When you order your tiles you should calculate how many you need and then add at least 10% for cuts and breakages. Please note that for larger tiles, it is advisable to actually calculate the exact number of tiles rather than the amount of m². If you get stuck talk to the people in your nearest showroom and they will be delighted to work it out for you.
To calculate first measure the wall or floor area that you are intending to tile, then measure any doors, windows or other features that you will not be tiling and subtract the latter from the former. For example in the diagram below A x B = E and C x D = F. E – F = sqm to be tiled. Add 10% to this total and you have the number of tiles that you need.