How the tile calculation works
Tiling comes down to two areas: the area you need to cover and the area each tile covers. Divide one by the other and you have your tile count.
room area = length × width
tile area = ( tile width × tile height ) ÷ 144
tiles = room area ÷ tile area × ( 1 + waste )
Tile sizes are usually given in inches, so the division by 144 converts square inches to square feet (12 × 12 = 144). The waste factor adds extra tiles for cuts, breakage and matching spares.
Worked example
A 12 ft × 10 ft room tiled with 12 in × 12 in tiles, 10% waste, 10 tiles per box:
Room area: 12 × 10 = 120 sq ft.
Tile area: (12 × 12) ÷ 144 = 1 sq ft each.
Tiles: 120 ÷ 1 × 1.10 = 132 tiles.
Boxes: 132 ÷ 10 = 13.2 → 14 boxes.
Tip: always round both the tile count and the box count up to a whole number. You cannot buy a fraction of a tile, and a partial box of spares is invaluable for future repairs.
Choosing a waste percentage
A straight grid layout in a simple rectangular room wastes the least — about 10%. Diagonal layouts, herringbone and rooms with many corners, nooks or fixtures create more cuts, so bump the allowance to 15-20%. Buying a little extra is cheaper than a second trip and a possible dye-lot mismatch.