If I want to brew a 5 gallon batch, how large of a kettle do I need to do so comfortably?
How about 10 gallons?
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Based on my own experience, an 8 gal. kettle will barely work for a 5 gal. batch. Even at that, you might have to boil a concentrated wort and add top up water after the boil. Remember that in addition to accounting for trub and evaporation, you also need some headroom in there to start with. I wouldn't recommend anything less than 15 gal. for a 10 gal. batch.
Well, it depends on the losses you assume along the way.
For 5 gallons, if you assume a trub loss of 0.5 gallons, and evaporation loss of 1.0 gallon, then you would need 6.5 gallons of wort to get 5 gallons into a fermenter. If you follow the oft-quoted rule of thumb that you want minimum headspace in your kettle equal to one-third of your boil volume, that means the minimum kettle size is around 8-2/3 gallons. I have heard of people using 8-gallon kettles. I recommend more like 10-gallons.
For 10 gallons, I suppose you could double that.