Thin-set for 120 ft² of 18×18″ tile on a concrete slab
| Bags (setting the tile) | 7× 50 lb |
|---|---|
| Total weight | 350lb |
| 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) |
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 120 ft² takes 7 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.
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 120 ft² that comes to 7 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.
Nearby sizes
- Thin-set for 100 ft² of 18×18″ tile on a concrete slab
- Thin-set for 140 ft² of 18×18″ tile on a concrete slab
- Thin-set for 80 ft² of 18×18″ tile on a concrete slab
- Thin-set for 160 ft² of 18×18″ tile on a concrete slab
- Thin-set for 120 ft² of 12×24″ tile on a concrete slab
- Thin-set for 120 ft² of 24×24″ tile on a concrete slab
- Thin-set for 120 ft² of 12×12″ tile on a concrete slab
Read more
- Trowel Size for Tile — The Notch Is Chosen by Coverage, Not by HabitThe right notch is not a rule of thumb. Set one tile, pull it up, and look at the back — that is the actual specification.
- Thin-Set Mortar Cost per Bag and per Square FootA bag of thin-set is nine dollars. How many bags you need is decided by the notch on your trowel, not by the tile.
- Waterproofing a Shower — Tile and Grout Are Not the WaterproofingThe most expensive failure in a bathroom is a shower that looks perfect and has been leaking into the framing since the week it was built.
- 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.