How much thinset you need is decided by the trowel, not the tile. A 50 pound bag of thinset covers 90 to 100 square feet spread with a quarter inch notch and 42 to 47 with a half inch one, on the same floor with the same tile. Pick the trowel and the bag count follows.
Coverage is a trowel specification
Thinset is combed into ridges and the tile is bedded into them. How much sits under a square foot is set by how deep and how far apart those ridges are, which is the notch on the trowel. Manufacturers publish coverage against notch size for exactly this reason. These are the figures from Custom Building Products for a 50 pound bag of VersaBond:
| Trowel | Square feet per 50 lb bag |
|---|---|
| 1/4 x 1/4 x 1/4 in square notch | 90 to 100 |
| 1/4 x 3/8 x 1/4 in square notch | 60 to 67 |
| 1/2 x 1/2 x 1/2 in square notch | 42 to 47 |
| 3/4 x 9/16 x 3/8 in U notch, held at 30 degrees | 42 to 47 |
| 3/4 x 9/16 x 3/8 in U notch, held at 45 degrees | 34 to 38 |
Look at the last two rows. Same trowel, same tile, same floor. The only difference is the angle you hold it at, and the coverage moves by about a quarter. That is not a rounding error on a big job, and it is the reason two tilers can give you different bag counts for the same room and both be right.
The trowel is not a free choice
You do not get to pick the small notch to save mortar. The notch has to leave enough material to support the tile once it is bedded, and the standard is written in terms of how much of the back of the tile ends up in contact with mortar.
CUSTOM puts the requirement at 80 percent coverage or better in dry areas and 95 percent or better in wet areas and outside, with the mortar behind the tile no thinner than three thirty seconds of an inch and no thicker than a quarter inch for a mortar without an ANSI H rating. A bigger or less flat tile needs a bigger notch to hit those numbers, which is why large format tile eats mortar.
The way to check is to pull a tile up after setting it and look at the back. If the ridges have not collapsed into something close to full contact, the trowel is too small or the mortar has started to skin over.
Two bags of thinset are not the same bag
Coverage is a property of the product as well as the trowel. At the same quarter inch square notch, CUSTOM gives VersaBond 90 to 100 square feet a bag and Mapei gives Ultraflex 2 75 to 90. That is up to a quarter less floor from a bag that looks identical on the pallet.
So take the figure off the bag you are actually buying rather than off a number you remember. Both of those are ordinary modified thinsets from major manufacturers, and they disagree by more than most people’s waste allowance.
How much thinset you need on a real floor
An 80 square foot bathroom, using the VersaBond figures and the middle of each range:
- Small tile on a flat floor, quarter inch notch: one 50 pound bag
- Mid size tile, quarter by three eighths: one and a quarter bags, so two
- Large format, half inch notch: two bags
The jump from one bag to two happens between the first and second line, on a floor that has not changed size and tile that has not changed price. Worth knowing before the second trip.
Buy it in bags you can finish
Thinset is the one tiling material where buying extra is not free. Once mixed it has a working life of a couple of hours, and mortar that has started to stiffen cannot be brought back with more water without ruining the bond. So a half used bag is fine, a half used bucket of mixed mortar is waste.
Mix what you can lay before it skins, particularly on a hot day or over a dry substrate. If the ridges have stopped being sticky by the time the tile goes down, that section has to come up.
The grout is a separate order and runs on completely different arithmetic, since it is decided by the size of the tile rather than the size of the trowel.
Start with the floor area
Frequently asked
Take it off the trowel, not the tile. A 50 pound bag of VersaBond covers 90 to 100 square feet with a quarter inch square notch, 60 to 67 with a quarter by three eighths, and 42 to 47 with a half inch notch. An 80 square foot bathroom is one bag at the small notch and two at the large.
Between about 34 and 100 square feet for a 50 pound bag, depending entirely on the trowel notch. The figure also varies by product: at the same quarter inch notch, CUSTOM rates VersaBond at 90 to 100 square feet and Mapei rates Ultraflex 2 at 75 to 90.
It is the main thing that changes it. Going from a quarter inch notch to a half inch notch takes a 50 pound bag from around 95 square feet to around 45, so the same floor needs twice the mortar. Even the angle matters: the same U notch trowel covers 42 to 47 square feet held at 30 degrees and 34 to 38 held at 45.
No. The notch has to leave enough mortar to support the tile, and the standard is written as coverage on the back of the tile: 80 percent or better in dry areas, 95 percent or better in wet areas and outdoors. Larger and less flat tiles need a larger notch to reach that, which is why large format tile uses more mortar.
For a mortar without an ANSI H rating, CUSTOM specifies no less than three thirty seconds of an inch and no more than a quarter inch behind the tile once it is bedded. That finished thickness is roughly half the notch depth, since the ridges collapse as the tile is set in.