A 12 × 10 ft floor laid with 12 × 24 in tile at ⅛ in joints takes 65 tiles, which is 9 boxes of 16 sq ft, once you add 10% for cuts and breakage. Divide the area by what one tile covers including its joint, add the waste allowance for your pattern, round up to whole tiles, then to whole boxes.
How the tile count is calculated
Each tile covers its own face plus half the grout joint on every side, so the area one tile accounts for is (tile width + joint) × (tile length + joint). Divide your area by that, multiply by 1 plus the waste allowance, and round up. Boxes come from the face area of those tiles divided by the coverage printed on the box.
tiles = area ÷ ((W + joint) × (L + joint)) × (1 + waste %), rounded up
boxes = tiles × tile face area ÷ box coverage, rounded up
Counting the joint matters on small tile and wide joints; on 12 × 24 in tile at ⅛ in it saves about one tile per 120 sq ft.
Worked example: a 12 × 10 ft kitchen floor
- Area: 12 ft × 10 ft = 120 sq ft.
- One tile with joint: (12 in + 0.125 in) × (24 in + 0.125 in) ÷ 144 = 2.031 sq ft.
- Tiles: 120 ÷ 2.031 = 59.1, × 1.10 = 65 tiles.
- Boxes at 8 tiles (16 sq ft) per box: 65 × 2 sq ft ÷ 16 = 8.1, so 9 boxes. The 7 spare tiles are your repair stock.
- Grout for ⅛ in joints on ⅜ in thick tile: 6.4 lb of cement grout, one 10 lb bag.
- Thinset: tile over 15 in wants a ½ × ½ in notch; at 38 sq ft per 50 lb bag of large-tile mortar plus a thin back-butter coat that is 3.9 bag-equivalents, so 4 bags.
- Backer board in 3 × 5 ft sheets: 9 sheets with 10% for cuts.
The layout preview draws this floor with 32 whole tiles and 28 cut ones. Because the grid is centered with a grout line on the middle of the room, both long walls get a 23 ¹¹⁄₁₆ in piece and both short walls get an 11 ⁷⁄₁₆ in piece, so nobody sees a sliver at the door.
How much extra tile to buy for each pattern
No standard sets a waste figure; these are the trade's usual allowances, filled in when you pick a pattern. The preview shows how many pieces your layout really cuts.
| Pattern | Usual allowance | Pieces in the 12 × 10 ft example |
|---|---|---|
| Straight | 10% | 60 (32 whole, 28 cut) |
| Brick offset, ⅓ | 10% | 65 (36 whole, 29 cut) |
| Diagonal | 15% | 78 (42 whole, 36 cut) |
| Herringbone | 20% | 87 (42 whole, 45 cut) |
Diagonal and herringbone cut more pieces than their allowance buys; they work because an offcut from one wall usually fits the opposite wall. For expensive or soon-discontinued tile, add 5%, and buy every box from one production lot (the shade code on the box).
Choosing the joint width
ANSI A108.02 (section 4.3.8) says the joint must be at least three times the actual size variation of the tile you received, and never under 1/16 in. For tile with any side over 15 in, section 4.3.8.1 sets an average minimum of ⅛ in for rectified tile and 3/16 in for calibrated tile. That is why the 12 × 24 in default uses ⅛ in.
Long tile bows slightly in the middle, so section 4.3.8.2 limits a running-bond offset to 33% when the offset side is over 15 in: use the 1/3 offset for 12 × 24 in and plank tile.
Floors and walls together
Add each floor and wall as its own surface: length × width for floors, length × tiled height for walls, minus what you will not tile (a vanity, a tub, a door). Wall rows in the preview start with a full tile on the floor line; the cut row belongs at the ceiling or under a shelf. For a room that is not a rectangle, split it into rectangles or total it in the room square footage calculator and enter that as one surface.
Calculators for each part of the job
- Shower: the shower tile calculator splits wall and floor tile and adds the niche, curb, bench, backer board, waterproofing and the floor slope to the drain.
- Kitchen backsplash: the backsplash tile calculator works from counter runs, subtracts windows, adds the range area and counts edge trim.
- Grout by brand: the summary above uses a generic cement grout at 100 lb per cubic foot of joint. The grout calculator has MAPEI, LATICRETE and CUSTOM coverage and pre-mixed buckets.
- Thinset and trowel: the thinset calculator picks the notch (¼ × ¼ in up to 8 in tile, ¼ × ⅜ in for 8 to 16 in, ½ × ½ in from 15 in, per CUSTOM's guide) and counts bags from the data sheets.
- Still choosing the floor: run the same room through the flooring calculator and compare.
Backer board comes in 3 × 5 ft, 32 in × 5 ft and 4 × 8 ft sheets (USG Durock sizes); leave a ⅛ in gap between boards and tape the seams. Over a plywood subfloor, ¼ in board is usual; on walls, ½ in.
Treat it as a tile layout planner: the drawing shows the cut tiles at the edges so you can shift the start line before you buy.