Calculating how many tiles you need
Measure the length and width of the first area (in the metric desired).
Repeat as required until all areas have been measured.
Multiply the first area's two measurements. Multiplying the length by the width provides you with the square footage of the project area. For example, if the room measures 12 feet wide by 12 feet long, multiply 12 by 12. The square footage of this room is 144 square feet.
Translate the square footage to the amount of tile you need. Tile typically comes in boxes, and you have to buy the whole box. Divide the total square footage of the room by the total square footage of the tile in the box. In our example, let's say each box contains 20 square feet of tile. So if you need 120 square feet, 20 goes into 120 6 times, so you'll need to buy at least 6 boxes.
Calculate 10% for coverage and breakages. You should never buy just the amount of tile you need, since you also need a bit extra for cuts, waste, breaks, and mistakes. Multiply the square footage of the room by 10%, then add this amount to the total square of the room. This is the total amount of square footage you should buy. To continue with the example we started above, .10 x 120 = 12, and 120 + 12 = 122. We'll need 122 square feet of tile.
Another reason to buy extra is if the style or color is discontinued and you need to replace a broken tile or two, you will need extras on hand to make the repairs. If you don't, then in order to fix it, you will have to replace the entire floor. The moral of the story is: Spend a little bit more up front to save money in the long run.
For information on how to tile a shower wall click here.