Carpet calculator

Broadloom comes off a fixed-width roll, not a spreadsheet. This works the order from the roll width — with seam count and the pile-direction rule that decides whether a room is one piece or three.

01 How the math works

There are two ways to count carpet, and they give different numbers. The first is the one every calculator uses: take the floor area, add a waste percentage, and divide by nine to get square yards. Carpet was priced by the square yard for a century, and one square yard is nine square feet, so the arithmetic is simple.

The second way is how the carpet actually comes off the roll. Broadloom is milled in a fixed width — 12 ft (3.66 m) is the standard, with 13'6" and 15 ft stocked in some lines. You lay full-width strips side by side, all with the pile running the same way, and seam them. The number of strips is the room's other dimension divided by the roll width, rounded up. That rounding-up is where the two methods part company: a percentage can be anything, but a roll-width is discrete, so the order jumps a full width at a time.

02 Worked example

Take a 12 ft × 15 ft bedroom (3.66 m × 4.57 m) — 180 ft² (16.72 m²), or 20 yd². The room's 12 ft dimension lands exactly on a 12 ft roll, so the whole floor comes off one 15 ft pull at the full roll width: 12 × 15 = 180 ft² = 20 yd², a single piece with no seam.

The flat-area method adds 10% and calls for 22 yd² — two yards of remnant the room never needed. Here the roll method orders less, because the room fits the roll and the only real waste is the net-fit trim. That is the good case. The next section is the other one.

03 When this calculator is wrong

Widen that same room to 14 ft × 15 ft (4.27 m × 4.57 m) — 210 ft² (19.51 m²), 23.33 yd² — and the shape of the answer changes. The room is now wider than a 12 ft roll in both directions, so it takes two roll-widths. The tighter layout runs strips the 14 ft direction: two strips of 14 ft, one full width and one cut to the remaining 3 ft, for 28 lin ft of 12 ft roll. That is 336 ft² (37.33 yd²), ordered as 38 yd², with one seam. The flat-area method gives 210 × 1.10 ÷ 9 = 26 yd² and would leave the job 12 yd² short.

The 9 ft × 14 ft piece left over from the second strip looks like free carpet. It is not usable in this room. Pile direction has to run the same way on every piece, and a piece rotated 90° reflects light as a different shade — pile reversal, also called shading. The offcut goes to a closet or the truck, not back into the floor. This is the constraint an area-only calculator cannot see: it treats carpet as a liquid that pours to fit, when it is a directional material milled to one width.

Two more places the number above needs a human:

• The roll width is a guess until your supplier confirms it. The order swings with it. That same 14 ft room on a 15 ft roll runs one 14 ft strip at the full width — 210 ft², no seam, zero fill. Pick the width the store actually stocks before trusting the seam count.
• It is one rectangle. Closets, L-shapes, and stairs are figured separately. A straight-run stair needs roughly 18 in of runner per step — the tread depth plus the riser height — and is usually cut from a single strip so the pile runs down the flight.
• Patterned carpet needs match length. A repeating pattern has to line up across every seam, which adds about one pattern repeat of length per drop. The calculator assumes plain or textured carpet with no repeat.

04 What to do with the result

Order the broadloom figure, in one cut, from one dye lot. Carpet is dyed in batches, and two batches of the same color rarely match exactly — a seam between dye lots shows. The cost of returning a remnant is small; the cost of running short and reordering into a different dye lot is a visible mismatch you live with. So when the roll order and the flat-area order disagree, trust the roll order: it is the one that knows the carpet comes in a fixed width.

Give your installer the room dimensions and the seam direction you want — seams run best toward the main window and along the traffic path, where the light and the footfall hide them. Let the supplier confirm the roll width they carry. A 15 ft roll can turn a two-seam room into one clean piece, and that is worth more than a few square yards either way.

05 Common questions

How many square yards of carpet do I need for a 12×15 room?

Off a 12 ft broadloom roll, 20 yd² — one 15 ft pull at the full 12 ft width with no seam (12 × 15 = 180 ft², ÷ 9 = 20). The flat 10% area method would call for 22 yd², two yards more than the roll needs.

How wide does carpet come?

Standard broadloom is 12 ft (3.66 m) wide. Some lines are also put up in 13'6" (4.11 m) and 15 ft (4.57 m). The width you can get decides how many seams a room needs, so it is the first thing to confirm.

Why is my carpet estimate more than the room's square footage?

Because the roll has a fixed width. A room one foot wider than the roll still buys a second full roll-width of length to fill that foot, and most of the second width becomes remnant. On a room wider than the roll in both directions, the purchased area can run well above the floor area — 38 yd² for a 210 ft² (23.33 yd²) room is normal.

Does carpet direction matter?

Yes. The pile (nap) must run the same way on every piece. A piece laid with the pile reversed reflects light differently and reads as a different shade, so fill pieces cannot be rotated 90° to save material. This is why carpet waste does not behave like tile or plank waste.

Which way should carpet seams run?

Along the length of the room, toward the primary source of natural light, and in the direction of the main traffic where the layout allows (CRI 105). A seam placed across the light and across traffic is both more visible and quicker to work loose.