Thin-set for 30 ft² of 18×18″ tile on cement backer board
| Bags (setting the tile) | 2× 50 lb |
|---|---|
| Total weight | 100lb |
| Trowelchosen by the 18″ tile, not by the room | 3/4″ × 3/4″ square notch |
| Coverage per bag25 ft² less 30% for back-buttering | 19ft² |
| Back-butteringover 15″ you cannot reach 80% coverage from the trowel alone | Required |
| Substrate | Cement backer board |
| Mortar type | Modified thin-set (ANSI A118.4) |
Notes
- The trowel decides the quantity, and the tile decides the trowel. A 3/4″ × 3/4″ square notch lays down enough mortar for 18″ tile; using it over 30 ft² takes 2 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.
- 18×18″ needs back-buttering. ANSI A108.5 wants 80% mortar contact for interior floors and 95% for wet or exterior work, and no trowel alone gets there under a tile this size. That is the 30% extra already added above.
- 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.
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 30 ft² that comes to 2 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.
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.
Nearby sizes
- Thin-set for 20 ft² of 18×18″ tile on cement backer board
- Thin-set for 40 ft² of 18×18″ tile on cement backer board
- Thin-set for 50 ft² of 18×18″ tile on cement backer board
- Thin-set for 30 ft² of 12×24″ tile on cement backer board
- Thin-set for 30 ft² of 24×24″ tile on cement backer board
- Thin-set for 30 ft² of 12×12″ tile on cement backer board
- Thin-set for 30 ft² of 18×18″ tile on a concrete slab
- Thin-set for 30 ft² of 18×18″ tile on a plywood subfloor
- Thin-set for 30 ft² of 18×18″ tile on existing tile
Read more
- 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.