How the tile count is calculated
Start with the area of the floor or wall in square feet: length x width for a rectangular room, or add up the sections of an L-shaped one and enter the total. Next, figure out how much floor one tile actually covers. A 12 x 12 inch tile is one square foot on its own, but once you add a 1/8-inch grout joint on two sides its footprint becomes 12.125 x 12.125 inches, or 1.021 sq ft. That small difference adds up over a room, so this calculator includes the joint width in the tile footprint rather than ignoring it.
Divide the area by the footprint to get the bare tile count, then add waste. Ten percent is the standard allowance for a straight or offset layout; diagonal, herringbone and rooms with lots of cuts around cabinets and doorways should use 15%. The result is rounded up to a whole tile, and if you enter how many tiles come in a box, it rounds up again to whole boxes. Always buy from the same dye lot and keep a few spares for future repairs; a matching tile five years from now is nearly impossible to find.
Worked example: a 10 x 12 foot bathroom floor
With the default inputs, a 10 x 12 room is 120 sq ft. A 12 x 12 tile with a 1/8-inch joint covers 1.021 sq ft, so the bare count is 117.5 tiles. Adding 10% waste gives 129.3, rounded up to 130 tiles, or 132 sq ft of material. If the store sells 12 x 12 tile in boxes of 10, that is 13 boxes. Switch to a 12 x 24 plank, which covers 2.03 sq ft with the same joint, and the bare count drops to 59.1, so you would buy 65 tiles. Subway tile at 3 x 6 inches with a 1/16-inch joint covers only 0.13 sq ft each, so a 120 sq ft wall takes 1,024 pieces with 10% waste.
For setting material, a 50-lb bag of thin-set mortar covers roughly 40-50 sq ft when spread with a 1/4 x 1/4 inch square-notch trowel, which is right for tile up to about 12 inches. Large-format tile (any side over 15 inches) needs a 1/2-inch notch and often back-buttering, cutting coverage to 30-40 sq ft per bag. The calculator uses 45 and 35 sq ft respectively and rounds up. Grout consumption varies enormously with joint width and tile size; a 10-lb bag will do 100 sq ft of 12 x 12 tile with a 1/8-inch joint but only about 40 sq ft of 3 x 6 subway tile.
Tips before you buy
Measure the room at floor level in two places in each direction and use the larger number. Check whether the area includes closets, a tub alcove or the space under a vanity that will be tiled anyway. If the tile has a directional pattern or you plan a border, add another 5%. Wall tile is measured the same way: perimeter x height, minus large openings like a window.
Order everything at once. Boxes of the same tile bought a few weeks apart can come from different production runs with slightly different shade and size, and dye-lot mismatches are the most common complaint on tile jobs. Most retailers accept returns of unopened boxes, so ordering one extra box is cheap insurance against running out with three tiles left to set.