Thinset & Mortar
The trowel decides how much mortar a job takes, and the tile decides the trowel. Room area is the least interesting input.
A ¼ × ¼″ square notch lays down enough for tile up to about 4 inches and covers around 95 ft² per 50 lb bag. A ¾ × ¾″ notch for large-format tile covers about 25. Same floor, nearly four times the mortar, because the tile changed.
Above 15 inches, back-buttering is required
ANSI A108.5 wants 80% mortar contact for interior floors and 95% for wet or exterior work. No trowel alone reaches that under a large tile — the ridges do not fully collapse and the corners end up hollow, which is where large tiles crack.
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 with gaps between.
Modified or unmodified is not a preference either
Over an uncoupling membrane, the mortar above the membrane must be unmodified. Modified thinset cures by drying as well as hydrating, and sandwiched between two non-porous layers it never fully does. This is the single most common expensive mistake in a membrane installation, and the membrane manufacturers all say so in print.
Everywhere else, modified thinset rated for the substrate is the answer.
Let it slake
Almost every bag says mix, wait five to ten minutes, then remix. That rest is what lets the polymers hydrate. Skipping it is why mortar sometimes will not hold tile on a wall, and it is invisible until it fails.
Only spread what you can set in fifteen minutes
Mortar that has skinned over looks completely normal and bonds to nothing. Touch it — if your finger does not come away wet, scrape it off and start that section again.
2 articles
Thin-set for 240 ft² of 6×6″ tile on a plywood subfloor
| Bags (setting the tile) | 4× 50 lb |
|---|---|
| Total weight | 200lb |
| Trowelchosen by the 6″ 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 | Plywood subfloor |
| Mortar type | Latex-modified thin-set rated for wood |
Notes
- The trowel decides the quantity, and the tile decides the trowel. A 1/4″ × 3/8″ square notch lays down enough mortar for 6″ tile; using it over 240 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 6″ 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.
- Wood moves; plywood alone is the most common cause of cracked floor tile.
- 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.
Thin-set for a tiled area
200 calculations · browse all
- Thin-set for 20 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 20 ft² of 12×12″ tile on a concrete slab
- Thin-set for 30 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 30 ft² of 12×12″ tile on a concrete slab
- Thin-set for 40 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 40 ft² of 12×12″ tile on a concrete slab
- Thin-set for 50 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 50 ft² of 12×24″ tile on a concrete slab
- Thin-set for 60 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 60 ft² of 12×24″ tile on a concrete slab
- Thin-set for 80 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 80 ft² of 12×24″ tile on a concrete slab
- Thin-set for 100 ft² of 4×4″ tile on a concrete slab
- Thin-set for 100 ft² of 18×18″ tile on a concrete slab
- Thin-set for 120 ft² of 4×4″ tile on a concrete slab
- Thin-set for 120 ft² of 18×18″ tile on a concrete slab
- Thin-set for 140 ft² of 4×4″ tile on a concrete slab
- Thin-set for 140 ft² of 18×18″ tile on a concrete slab
- Thin-set for 160 ft² of 6×6″ tile on a concrete slab
- Thin-set for 160 ft² of 24×24″ tile on a concrete slab
- Thin-set for 180 ft² of 6×6″ tile on a concrete slab
- Thin-set for 180 ft² of 24×24″ tile on a concrete slab
- Thin-set for 200 ft² of 6×6″ tile on a concrete slab
- Thin-set for 200 ft² of 24×24″ tile on a concrete slab
- Thin-set for 240 ft² of 12×12″ tile on a concrete slab
- Thin-set for 280 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 280 ft² of 12×12″ tile on a concrete slab
- Thin-set for 320 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 320 ft² of 12×12″ tile on a concrete slab
- Thin-set for 360 ft² of 2×2″ mosaic tile on a concrete slab
- Thin-set for 360 ft² of 12×24″ tile on a concrete slab
- Thin-set for 400 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 400 ft² of 12×24″ tile on a concrete slab
- Thin-set for 450 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 450 ft² of 12×24″ tile on a concrete slab
- Thin-set for 500 ft² of 3×6″ subway tile on a concrete slab
- Thin-set for 500 ft² of 18×18″ tile on a concrete slab
- Thin-set for 600 ft² of 4×4″ tile on a concrete slab
- Thin-set for 600 ft² of 18×18″ tile on a concrete slab
- Thin-set for 700 ft² of 4×4″ tile on a concrete slab
- Thin-set for 700 ft² of 18×18″ tile on a concrete slab
- Thin-set for 800 ft² of 4×4″ tile on a concrete slab
- Thin-set for 800 ft² of 24×24″ tile on a concrete slab
- Thin-set for 1000 ft² of 6×6″ tile on a concrete slab
- Thin-set for 1000 ft² of 24×24″ tile on a concrete slab
- Thin-set for 1200 ft² of 6×6″ tile on a concrete slab
- Thin-set for 1200 ft² of 24×24″ tile on a concrete slab
- Thin-set for 1500 ft² of 6×6″ tile on a concrete slab
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.