Volumes, 5 gallon batch, 6.5 fermenter...??

[Keqwow]

Another silly newbie question: Volumes. Most recipes are for 5 gallon batches. Most fermenting pails I see are 6.5 gallon volumes. I know some volume is lost during the fermentation process (not sure how much?). So can someone clarify what volume ends up going into the fermentation pail to produce what volume?

 

[BiscayneBrewing]

The extra space in the fermenter is referred to as 'head space'. It's needed because once fermentation takes place the beer starts foaming/bubbling as the yeasties start eating the sugars and emitting CO2. Even with a 6.5 gallon bucket many people use a 'blow off' tube because krausen would normally clog an air lock creating a mini-beer explosion.