The formula
Floor tile math has five steps. One: room area in square feet, which is length times width in feet. Two: coverage of one tile in square feet, which is tile length times tile width in inches divided by 144. Three: divide area by tile coverage for the raw tile count. Four: multiply by 1.10 for the straight-lay waste allowance and round up. Five: divide by the tiles per box and round up to whole boxes. Steps four and five both round up, because a fraction of a tile and a fraction of a box are both useless at the store.
For L-shaped or irregular rooms, split the floor into rectangles, compute each area, and add them. Do not subtract permanent fixtures like a kitchen island from the area unless the island is truly permanent and tiled around rather than under; most installers tile under movable islands and vanities, and the waste allowance absorbs the rest. For the layout side of the job, read our floor tile layout guide before you buy.
Worked example 1: a 12 by 12 room with 12 by 12 tile
Area: 12 times 12 is 144 square feet. One 12 by 12 inch tile covers exactly 1 square foot, so the raw count is 144 tiles. Add 10 percent: 144 times 1.10 is 158.4, so buy 159 tiles. At 10 tiles per box: 159 divided by 10 is 15.9, so buy 16 boxes.
Cost check: at 24.99 dollars per box, 16 boxes cost 399.84 dollars. This is the canonical example because the numbers are clean, and it is the sanity check for every other calculation: if your answer for a 12 by 12 room with 12 inch tile is far from 16 boxes, recheck the tile coverage first.
Worked example 2: a 15 by 10 room with 12 by 24 tile
Area: 15 times 10 is 150 square feet. One 12 by 24 inch tile covers 288 square inches, divided by 144 is 2 square feet. Raw count: 150 divided by 2 is 75 tiles. Add 10 percent: 75 times 1.10 is 82.5, so buy 83 tiles. Large format tile often comes 5 to a box: 83 divided by 5 is 16.6, so buy 17 boxes.
Notice how box size changes the waste cushion. Seventeen boxes hold 85 tiles, which is only 2 tiles beyond the order. If the room has a closet or an angled wall, consider an 18th box. Big tile breaks into big losses: one cracked 12 by 24 is two square feet of the order gone.
Worked example 3: an 8 by 6 bathroom with 6 by 6 tile
Area: 8 times 6 is 48 square feet. One 6 by 6 inch tile covers 36 square inches, divided by 144 is 0.25 square feet. Raw count: 48 divided by 0.25 is 192 tiles. Add 10 percent: 192 times 1.10 is 211.2, so buy 212 tiles. At 10 per box: 212 divided by 10 is 21.2, so buy 22 boxes.
Small bathrooms are where the fixture question matters most. If the vanity is staying and you will tile around it, subtract its footprint: a 4 by 2 foot vanity is 8 square feet, so the tiled area drops to 40 square feet, which is 160 tiles plus 10 percent, or 176 tiles and 18 boxes. Measure what you will actually tile, not the room.
The quick answers people ask for
How many tiles for 100 square feet? With 12 by 12 tile: 100 plus 10 percent is 110 tiles, or 11 boxes of 10. With 24 by 24 tile at 4 square feet each: 25 plus 10 percent is 27.5, so 28 tiles. With 6 by 6 tile at 0.25 square feet each: 400 plus 10 percent is 440 tiles. Tile size dominates the piece count, not the area.
Run any of these through the tile calculator to confirm: enter the room dimensions, pick the tile size, and it applies the waste factor, box rounding, and an optional cost estimate in one pass. For showers, which need wall-by-wall math, use the method in our shower tile guide instead.