To estimate how many tiles you'll need for your job, you can do two things. Firstly - ask your tiler. Done. Secondly - work it out.
To work it out, you'll need to assess the following:
You're aiming to work out the surface area. To do this you need to look at the areas you want to tile and split them up into square or rectangular areas. Then measure the length and breadth of each square or rectangular area. Of course, if you're just doing the floor then the calculation is simple - but if you're doing walls as well, then each area needs to be done separately.
If you're using tile sheets your grout area will already be factored in but don't forget to allow for the spacing between the tiles.
The more separate areas you have, the more tiles you are likely to waste. Unconventional layouts will also cause increased wastage.
This is usually easier as it's just one measurement, and is often linear. Simply measure each line of trim and add the space between each tile. Trim tiles are far more often found in strips which makes it easier to purchase.
Tip: Tiles are mostly sold in packs with the floor coverage area written on the pack.