Carpet Calculator

How to use the carpet calculator

Measure the room wall to wall at its longest and widest points and round each up to the next half foot. Include any doorway the carpet runs through and any alcove getting the same floor, because those are cut out of the same run and they change the length you have to pull off the roll.

Then pick the roll width. This is the input that makes carpet different from every other floor covering on this site. Broadloom is manufactured in fixed widths, so the roll is not cut to your room: your room is cut out of the roll, and any width you do not use is bought and paid for anyway.

There are two ways to run the carpet and the calculator takes the cheaper one. On the default room the runs cross the 20 ft length, so each run is 14 ft plus trim rather than 20 ft plus trim. Turn the same room the other way and it still takes 2 runs, but each one is longer, and that difference is real money.

A pattern repeat raises the number again. To make the pattern line up across a seam, each run has to be cut to a whole number of repeats, so an 18 in repeat rounds the default room 14.7 ft cut up to 15 ft. That is 360 square feet instead of 352.8, and the bigger the repeat the bigger the jump.

The Advanced panel holds the trim allowance and the pattern repeat. Trim covers doorways, out of square walls and the odd miscut, and it is added to each run before the pattern rounding, because rounding to a whole repeat has to be the last thing that happens to the cut length.

Why carpet gets quoted per square yard

Carpet is one of the last US trades still quoting per square yard while the material itself is sold and installed per square foot. A square yard is 3 ft by 3 ft, so it is 9 square feet, and that is where the mistake lives: people reach for 3 because a yard is 3 feet, and land a third of the way to the right answer.

The practical version: when a quote and a product page disagree by roughly a factor of nine, nobody is lying to you, the two numbers are in different units. Convert both to the same unit before comparing, and do it on the area coming off the roll rather than on the floor area, because the roll is what gets invoiced.

Where the seam lands, and why the roll width decides it

A seam is not a failure, it is arithmetic. If the room is wider than the roll in both directions, no single strip reaches, so the floor takes two runs and they have to meet somewhere. The calculator puts that meeting point exactly one roll width in from the wall the first run starts at, which is where an installer would put it too.

That gives you one decision worth making before anything is cut: which wall the first run starts from. Start it at the window wall and the seam lands 12 ft into the room, in the light, where it is most visible. Start it at the opposite wall and the same seam sits under a sofa or in the traffic lane nobody looks at.

Roll width is also worth shopping. On the default 20 by 14 ft room a 15 ft roll buys 35 square yards seamless while a 12 ft roll buys 39.2 with a seam, so the wider roll wins twice. Flip to a 16 by 12 ft room and it reverses: 21.33 square yards off the 12 ft roll against 26.67 off the 15 ft, because the room already fits the narrow roll exactly.

The formula, worked line by line

The floor area is the easy half. Length times width gives square feet, and dividing by 9 gives square yards, since a square yard is 3 ft by 3 ft. Everything interesting happens after that, when a rectangular room has to be cut out of a roll that comes in one fixed width and unlimited length.

Divide the room span you are crossing by the roll width and round up, and you have the number of runs. Round up rather than down because half a run does not exist: a room 8 ft into its second roll width still needs that whole second width delivered, and you pay for all 12 ft of it.

Everything else falls out of those runs. The offcut is the roll width you bought less the room span you covered, the seams are one fewer than the runs, and the first seam sits one roll width in from the wall the first run starts at.

floor area = length ft × width ft
square yards = square feet ÷ 9
runs = room span ÷ roll width, rounded UP to a whole run
cut length = run length × (1 + trim% ÷ 100)
with a pattern: cut length rounded UP to a whole repeat
carpet off the roll = runs × cut length × roll width
offcut width = runs × roll width - room span
seams = runs - 1, the first one a roll width from the wall

The pattern repeat, when there is one, is applied last. Trim goes on first because it is a percentage of the run, then the result rounds up to the next whole repeat, because the pattern only lines up across a seam if every run holds a whole number of repeats.

An 18 in repeat turns the 14.7 ft cut into 15 ft and the order into 360 square feet, 40 square yards. Larger repeats jump further, which is why a bold pattern in a big room is a genuinely more expensive floor than the same carpet in plain.

What this figure does not include: underlay, gripper rod, door bars, and fitting. Underlay is sold in rolls with its own width and its own coverage and should be sized off the floor area rather than the roll area, because it does not need to match a pile direction or a pattern.

For an irregular room, split it into rectangles, run the calculator on each, and check whether the offcut from the large rectangle can supply the small one with the pile running the same way. That is the judgement an installer brings, and it is worth a measure and a quote on anything larger than a single room.

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