How the tile math works
Tile is counted in whole pieces, so the math is: measure your area in square feet, add a waste allowance for cuts, then divide by the area one tile covers and round up. A tile's coverage comes from its size in inches — width × length ÷ 144 gives square feet per tile, so a 12 × 24 in tile covers exactly 2 sq ft and a little 3 × 6 subway tile covers just 0.125 sq ft. Boxes work the same way: divide your with-waste area by the coverage printed on the box and round up again. You always round up, never down — a project that comes up one tile short stalls until you can get back to the store (and hope the same dye lot is still in stock).
The formula
Area is the space to tile in square feet (length × width for a rectangle), Waste% is your allowance for cuts (10-20%), and tile W × L is the tile's size in inches — dividing by 144 converts it to square feet per tile. The ⌈ ⌉ brackets mean round up to the next whole tile.
Worked example
A 10 × 12 ft kitchen floor is 120 sq ft. With a straight lay of 12 × 24 in tile, add 10% waste: 120 × 1.10 = 132 sq ft to buy.
Each 12 × 24 tile covers 2 sq ft, so 132 ÷ 2 = 66 tiles. If the box label says 15.5 sq ft per box, that's 132 ÷ 15.5 = 8.52 → 9 boxes.
Why the waste factor depends on your layout
Waste isn't about clumsiness — it's geometry. Every tile that meets a wall, cabinet, or doorway gets cut, and the offcut usually can't be used anywhere else because its cut edge or its size is wrong. A straight lay in a simple rectangular room wastes the least, so 10% covers it. Lay the same tile diagonally and every perimeter tile becomes a 45° cut with a triangular offcut — that's where 15% comes from, and patterns like herringbone are similar. Small rooms, hallways with lots of doorways, and spaces with niches or angled walls push the cut count higher still, hence 20%.
Two more things before you head to the store. First, buy every box from the same dye lot (the batch code stamped on the box) — tiles from different firing batches can differ visibly in shade and even size. Second, don't return the leftovers: a few spare tiles in the garage are the only way to invisibly repair a cracked tile years from now, when the line is discontinued. And remember this calculator counts tile only — grout and thinset are separate purchases, sized by the coverage charts on their bags.