Room area in square feet divided by the face of one tile, then a waste percentage for cuts, is the method installers use. Enter the room in feet and the tile in inches.
Twelve-by-twelve tile is one square foot. A 10 percent waste factor is typical for straight layouts. Use about 15 percent for diagonals or rooms with many jogs.
If you know tiles per box, round the purchase up so you do not leave the aisle short. Open the Tile Calculator when you want the arithmetic done for you.