Carpet is the one flooring where square footage barely decides the price. It comes off a roll of fixed width, and whether your room fits that width matters far more than its area.
Broadloom carpet in the United States comes predominantly in 12 foot widths. Some manufacturers also offer 13 foot 6 inch and 15 foot rolls.
That means the important measurement is not your room's area. It is your room's narrow dimension, and specifically where it falls relative to those widths.
This is why two rooms with identical square footage can be priced very differently. A room 11 by 20 feet takes one clean 12 foot width run down its length. A room 13 by 17 feet has almost the same area and is far more awkward.
Measure both dimensions carefully, then check the narrow one against 12 ft and 15 ft. If you are at 12 ft 3 in, it is worth knowing, because that three inches is the difference between a seamless 12 foot run and paying for a 15 foot roll.
This is the opposite of the advice for hard flooring, and it catches people who have measured for laminate before.
With hard flooring you subtract the chimney breast because you genuinely cut around it and save material. With carpet you usually do not, because the roll still has to run past it. The carpet that would have covered that alcove gets cut off and discarded rather than saved.
So measure the maximum length and the maximum width of the room, including into doorways and alcoves, and treat it as a rectangle. Add the doorway thresholds, because carpet normally runs into the doorway to the centre of the door.
If your room needs a seam, its position is decided before the carpet is ordered, which makes it something to think about while measuring rather than on fitting day.
Two rules installers work to. Seams should sit in low traffic areas, away from doorways and the main walking line. And seams should run parallel to the primary light source, usually the main window, because a seam running across the light casts a faint shadow line that catches the eye.
Both of those constrain which direction the carpet runs, which in turn decides which of your two room dimensions has to fit the roll width. Measure the room, note where the window is and where people walk, and the fitter has what they need to work it out.
Do not fold stairs into the room figure. They are quoted on their own and measured differently.
For each step you need the tread depth, the riser height, and the width. Then add roughly an inch and a half per step for the nosing, the rounded front edge that the carpet wraps over. A rough working figure is about 18 inches of carpet run per standard step, but measure rather than assume, because tread depths vary considerably between older and newer houses.
Winder steps, the wedge shaped ones on a turn, need each one measured individually at its widest point. They are the single most common source of a shortfall on a stair order.
Industry guidance for straight laid carpet in a simple rectangular room is typically 5 to 10 percent over. Rooms with multiple angles run nearer 15 percent because there are more cuts and the offcuts are less reusable.
Note that this waste factor sits on top of the roll width waste, not instead of it. If your 13 foot room is being covered by a 15 foot roll, you are already discarding two feet across the length before any allowance for cutting.
Underlay is ordered by the same area but is not width constrained in the same way, so it is closer to a straight square footage calculation with a small overage.
The practical upshot is that carpet rewards accurate measurement of the two room dimensions far more than accurate area. Get the width right to the inch. The general room method is here, and hard flooring follows different rules.
If none of the above is quite right, the eight we recommend most often are on our front page.