You don't state your brewing process. If you are using a standard mash tun (cooler or otherwise) then what Kevin58 stated is most likely correct. Much also depends upon your boil off rate, standard time of boiling, and amount of trub you leave behind in your kettle upon transferring to the fermenter.
If you are doing BIAB, however, the calculations change quite a bit. A good general recommendation for BIAB is o size the kettle at least 2x your batch size to accommodate the grain and bag which will be in the kettle during the mash.