2" ID silicone tube as a blow-off so that you don't get a big mess through your bubbler in the event of a beer volcano type of event! Always best to use a blow off tube in the first few days of primary and our lid hole on the Bucket is perfectly and intentionally designed to take a 1/2" silicone tube stuffed right into the hole. We sell the tubing on our site in the accessories section BTW!