You should have a clear idea of the size and shape of the area you want to tile and how it's going to be used. It's helpful to first draw your area on paper. For example, two walls and one floor. Then, determine if the tiles are to cover the whole area or part of the area and mark the area to be covered.
Make sure you settle on a tiling pattern early like straight, pin wheel or herringbone, and your grout width. Remember to take out the spaces for windows but add in any sills or skirting you plan to tile. Also think about the fixed accessories you want incorporated with your tiling. Plus, you should always order an extra 10 to 15% because even if you don't use them for the project, they may come in handy at a later date.
If it's a floor you're tiling, use a measuring tape to measure the length of one side of the room and then the width of another side of the room. For walls, measure the length of the wall up and down and the width left to right.
Multiply the length and width measurements you obtained, which will give you the total square meterage of the wall and/or floor areas. So, if the wall or floor measures 5m long by 4m wide, multiply 5 by 4 and your calculation will be 20 square metres. Do this for each wall or floor area you need tiled, and then add all of the numbers together.
So now you have the square meterage, but it's important to buy more tiles than you need to cover the space as you should factor in cuts, waste and breaks. The extra tiles will also help if your supplier runs out of stock mid-project.
Multiply the square meterage you have by 15%, this will be the actual amount of square meterage you should buy tiles for.
Tiles usually come in boxes with their amount labelled, so divide the total square meterage required by the total square meterage of the tiles in the box. For example, if the room is 120 square metres and each box has 10 square metres of tile inside, then you'll need 12 boxes.