Count the tiles needed for a floor or wall including waste allowance.
Tile counting is area divided by the area of one tile, and every part of that has a trap. Tile sizes are nominal: a tile sold as twelve inch usually measures a little under, with the difference made up by the grout joint, so the count comes out about right if you use the nominal figure. The waste allowance is what people get wrong. Ten percent covers a simple rectangular room laid straight; a diagonal layout needs fifteen, and a room with alcoves, a large pattern repeat, or tiles you cannot re-order needs twenty. Then buying happens in whole boxes, so the practical order is nearly always more than the calculated count — which is welcome, since a few spare tiles from the same batch are the only way to repair a cracked one later without a visible shade mismatch.
Tile count with waste and box rounding
Area = length x width; tile area in sq ft = tile inches squared / 144; tiles = ceiling(area / tile area x (1 + waste / 100)); boxes = ceiling(tiles / tiles per box)
Ten percent for a straightforward rectangular room, fifteen for diagonal or herringbone layouts, and twenty where the room has many cuts or the tile has a large pattern repeat that must be matched.
Because tile is made in dye lots and shades drift between batches. A single cracked tile years later is invisible if you have one from the original run and obvious if you do not — keep at least a few square feet.
Yes, the arithmetic is identical; just enter the wall dimensions. Subtract large openings such as windows separately, but do not deduct small ones, since the offcuts rarely prove usable.