Short answer: divide the area by the area of one tile including its grout joint, add waste for your layout, then round up to whole boxes.
Step 1 — Measure the area
Floors: length × width. Walls: width × tiled height, minus windows, niches and large openings. For an L-shaped room, split it into rectangles and add them up.
Step 2 — Area of one tile with its joint
tile footprint = (tile width + joint) × (tile length + joint)
A 12 × 12 in tile with a 1/8 in joint covers 12.125 × 12.125 = 147 in², or 1.02 ft².
Step 3 — Add waste for the layout
| Layout | Waste |
|---|---|
| Straight (grid) | 10% |
| Offset / brick | 12% |
| Diagonal | 15% |
| Herringbone | 20% |
Step 4 — Count tiles and round up to boxes
tiles = ⌈area × (1 + waste) ÷ tile footprint⌉
boxes = ⌈tiles ÷ tiles per box⌉
boxes = ⌈tiles ÷ tiles per box⌉
Example: 100 ft² straight layout, 12 × 12 in tiles → 110 ÷ 1.02 = 108 tiles.
Quick reference: 100 ft², straight layout, 1/8 in joints, 10% waste
| Tile size | Tiles |
|---|---|
| 6 × 6 in | 423 |
| 12 × 12 in | 108 |
| 18 × 18 in | 49 |
| 12 × 24 in | 55 |
| 24 × 24 in | 28 |
Thinset and grout
- Thinset: a 50 lb bag covers about 90 ft² with a 1/4 × 1/4 in notch, 60–70 ft² with 1/4 × 3/8 in, and 40–45 ft² with 1/2 × 1/2 in. Large tiles need bigger notches and back-buttering.
- Grout: depends on joint width and tile thickness. 12 × 12 in tile with 1/8 in joints uses about 0.07 lb per ft² — a 10 lb bag covers roughly 150 ft². Small tiles and wide joints use far more.
Common mistakes
- Ignoring the joint in the tile area — fine for big tiles, but mosaics add up.
- Forgetting waste in rooms with many cuts, doors and corners.
- Buying in two batches. Dye lots differ, so buy everything at once.