Tile
Thinset coverage by trowel size: how many bags of mortar?
The notch on the trowel decides how much mortar goes under each square foot, and so how many bags you need. A floor of large tile can take three times the mortar of the same floor in small tile.
Why the trowel decides the bags
Thinset is spread with a notched trowel, and the notches leave ridges of mortar that collapse under the tile. The deeper the notch, the more mortar per square foot, and the fewer square feet one bag covers. Manufacturers print a coverage chart on every bag: for a 50 lb bag, the number of square feet for each common notch. That chart, not the area alone, is what turns a floor into bags.
The notch is chosen to give full contact between the tile and the mortar. The tile industry’s installation standard, ANSI A108, asks for at least 80% contact on dry interior floors and 95% in wet areas and outdoors. Bigger tiles bridge more irregularity in the floor and the tile’s own back, so they need a deeper bed to reach that contact.
Coverage by notch
| Trowel notch | Typical tile | sq ft per 50 lb bag | lb per sq ft |
|---|---|---|---|
| 3/16 × 5/32 in V-notch | mosaic, wall tile up to 4 in | about 120 | 0.42 |
| 1/4 × 1/4 in square | tile up to 8 in | about 90 | 0.56 |
| 1/4 × 3/8 in square | 12 to 16 in | about 63 | 0.79 |
| 1/2 × 1/2 in square | 18 in and larger, large format | about 40 | 1.25 |
These are the mid-points of the ranges printed on common US polymer-modified thinsets, which vary by brand by about 10% either way. The thinset calculator uses them by default and lets you type the figure from your own bag. The grout and thinset coverage reference gathers the same figures with the grout table.
Back-buttering
Back-buttering is spreading a thin, flat coat of mortar on the back of each tile before setting it. It fills the hollows in the tile’s back, raises contact and is recommended for tile 15 inches and larger and for all natural stone. It uses about 30% more mortar than the bed alone, so it moves the bag count as much as a change of notch.
Three floors of the same size
A 168 sq ft kitchen in three tile sizes
The same 14 × 12 ft floor, with 10% for mixing and mortar left in the bucket:
- 6 × 6 in tile, 1/4 in square notch: about 103 lb, 3 bags
- 12 × 24 in tile, 1/4 × 3/8 in notch: about 147 lb, 3 bags
- 24 × 24 in tile, 1/2 in notch, back-buttered: 168 sq ft plus 50 sq ft of buttering plus 22 sq ft of allowance is 240 sq ft at 40 sq ft per bag, 7 bags, 300 lb
The large-format floor takes nearly three times the mortar of the small tile. The first two round to the same three bags only because both land just under a whole bag.
A 48 sq ft backsplash in small wall tile set with a V-notch, by contrast, is about one 25 lb bag. The floor tile calculator gives the tile count for the same floor.
Which thinset
Use a polymer-modified mortar (ANSI A118.4 or A118.15) for porcelain and for most floors; porcelain’s dense body grips modified mortar better. Large and heavy tile needs a mortar labelled for large and heavy tile (LHT), which holds a thick bed without slumping. Some membranes and glass tile call for a specific product; the membrane’s instructions override any rule of thumb.
Mortar over backer board
Cement board is set in mortar too. Under the board, a bed of thinset is spread with a 1/4 inch square notch to fill voids between board and subfloor, and the board is then screwed down. That is another 90 sq ft or so per bag, which many people forget. The cement board calculator counts the sheets, screws and tape; add the bed mortar from the thinset calculator at the 1/4 inch notch.
Checking coverage on the job
The only reliable check is to lift a tile. Set one, press it in, then pull it up and look at its back: mortar should cover the back almost completely, with the ridges collapsed. If the ridges still stand or the corners are bare, use a deeper notch or back-butter. Do this on the first few tiles and again when you change areas of the floor. A deeper notch than planned changes the bags, so keep a spare bag on hand; an unopened bag can usually be returned.
Mixing in batches
Thinset has a pot life of one to four hours once mixed. Mix only what you can use in that time, typically half a bag. Mortar that skins over in the bucket is waste, which is why the allowance is 10% and not less. The spacers and clips calculator counts the leveling clips large-format tile also needs.
The short version
Read the coverage chart on your bag for your notch. Add 30% for back-buttering and 10% for mixing. Divide and round up. Large-format tile needs the deepest notch and buttering, and the bag count shows it. The guide to tile size and grout joint covers the rest of the materials that move with tile size, and the large-format tile guide covers the floor preparation.