18" tiles need a pretty flat floor to get all the sides to line up with each other and avoid lippage from one tile to the next. The smaller the grout lines, the tougher it is to stay away from lippage, unless the floor is flat. Before deciding on a grout width, you may want to check your floor for flatness using the longest straight-edge you have (a 10' piece of 1x4 or 2x4 works ok if it is very straight). Run around the floor and see how much air space is under the straight-edge when it's on the floor. The normal goal is to to have less than 1/4" out of flat over 10' in distance, with no more than 1/16" out of flat in any given foot. Your larger tiles will install easier if your floor is well inside those figures.
Groutlines up to 1/8" use unsanded grout.
Groutlines 1/8" and wider use sanded grout.
I'd recommend thinking about 3/16" to 1/4" width. I'm not a big fan of wider grout lines unless you're looking for a more rustic look. And before commiting to those discontinued grout bags, I'd recommend checking the date codes on them to make sure they aren't more than a year old? Old grout doesn't work so good.
Tonto Goldstein….. but my friends call me Bubba
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