If you have taken by suggestion seriously, and have visited your local tile supplier, this is the most accurate and professional opinion of the amount of tile, mortar and grout you should purchase. The amount of tile will be based upon the type of tile being installed, as well as the square footage of surface that will be covered. Square footage, is the area of the surface to be covered, and is easily computed by measuring the length and the width of the space. These dimensions are multiplied together and this is the square footage of the area. If there are several different areas of flooring or wall space that will be covered, it is best to break each area up into simple squares or rectangles, take the measurements of each area and simply add them up.
The amount of grout material will depend on the width of the joints, as well as the size of the tile. However, simply buy more than you need, because the grout is very inexpensive, and easily used later on for joint repairs, etc. The thin set, or mastic under the tile, is based upon the square footage of tile, and will be noted on the product packaging in relationship to coverage.
I would suggest that a minimum of 10% overage be ordered for the tile. This will allow cutting, an occasional broken tile, etc. However, dependent upon the tile, your supplier will guide you regarding the amount of tile required to accommodate the square footage, the layout, as well as the type of tile selected. It is always better to have more tile than not enough. In many instances, the tiles are produced in what are called, batches. This refers to the process involved when the actual tiles were manufactured. If additional tile is required to complete the job, the same batch should be used. Unfortunately, this is not as simple as it sounds, and in many cases, the additional tile ordered, after the initial order, will not be the same batch, and will not be exactly the same color. This mixing of different batching, will cause a problem when the newly ordered tile is installed alongside the older tile, there will be a color difference. This is the reason why, it is always best to order enough tile the first time.