How the tile count works
Tile sizes are quoted in inches, rooms in feet — convert first, then divide:
Tile area (ft²) = tile length × width ÷ 144
Tiles = room area ÷ tile area × (1 + waste%)
The waste allowance matters more for tile than for most materials because cuts are inevitable and breakage happens in transit and at the wet saw. Standard practice: 10% waste for straight-lay in rectangular rooms, 15% for diagonal, herringbone or brick-bond layouts with lots of edge cuts, and a touch more on very small rooms where the fixed perimeter eats a bigger share.
Worked example
A 10 × 8 ft bathroom floor (80 ft²) in 12 × 24 in planks: each tile covers 288 ÷ 144 = 2 ft², so the exact count is 40 tiles. Adding 10% waste gives 44 → buy 44 tiles. If they come 8 to a box, that is 5.5 boxes → 6 boxes, which also stocks your repair shelf.
Don't forget the consumables
Tiles are one line of the tiling bill. Thinset mortar runs roughly 1 lb per square foot for standard trowels; grout depends on tile size and joint width (a 12×24 with 1/8 in joints uses about 1.5 lb per 10 ft²). And always buy tile from a single dye lot — check the carton codes before leaving the store, not after the first row is set.