Thin-set for 280 ft² of 12×12″ tile on cement backer board

Thin-set needed

450 lb bags

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

Bags (setting the tile)	4× 50 lb
Total weight	200lb
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	Cement backer board
Mortar type	Modified thin-set (ANSI A118.4)

Area: 280 ft²
Tile size: 12×12″
Substrate: cement backer board

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 280 ft² that comes to 4 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.

Cement backer board

Mortar for this substrate: Modified thin-set (ANSI A118.4).

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.

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 280 ft² takes 4 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.
Cement backer board is a straightforward substrate for this. Clean, flat and structurally sound matters more than the mortar you choose.
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.

Nearby sizes

What a 25 lb Bag of Grout Actually CoversA bag's coverage figure is real. It is also stated for one specific tile size, joint width and thickness — change any of the three and the number moves by a lot.
Grout for Subway Tile: Three Times the 12-Inch FigureSubway tile is the highest grout consumption per square foot of any common format. If you sized the order from a floor tile figure, you are about a third of the way there.
Grout for 12 × 24 Tile: Why the Big Format Uses LessLarge format tile is the one place where the bigger product costs you less of something. Grout is that something, and the saving is exactly a quarter.
Sanded or Unsanded: The Joint Width That Decides ItThe dividing line is 1/8 inch and it is not a style guideline. Using the wrong one on either side of it produces a specific, predictable failure.