An 10 ft × 8 ft tiled wall (80 sq ft) using subway tile that covers about 10.5 sq ft per box needs about 9 boxes, including a 10% cut-and-waste allowance. Bump the waste up for diagonal or herringbone layouts.
Give the area you're tiling. Add extra waste for busy layouts or lots of cuts.
Cuts at edges, around outlets and at corners turn whole tiles into offcuts, and patterned layouts waste more. Ten percent is the floor for a simple straight lay; diagonal, herringbone and large-format on uneven walls can need 15–20%. Buy it all up front so the shade (the tile's "lot") matches.
Like wallpaper, tile is produced in batches and colour can shift between them. Order every box for a wall in one lot, and keep a spare box after the job — future repairs are almost impossible to match otherwise.
Measure the wall area (width × height), add a waste allowance of 10–20%, then divide by the coverage printed on the tile box. Round up. The calculator above does this and lets you pick common box coverages.
At least 10% for a straight lay, and 15–20% for diagonal, herringbone or large-format tile on uneven walls. Buy it all at once so the lot matches, and keep a spare box for repairs.
For a rough count you can tile the full wall area and let the waste allowance absorb small openings. For a large opening, subtract its area before applying waste.
Tile is fired in batches and shade varies by lot. Always order boxes from the same lot number, and blend from several open boxes as you work to average out any variation.