Thin-set for 180 ft² of 12×12″ tile on existing tile

Thin-set needed

350 lb bags

180 ft² of 12×12″ with a 1/4″ × 3/8″ square notch

Bags (setting the tile)	3× 50 lb
Total weight	150lb
Trowelchosen by the 12″ tile, not by the room	1/4″ × 3/8″ square notch
Coverage per bag75 ft² at this notch depth	75ft²
Back-butteringtrowel ridges alone will collapse to full coverage	Not needed
Substrate	Existing tile
Mortar type	Modified thin-set made for tile-over-tile

Notes

The trowel decides the quantity, and the tile decides the trowel. A 1/4″ × 3/8″ square notch lays down enough mortar for 12″ tile; using it over 180 ft² takes 3 bags. The same floor in 2″ mosaic would need a fraction of that, and a generic "bags per square foot" figure is wrong for both.
At 12″ the trowel ridges collapse into full contact when you set and slide the tile. Pull one up early in the job and look at the back — you want the ridges flattened, not standing.
The old tile has to be sound, clean and de-glossed — a bonded layer is only as good as what it sits on.
Mix it and let it slake. Almost every bag says mix, wait 5–10 minutes, then remix — that rest is what lets the polymers hydrate. Skipping it is why mortar sometimes will not hold a tile on a wall.
Only spread what you can set in about 15 minutes. Mortar that has skinned over looks fine and bonds to nothing; touch it with a finger and if it does not come away wet, scrape it off.

The trowel decides the quantity

Not the room. A 1/4″ × 3/8″ square notch is what 12×12″ tile needs, and at that notch depth a 50 lb bag covers about 75 ft². Over 180 ft² that comes to 3 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: not needed

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.

Nearby sizes

How Much Extra Tile to Buy: 10%, 15% or 20% — and What Sets the NumberWaste percentage is not a safety margin. It is a prediction about how many cut tiles produce a usable second piece, and the pattern decides it.
How Many 12×12 Tiles in 100 Square Feet? 100 — Then Buy 110The arithmetic gives a round number that is right and not enough. Two things separate the count from what you should put in the trolley.
What Size Trowel for Tile? Match the Notch to the Tile, Then Check the BackThe chart gets you to the right aisle. What settles it is lifting one tile and looking at how much mortar actually touched it.
Thinset Coverage per Bag: 90 Square Feet or 25, and the Trowel DecidesThe bag prints a coverage figure that assumes a trowel size it does not name. Change the notch and the same bag covers a third as much floor.