Multiply length by width for a rectangle. For an L-shaped room, walls or several areas, split them into rectangles, add the areas and enter the net total above. Subtract permanent openings that will not be tiled.
Prefer the coverage per box printed on the package or product page. If only tiles per box is known, enter the tile dimensions and tile count. When both are entered, box coverage takes priority.
Ten percent is a common starting point, not a universal rule. Diagonal layouts, complex patterns, many cuts or keeping same-lot spare tiles can require a higher allowance.
Enter length and width or a known net area. Add tile dimensions for piece counts, or use the printed coverage per box for whole boxes, leftover coverage and cost.