Eighteen-inch Tile

• An 18-by-18 inch tile is 2.25 square feet. After dividing 138 by 2.25, you get 61 pieces of 18-inch tile. To account for waste and cutting, experienced tile setters add an extra 10 percent to the amount. Therefore, you should buy 67 pieces.
  
  

Twelve-inch Tile

• A 12-inch piece of tile is exactly one square foot. After adding 10 percent to the amount, a do-it-yourselfer will need approximately 152 pieces to cover a 138-square-foot area
  
  

Eight-inch Tile

• An 8-by-8-inch tile is 0.444 square feet. Divide 138 square feet by 0.444. After rounding, the figure is 311 pieces of 8-inch tile. Add 10 percent for waste and cutting and you will need 342 pieces.
  
  

How to Figure Square Footage

• Figuring the correct amount of tile is simpler than one may suspect. For floors and walls, simply measure the length and width and multiply the two numbers. Separate an L-shaped room into two separates rectangles, figure the square footage amounts for each part and add them together to get the square footage. Also, separate each wall and floor area in a complicated bathroom area to get the square footage figure.