Carpet is bought off a roll, not by the square foot
Broadloom carpet is cut from rolls of a fixed width — Shaw says most carpet is made 12 ft wide, with 15 ft on some styles; a few styles come 13 ft 6 in. The store cuts a length off the roll, so what you pay for is roll width × length cut, then quoted in square yards (sq ft ÷ 9). A room that is 1 ft wider than the roll needs a second full-width drop, which is why two rooms with nearly the same area can need very different amounts of carpet.
Worked example — a 13 × 15 ft room: on a 12 ft roll the cheapest layout runs the roll along the 13 ft wall. That needs two drops (13 ft across is more than one 12 ft width), each 13 ft + 3 in trim at both ends = 13 ft 6 in. Two × 13.5 × 12 = 324 sq ft = 36 sq yd to cover 195 sq ft, with one seam. The same room on a 15 ft roll is a single 13 ft 6 in cut: 15 × 13.5 = 202.5 sq ft = 22.5 sq yd, no seam.
Carpet for common room sizes
| Room | Area | 12 ft roll | 15 ft roll |
|---|---|---|---|
| 10×12 ft | 13.3 sq yd | 14 sq yd | 17.5 sq yd |
| 12×12 ft | 16 sq yd | 16.7 sq yd | 20.8 sq yd |
| 12×14 ft | 18.7 sq yd | 19.3 sq yd | 20.8 sq yd |
| 13×15 ft | 21.7 sq yd | 36 sq yd, 1 seam | 22.5 sq yd |
| 14×16 ft | 24.9 sq yd | 38.7 sq yd, 1 seam | 27.5 sq yd |
| 15×20 ft | 33.3 sq yd | 41.3 sq yd, 1 seam | 34.2 sq yd |
All figures include 3 in of trim at each cut end. Check that your style is sold in the wider width before you plan around it.
Where the seams go
Every piece has to keep the same pile direction (CRI 105), so fill strips are cut lengthwise from extra drops rather than turned sideways. When there is a choice, CRI 105 places seams so they run the length of the room, parallel to foot traffic, and never across a doorway. Pad seams should sit at right angles to the carpet seams or at least 6 in away from them.
Pad and tack strip
- Pad: buy the room's area plus about 5%. CRI 105 caps residential cushion at 7/16 in; Shaw's 2026 warranty allows up to 1/2 in. A thicker or softer pad than the carpet maker allows can void the carpet warranty.
- Tack strip: runs along every wall but not across door openings. For walls longer than 30 ft, CRI 105 calls for architectural (3-row) strip or two rows of standard strip.
- Transitions: every doorway where carpet meets a hard floor needs a transition strip.
Carpet for stairs
Each straight step takes a piece as wide as the stair and as long as the tread + riser + about 1 in to wrap the nose + about 1 in of allowance. On a 12 ft roll, 36 in stairs fit three pieces across, so 13 steps need five rows: 5 × 19.5 in = 8.1 linear ft, or 97.5 sq ft. That covers about 4.6 sq ft per step — close to Fixr's 4.5 sq ft — but you buy 7.5 sq ft per step. CRI 105 requires the stair nose to be rounded 3/4–1 in, and most manufacturers want the pile running down the stairs.
What carpet costs installed
This Old House (June 2026) puts carpet labor at $0.50–$1 per sq ft, pad at $0.75–$2 per sq ft installed, and stairs at $11–$26 extra per step; tearing out old carpet runs $2–$3 per sq ft. Fixr quotes $36–$65 per step for carpeted stairs all in. "Free installation" promotions often exclude stairs, removal and furniture moving, so ask for those lines separately. Let carpet and pad acclimate at least 24 hours at 65–95°F (CRI 105).
Common mistakes
- Ordering by floor area: a 13 ft wide room on a 12 ft roll needs far more than its area.
- Forgetting the 3 in trim at each end, or measuring a room that is exactly roll-width with no side allowance.
- Seams in doorways or across the main walkway.
- A pad thicker than the carpet warranty allows.
Doing the rest of the house? Use the flooring calculator for vinyl plank or hardwood, the baseboard calculator for new trim, and the paint calculator before the carpet goes down.