Thin-set for 1500 ft² of 18×18″ tile on existing tile

The trowel decides the quantity

Not the room. A 3/4″ × 3/4″ square notch is what 18×18″ tile needs, and at that notch depth a 50 lb bag covers about 19 ft². Over 1500 ft² that comes to 78 bags.

The same floor in 2″ mosaic would take a fraction of this. Any figure quoted as "bags per hundred square feet" without asking the tile size is a guess.

Back-buttering: required

ANSI A108.5 wants 80% mortar contact for interior floors and 95% for wet or exterior work. Above about 15″, no trowel alone gets there — the ridges do not collapse fully under a tile that size, and the corners end up hollow. Where it is required, the extra 30% is already in the bag count above.

Pull a tile up early in the job and look at the back. You want flattened ridges across the whole face, not a stripe pattern.

Existing tile

Mortar for this substrate: Modified thin-set made for tile-over-tile.

Two habits that decide whether it holds

Let it slake. Almost every bag says mix, wait five to ten minutes, remix — that rest is what lets the polymers hydrate, and skipping it is why mortar sometimes will not hold tile on a wall.

Only spread what you can set in fifteen minutes. Mortar that has skinned over looks perfectly fine and bonds to nothing. Touch it; if your finger does not come away wet, scrape it off and start again.