Before you begin a tiling project, it's important to have enough tiles on hand to finish the job. If you run out before you're finished, you may find that the supplier has run out of the tiles you're using or that the ones in stock are slightly different from yours. Calculating your tile requirements isn't complicated -- all it takes are a few careful measurements and some simple math. If math isn't your forte, you can always make use of one the online calculators available on the websites of tile suppliers and manufacturers.
Measure the length and width, or height and width, of the floor or walls you're going to tile. Take all measurements in inches. Include the depth of the sides of niches for showers. Divide irregular shapes, such as walls that wrap around cabinets or turn corners, into smaller rectangles and measure the length and width of each rectangle.
Calculate the area of each rectangle you measured by multiplying the length and width of the measurement. Add the areas of all the rectangles to determine the total area you need to tile. This number is in square inches.
Multiply the length and width of the tile to determine its area. For example, a 4-inch by 4-inch tile is 16 square inches.
Divide the area of one tile -- in square inches -- into the total you just calculated to determine the number of tiles you need. For example, if you're tiling a 10-by-10-foot wall -- 120 by 120 inches -- and using 4-by-4-inch tiles, divide 16 square inches into 14,400 square inches to find you need 900 tiles.
Convert the area you measured to square feet, if buying your tile by the box, by dividing by 144 to determine the number of boxes you need. Alternatively, measure in feet to begin with. The coverage of each box is marked on its front in square feet. For example, say the area of your 10-by-10-foot wall is 100 square feet. If each box of tiles covers 20 square feet, you need five boxes to tile it.
Include an overage in your calculations. If you plan to have the tiles professionally installed, add 5 percent, but if you plan to do it yourself, and you aren't experienced, it's better to add 10 percent. If the area you're tiling is irregularly shaped, requiring multiple angled cuts, add 15 percent.