The calculation
Tile area is simply tile length × width. The result is rounded up to a whole tile, then divided by tiles-per-box (also rounded up) so you're ordering in full boxes rather than a fractional count that doesn't match how tile is actually sold.
Picking a wastage allowance
10% is a common default for a straightforward grid layout with no unusual cuts. Diagonal (herringbone or 45°) layouts, small rooms with lots of edge cuts, or a first-time DIY installer generally warrant 15-20% instead, since cut-off pieces from one edge often can't be reused elsewhere.
Frequently asked questions
Does grout joint width affect the tile count?
Only slightly, through the effective coverage of each tile — this calculator uses the tile's face dimensions directly, which is accurate enough for ordering purposes. Grout joint spacing has a much bigger effect on grout quantity than on tile count.
How much extra should I order for a diagonal layout?
Bump the wastage allowance to 15-20% for diagonal or herringbone patterns — the offcuts from diagonal cuts are far less reusable than a simple grid layout.
Should I round tile size to the nearest common size?
No — enter the exact tile dimensions from the box or spec sheet. Even small rounding errors compound over a large area.