This seems to give a lot of people headaches but, when you know the really simple method outlined below, it's actually really easy. You will need some measuring tape, pencil and paper, and maybe a calculator.
Draw out each wall on a piece of paper (it doesn't need to be to scale) and take each square section as a separate area.
Take a width measurement in millimetres and divide by the width of the tile you have chosen. This will tell you how many tiles wide your area is.
Decide how high you would like to tile (it can be all the way up the ceiling. Measure this height in millimetres and divide by the height of your tiles. This tells you how many tiles high your area is.
Multiply the number of tiles wide by the number of tiles high.
Repeat these steps for each wall drawn on your sheet.
Add each number of tiles together and add an extra 10% as an allowance for cuts, breakages, and spares.
Floor tiles are worked out in the same way as wall tiles, above.
We find it's best to measure out your areas than trusting measurements given to you on a plan (which will need to be converted into millimetres anyway). If you're having a hard time figuring out the floor quantities (especially thanks to a strange shape room), one of the friendly Beaumont Tiles consultants in your local branch will be happy to help out.
Don't forget to ask us about step treads (if you have stairs) and brass angles (if your tiles butt up against another type of flooring).
When we work out quantities, we always allow an extra 10% for cuts, breakages and spares. At Beaumont Tiles we have dozens of people each week chasing old tiles to replace where a pipe has burst, or to extend, and so often the tiles have been out of production for years. (It pays to keep spares!) Put them across the joists in your ceiling space.