Thin-set for 80 ft² of 18×18″ tile on a concrete slab
| Bags (setting the tile) | 5× 50 lb |
|---|---|
| Total weight | 250lb |
| 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 | Concrete slab |
| Mortar type | Standard modified thin-set (ANSI A118.4) |
- Area
- 80 ft²
- Tile size
- 18×18″
- Substrate
- a concrete slab
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 80 ft² that comes to 5 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.
Concrete slab
Mortar for this substrate: Standard 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 3/4″ × 3/4″ square notch lays down enough mortar for 18″ tile; using it over 80 ft² takes 5 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.
- Concrete slab 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
- Thin-set for 60 ft² of 18×18″ tile on a concrete slab
- Thin-set for 100 ft² of 18×18″ tile on a concrete slab
- Thin-set for 50 ft² of 18×18″ tile on a concrete slab
- Thin-set for 120 ft² of 18×18″ tile on a concrete slab
- Thin-set for 80 ft² of 12×24″ tile on a concrete slab
- Thin-set for 80 ft² of 24×24″ tile on a concrete slab
- Thin-set for 80 ft² of 12×12″ tile on a concrete slab
- Thin-set for 80 ft² of 18×18″ tile on cement backer board
- Thin-set for 80 ft² of 18×18″ tile on a plywood subfloor
Read more
- 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.