Need to figure the square footage of the room and then the square footage of the tile you have chosen and do the math. It is all math.

You need the square footage of the floor you want to tile and then add 10% to the square footage in case tiles break during the cutting and installation. This is what my tiler told me and my two bathroom and hall tiled areas came out great. Also, you can call up any reputable tile store for additional advice.

"then add 10% to the square footage in case tiles break"

That may be but to declare it a blanket statement is impossible without knowing the room layout and tile layout. It could 15% (which is typically the minimum standard allowance for overage) or 50% or more depending.

Also the extra is good for tiles with pattern. You have of a choice of ones you like.