Calculating the number of tiles you need for a given room isn't simply a matter of taking the square footage. Tiles have to be cut for the edges of room, which means a whole tile will be used to fill a smaller space, creating wasted tile --- and that's not accounting for any mistaken cuts you might make. Calculating the number of tiles by inches rather than feet, then adding 10 per cent to account for waste, is a good general rule for a typical grid-pattern tile floor.
Measure along each side of the floor, marking the middle. Pull the string from your chalk box from one mark to the mark across from it, with the help of an assistant. Snap the string against the floor to leave a line of chalk.
Repeat the process between the two remaining marks, so the string intersects the first line. Use a square to ensure the lines are exactly perpendicular to each other. Snap the line. The floor should be divided into four square sections, meeting at the middle of the room.
Measure one line from wall to wall and divide by the length of a tile. Round up the resulting number to the next full tile size. So if the line is 184 inches across, and your tiles are 12 inches across, the line will fit 15 tiles plus 4 inches, so round up to 16 tiles. Repeat for the other line.
Multiple the total number of tiles in the two lines by each other. So if one line has 16 tiles and the other has 12, your total is 192 tiles.
Add another 10 per cent, since some cuts will waste most of a tile. For 192 tiles, 10 per cent would be 19.2, or 20 rounded up. So your total for the room is 212 tiles.
If the room isn't square but has various offset sections, divide it visually into square sections and calculate each one separately.