Carpet arrives in a fixed-width roll. What you pay for is the rectangle you cut it from, and a room an inch wider than the roll costs you an entire second run.
Run direction is chosen automatically — whichever way needs less carpet. Pattern or sight lines may override it.
A pattern has to line up across every seam, so each run gets rounded up to a whole number of repeats.
Carpet is not sold as a quantity of surface. It is sold as a length cut from a roll of fixed width — almost always 12 ft in North America, sometimes 15 ft, and 4 m where metric widths are stocked.
So the question is not how many square yards the floor is. It is how many full roll widths the room needs and how long each has to be. You buy that rectangle whether or not you use all of it.
A 12 × 18 room takes one 18 ft run off a 12 ft roll: 24 sq yd, and almost nothing wasted. A 13 × 18 room takes that same run plus a 1 ft strip — which still has to be cut from a full 12 ft width. The purchase jumps to 48 sq yd to cover 26 sq yd of floor.
One extra foot of room width nearly doubles the order. That is the number this page exists to show you.
Where a room is wider than the roll, there will be a seam. Good practice puts it running the length of the room, aligned with the main sight line, away from doorways and traffic paths, and not landing where furniture legs will work at it.
Every piece must also run the same pile direction. Carpet reflects light differently along the nap and against it, so a piece laid backwards reads as a different colour under the same light. This is not a manufacturing fault and it cannot be fixed after fitting.
A patterned carpet has to line up across the seam, so each run gets extended to a whole number of repeats. A 36″ repeat on an 18 ft run means buying 18 ft rounded up to the next multiple of 3 ft, and doing it on every run.
That is why patterned carpet costs more to fit than its price per yard suggests.
Sometimes, and a good fitter will plan for it. It has to be the same roll, the same dye lot and the same pile direction, and it has to be big enough for the second room to take it in one piece.
In a room between 12 and 15 ft wide, absolutely — it can turn a two-run job into one, remove the seam entirely and cost less overall. Selection is narrower, so it is worth checking early rather than after choosing a style.
Stairs are figured separately, per tread and riser, and want the pile running down the flight. They are not part of a room calculation.