First MIAB water volume question

So I’ve got a 7 gallon kettle and am considering my first MIAB, the question is: can/should I mash with 5 gallons and then top off for a full volume? Im sure it will change efficiency and pre-boil gravity, but how significantly? Or am I best off just boiling with 6 gallons? My concern is that with 6 gallons of water, and the 12 lbs. of grain in the bag, I’ll run out of room in the kettle?

Thanks guys.

No-sparge efficiency is just the volume of wort in the kettle divided by the volume of water used to mash. Topping up for the boil is the only way you’re going to be able to make a 5-gal batch of a stronger beer and your efficiency will suffer a little, or you can add some DME to the kettle instead.

Thanks, I was planning on adding some sugar after the mash to bring the gravity up. I assume that would work just as well???

So does that mean that I’m okay doing like a 5 gallon boil then top off?

Adding sugar is not the same as adding DME - sugar thins the body and adds nothing to the flavor. If you have a 7-gal kettle, I would do a 6.5-gal boil by mashing and then topping up in the kettle, but if you want to do a partial-boil that’s always an option.