Generally, tile and stone for a "field" area is figured by the square foot. To figure square footage, measure the area that you will be putting the tile with a tape measure. Examples:
1.You measure your room and it comes out to be 10-1/2 feet wide by 15-3/4 feet long.you would calculate it by multiplying 10.5x15.75 = 165.38. Round this number off to the closest whole number (165). I suggest adding on a 10-15% overage to plan for cuts, breakage and a little extra to store in case a piece needs to be replaced later in time.
2.You measure your wall area to be covered and you get 18" tall and 33" long. Multiply 18x33 = 594". Now divide that number by 144, resulting in 4.13sf. (There are 144 sq. inches per square foot). Again, round it off and add on for overage.
When figuring for decorative tile or trim, it is usually by the piece and calculated by the lineal foot.
For slabs, it is best to let your fabricator figure the quantity for you.