Volume in fermenter = original volume – losses
Losses can be from grain absorption, mash tun dead space, evaporation during boil, dead space in boil kettle, trub, and dead space in fermentor.
Everybody’s system is different in their losses so you should learn how your system performs. Like MDixon suggested, do a test run on your system using a small volume.
There may be some other losses I’m not thinking of offhand.