The short answer, then the reason it is not 40
A standard 8 by 10 foot bathroom floor is 80 square feet. A 12×24 tile covers 2 square feet, so the bare count is 40 pieces. Order 47 tiles, which for most 12×24 lines means 6 cartons at 8 pieces each. The gap between 40 and 47 is not padding for clumsiness. It comes from the way a two foot long tile lands in a small rectangular room.
We ran this floor both ways before settling on the layout below, and the deciding factor was not tile count. Turning the 24 inch dimension to run with the long wall cut the number of edge pieces almost in half, because the field lands on a full tile at one end instead of a two inch sliver at both. The box count barely moved. The cut list, and the day, got a lot shorter.
With a 4 inch tile you can nudge a layout anywhere and nobody notices. With 12×24 the module is coarse. Two inches of error at one wall shows up as a two inch sliver at the other, and there is no hiding it behind a toilet.
Work the layout module, not just the area
The number you actually need is how many tile positions fit, row by row. That depends on the tile plus the grout joint, which tile setters call the module.
Module = tile dimension + joint width
Positions per run = ceiling of (wall length / module)
Last cut size = wall length minus (full positions x module)
For a rectified 12×24 porcelain at a 1/8 inch joint, the modules are 12.125 and 24.125 inches. Our 8 by 10 room is 96 by 120 inches. Running the long dimension of the tile down the 10 foot direction:
| Direction | Wall length | Module | Full tiles | Total positions | Final cut |
|---|---|---|---|---|---|
| Across (8 ft) | 96 in | 12.125 in | 7 (84.875 in) | 8 rows | 11.1 in strip |
| Along (10 ft) | 120 in | 24.125 in | 4 (96.5 in) | 5 per row | 23.4 in piece |
Eight rows of five is 40 positions, which matches the area math. But look at the final cut in the long direction. It is 23.4 inches off a 24 inch tile. That tile is consumed for a piece nearly its own size, and the sliver that falls off is trash. That happens eight times, once per row.
The worked example
- Floor area: 8 x 10 = 80 sq ft. We tile under the vanity and behind the toilet, so nothing gets deducted.
- Tile coverage: 12 x 24 / 144 = 2.0 sq ft per piece.
- Base count: 80 / 2.0 = 40 pieces.
- Cut loss on a 1/3 offset in a room with two cut walls: 12 percent. 40 x 0.12 = 4.8 tiles.
- Handling, culling and bad cuts on large format: 5 percent. 40 x 0.05 = 2.0 tiles.
- Total: 40 + 4.8 + 2.0 = 46.8, round to 47 tiles.
- Cartons at 8 pieces (16 sq ft): 47 / 8 = 5.875, so 6 cartons, 48 tiles, 96 sq ft.
One spare tile is thin attic stock for a 12×24. Buy a seventh carton and store it. Dye lots do not come back.
The 1/3 offset rule, and why half offset ruins 12×24
Nearly every 12×24 manufacturer prints a maximum 33 percent offset in the installation instructions, and it is there for a physical reason. Pressed porcelain in that shape almost always has a slight crown, meaning the middle of the tile sits a hair higher than the ends. In a half offset brick pattern, the high middle of one tile lands right against the low end of its neighbor, and the two errors add. Pull the offset back to a third and the mismatch drops to roughly a third of the crown.
If you want the brick look anyway, dry lay two rows and run a straightedge across the joint. If you can feel a ledge with a fingernail, grout will not fix it.
Flatness and mortar, the two things that decide lippage
Large format tile does not bend to follow a hump. Most manufacturers ask for a substrate flat within 1/8 inch in 10 feet and 1/16 inch in 2 feet, which is tighter than the 1/4 inch in 10 feet people are used to for small tile. Check your floor with a 6 foot level before you buy a single tile. A bag of patch or a small pour of self leveler costs less than a day of fighting lippage.
Use a large and heavy tile mortar, sometimes labeled medium bed, rather than a standard thin set. Standard thin set is formulated to be under about 3/16 inch after the tile is beaten in, and it shrinks if you build it thicker. Medium bed holds a thicker bed without slumping, which is exactly what you need when you are dialing a 24 inch tile flat.
| Trowel notch | Typical use | Approx coverage per 50 lb bag |
|---|---|---|
| 1/4 x 1/4 square | Mosaic, small wall tile | 90 to 100 sq ft |
| 1/4 x 3/8 square | 12×12 floor tile | 85 to 95 sq ft |
| 1/2 x 1/2 square | 12×24 and larger | 40 to 50 sq ft |
| 1/2 x 1/2 plus back butter | 12×24, wet areas | 30 to 35 sq ft |
For our 80 square foot bathroom with back buttering, that is roughly 80 / 32, so three 50 pound bags. Back buttering is not optional on this size tile if you want the coverage a bathroom floor deserves.
Practical cut planning
- Split the difference. If a wall leaves you an 11 inch strip on one side and a full tile on the other, shift the field so both sides get about 5.5 inches. Symmetry reads as intentional.
- Never leave a cut under 2 inches. Slivers snap on the saw and they telegraph every out of square wall.
- Use leveling clips. Two per long edge, one per short edge. On 12×24 they are cheap insurance, not a crutch.
Comb your mortar ridges in one direction and set the tile across them so trapped air can escape. Pull a tile after the first few and look at the back. If you are not seeing near full contact, change trowels before you go any further.
Sources and Further Reading
Run your own numbers with the Tile Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 7, 2026
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