How to Calculate Tile Using Two Different Sizes
Whether you are tiling a wall or a floor, the same formula applies to calculate your tile needs. Your project could be a ceramic tile backsplash, the interior of a shower, the foyer in your home or a stone patio. Using tile of two different sizes within your project only requires the designer to make a few more calculations. All you need to do is multiply, divide and subtract.

Choose your tile. Notate the exact width, depth and length of your selections.

Measure the width and length of the area you plan to cover with tile. Work in feet or convert your measurements to inches for greater accuracy. Multiply the width times the length of the area. For instance, if you are installing an 18inch deep backsplash, measure each area that will be covered with tile. If one wall of the backsplash is 12 feet wide, you would multiply 12 x 1.5 to calculate the total square feet. In this example, the answer is 18 square feet. Because there are 144 square inches in a square foot, you will need 2,592 square inches of tile (18 times 144).

Calculate how much tile you will need for your border or accent pieces. For this example, assume that you have selected a 4inch by 2inch border. To cover 12 feet (or 144 inches), divide 4 (the tile length) into 144. You will need 36 pieces of tile for your border. Each 4inch by 2inch border tile is 8 square inches. Multiply the 36 by 8 to get the total number of square inches used by your border. In this example, the answer is 288 square inches.

Subtract the total number of square inches for the border from the total square inches of tile needed. Using the same example, 2,592 minus 288 is 2,304 square inches. For this example, 4inch by 4inch squares have been selected for the tile field. Each 4inch by 4inch field tile is 16 square inches. Divide 16 into 2,304. You will need 144 squares to complete this section of backsplash.

Figure out how much extra tile you will need to account for tiles lost due to waste, such as improperly cut tile or breakage. If you use a professional installer, plan for five percent waste. Multiply the total tiles needed, in both sizes, by 1.05. If you are installing your own tile, plan for 10 percent waste. Multiply the total tiles needed in both sizes by 1.10. If you are doing a diagonal installation, have lots of corners, jogs or curves, plan for 15 percent waste. Multiply the total tiles needed in both sizes by 1.15.
Things You Will Need
 Tape measure
 Calculator
 Paper
 Pencil
Tip
 If you are planning to inset a different sized tile within the overall field of tile, create a line drawing so you or the tile installer can use it as a guide.
Warnings
 Never skimp when you order your tile. Ensure that you have enough to complete the job.
 Make sure that the tile matches perfectly. Die lots and tones can vary among shipments.