Thinset coverage is set by the trowel, not the tile. The same bag covers 100 square feet under mosaic and 45 under large-format porcelain, because the notch depth decides how much mortar stays on the floor.
Matching notch to tile
A quarter-inch square notch is the default for tile up to about twelve inches. Bigger tile needs a bigger notch to bridge the slight cupping every large tile has.
The test is coverage on the back of the tile: pull one up after setting it. You want 80 percent of the back covered for a dry area and 95 percent for a shower or anywhere wet. Less than that, go up a notch size.
Back-buttering large format
Anything over fifteen inches on a side should get a thin skim of thinset on the back of the tile as well as the notched bed on the floor. That is additional material beyond the coverage figure above.
Budget roughly 20 percent more when back-buttering. It is also the difference between a tile that sounds solid and one that sounds hollow when tapped.
Modified or unmodified
Modified thinset has polymers added for bond strength and flexibility, and is the normal choice for porcelain over a wood subfloor.
Unmodified is required over some uncoupling membranes — the membrane cannot breathe, and modified thinset trapped between two impervious layers never fully cures. Follow the membrane manufacturer's instruction over anything else.
Open time is the real constraint
Thinset starts curing the moment water hits it, and the clock that matters is not the bucket — it is the trowel ridges on the wall. Combed thinset skins over in as little as ten or fifteen minutes in a warm, dry room.
Once it has skinned, a tile pressed into it sticks to the skin rather than bonding through it. The tile looks set, it sounds fine, and it releases years later.
The test is to touch a ridge with a finger. If thinset transfers to your finger it is still open. If it feels tacky but leaves nothing, scrape it off and recomb it. That is why professionals spread small sections at a time and why a first tiling job always spreads too much.
Coverage is what an inspector looks for
The standard most tile work is judged against is eighty percent coverage on a dry interior floor and ninety-five percent in a wet area or outdoors. That means the back of the tile is in contact with mortar over that proportion of its surface.
The way to check is to set a tile, pull it straight back up, and look. Ridges that are still standing in peaks with bare valleys mean the notch is too big or the tile was not pressed and slid into place.
Do this once at the start of every job, and again when you change tile size or switch from floor to wall. It costs one tile and it is the difference between a floor that lasts and one that hollows out.