Exact count is not the buy count
The calculator gives two numbers: the mathematical tile count for the area, and the realistic buy count with a waste allowance. Tiles crack in the box, cuts go wrong, and future repairs need spares — 10% waste is the standard for a straight lay, 15% for diagonal, herringbone or rooms with many jogs.
Dye lots are forever
Tile from different production runs varies slightly in color and texture. Buying the full quantity — plus the waste — from one dye lot matters more than any other tile-buying rule, because you cannot top up from a different lot without seeing the seam. This is why you buy all at once, not in stages.
Measure the awkward bits
Break L-shaped rooms into rectangles and add them. For walls (kitchen backsplash, shower surround), the same math applies: width × height of each section, minus window and niche openings, then the waste factor.