How the tile count is worked out
The area to cover is length × width in square feet. Each tile covers its own size plus one grout joint in each direction, so a 12 × 12 inch tile with a 1/8 inch joint covers 12.125 × 12.125 inches, about 1.02 square feet. The area is increased by the waste percentage and divided by that coverage, then rounded up, because you cannot buy part of a tile.
If you enter how many tiles come in a box, the calculator rounds up to whole boxes. The price is per square foot of tile, the way most stores label it, and is charged on every tile in the boxes you buy, not just the ones you lay.
Choosing the waste percentage
The calculator starts at 10% waste. That is a common rule of thumb for a straight or brick lay in a simple rectangular room: it covers the cut pieces along the walls and a few broken tiles. Diagonal and herringbone layouts cut far more tiles at the edges, so 15% is the usual starting point for those. Raise it further for large format tiles in a small room, rooms with lots of corners or a floor that has to work around cabinets and fixtures. These are starting points, not rules; if your installer gives you a number, use theirs.
Grout joint width
The default joint is 1/8 inch. Rectified porcelain is often set with a 1/16 inch joint, while handmade tile, terracotta and natural stone with uneven edges often need 3/16 or 1/4 inch. Check the tile manufacturer's recommendation and enter that. The joint has little effect on the count for large tiles but a noticeable one for small tiles, because there are many more joints per square foot.
Rooms that are not rectangles
Split an L-shaped or irregular floor into rectangles, run each one, and add the tile counts together before rounding to boxes. For a shower or backsplash wall, enter the wall width as the length and the tiled height as the width, and leave out large openings like windows only if they are big enough that the cut pieces will not be reused.
What this does not cover
The calculator counts field tile only. Bullnose or trim pieces are sold by the linear foot, so measure the edges separately. Thinset, grout, backer board, spacers and sealer are separate purchases; check the coverage printed on each bag or bucket against the area shown above.