How much thinset do I need?
The trowel decides it, not the tile.
A 50 lb bag covers about 90 sq ft with a 1/4 x 1/4 in notch and about 43 sq ft with a 1/2 x 1/2 in. Same bag, same floor, less than half the coverage — because a deeper notch leaves more mortar behind.
Coverage by trowel
| Trowel | For | Per 50 lb bag | 100 sq ft | 500 sq ft |
|---|---|---|---|---|
| 3/16 in V-notch | mosaic and tiles under 4 in | 100 sq ft | 1 | 5 |
| 1/4 x 1/4 in square | tiles from 4 to 8 in | 90 sq ft | 2 | 6 |
| 1/4 x 3/8 in square | tiles from 8 to 12 in | 65 sq ft | 2 | 8 |
| 1/2 x 1/2 in square | large format, 12 in and over | 43 sq ft | 3 | 12 |
| 3/4 x 9/16 in U-notch | very large or heavily warped tile | 32 sq ft | 4 | 16 |
These are typical figures and brands differ, so check the bag. What does not differ is the direction: every step up in notch size costs roughly a third more mortar, and the step from 1/4 in to 1/2 in more than doubles it.
Which trowel for your tile
| Tile | Trowel | Per 50 lb bag | Bags per 100 sq ft |
|---|---|---|---|
| Mosaic (1x1, 2x2) | 3/16 in V-notch | 100 sq ft | 1 |
| Subway 3x6 | 1/4 x 1/4 in square | 90 sq ft | 2 |
| 4x4 | 1/4 x 1/4 in square | 90 sq ft | 2 |
| 6x6 | 1/4 x 1/4 in square | 90 sq ft | 2 |
| 8x8 | 1/4 x 3/8 in square | 65 sq ft | 2 |
| 12x12 | 1/4 x 3/8 in square | 65 sq ft | 2 |
| 12x24 | 1/2 x 1/2 in square | 43 sq ft | 3 |
| 18x18 | 1/2 x 1/2 in square | 43 sq ft | 3 |
| 24x24 | 1/2 x 1/2 in square | 43 sq ft | 3 |
| 24x48 | 3/4 x 9/16 in U-notch | 32 sq ft | 4 |
The rule behind the table: the bigger and flatter the tile, the deeper the notch, because a large tile spans further and needs more mortar to bridge the dips in the floor. Undersize the trowel on a 12x24 and you get hollow spots that sound like a drum and crack under a dropped pan.
What actually changes the number
Coverage on the back of the tile. The standard is 80% for a dry floor and 95% for a wet area or anything outdoors. Lift a tile you have just set and look — if the ridges have not collapsed into a bed, the notch is too small or the mortar has skinned over.
Back-buttering. Large-format tile usually needs a skim on the back as well as ridges on the floor. That is not optional at 12x24 and up, and it adds roughly a third to the mortar you use. The tables above do not include it.
How flat the floor is. Thinset is an adhesive, not a levelling compound. A floor out by more than 1/4 in over 10 ft wants patching first — trying to make up the difference in mortar gives you tiles that lip at the edges and a bag count nobody can predict.
Common questions
- How much thinset for 100 square feet?
- One to three 50 lb bags depending on the notch: 2 at 1/4 x 1/4 in, 2 at 1/4 x 3/8 in, 3 at 1/2 x 1/2 in. Add a third if you are back-buttering.
- What size trowel for 12x24 tile?
- 1/2 x 1/2 in square notch, and back-butter as well. A 12x24 is long enough that any dip in the floor becomes a hollow under the middle of the tile, which is where it will crack.
- How thick should the thinset be?
- Thin — the name is the specification. Ridges collapse to roughly 3/32 to 3/16 in once the tile is set and beaten in. A bed thicker than about 1/4 in shrinks as it cures and pulls the tile down unevenly.
- How much thinset under cement board?
- Full coverage with a 1/4 x 1/4 in notch, so about 2 bags per 100 sq ft. It is not there to glue the board down — the screws do that — it is there to fill voids so the board cannot flex.
- How much should I mix at a time?
- As much as you can lay before it skins over, which is 20 to 30 minutes in a warm room. Mortar that has started to set cannot be brought back with water; retempering it looks like it works and halves the bond strength.
- Is tile adhesive the same as thinset?
- No. Premixed mastic in a tub is for dry walls and small tile. Thinset is cement-based, mixed from powder, and is what floors, wet areas and large tile need. Mastic in a shower never fully dries and eventually lets go.
Related answers
Or work out the area first with the tile calculator.