Carpet is sold by the length, not the area
A roll is a fixed width — usually 12 feet — and you cover a room in
full-width strips cut off it, called drops. So the answer is a length of
roll, and the strip left over down one side is bought whether you use it
or not. Dividing the floor area by nine and calling it square yards
under-orders every room that is not exactly a roll width wide.
drops = ⌈room dimension ÷ roll width⌉
each drop = the other dimension + trim
linear ft = drops × drop length
20 × 14 ft room, 12 ft roll, 3 in trim
drops along the 20 ft 2 × 20.25 = 40.5 ft
drops along the 14 ft 2 × 14.25 = 28.5 ft ← cheaper
Same room. 144 ft² of difference.
Which way you lay it is the biggest lever
It is worth saying plainly, because no calculator that starts from an
area can tell you: turning the layout ninety degrees changes what you buy.
On the example above it is 144 square feet — real money on
a real living room — and on some rooms it is the difference between two
drops and three. This page works both and leads with the cheaper one.
Common rooms off a 12 ft roll
From this calculator with a 3 in trim allowance on each drop. The last column is what the worse orientation would cost you in extra carpet.
Every drop runs the same way
Carpet has a pile direction. Two drops laid against each other catch the
light differently and read as two different colours in the same room —
obvious in daylight and not fixable afterwards. So you cannot cleverly
rotate one strip to fill a gap, and the choice of orientation is made
once for the whole room. That constraint is exactly what makes the
layout worth calculating.
The offcut is the shape of the problem
On the worked example the offcut is 62 square feet, or
22% on top of the room. That is not careless
measuring — it is what happens when a room is not a whole number of roll
widths wide. It is also why a room 12 feet wide or under is dramatically
cheaper to carpet per square foot than one 13 feet wide, and worth knowing
before you compare two quotes on price per yard.
Patterns cost more than their price
A pattern repeat rounds every drop up to a whole repeat, because two
drops seamed together have to show a full pattern — the manufacturer's
instruction is to always cut on the pattern. On a large repeat that adds
to every drop, so a patterned carpet installed costs more than its per
yard price against a plain one. Enter the repeat and the tool carries it.
Not in this figure
- Closets and doorways. Measure them as their own rectangles; carpet has to run through an opening, not stop at it.
- Stairs. Tread plus riser plus a nosing allowance per step, and a flight takes far more than its plan area.
- Underlay, gripper and seam sealer. Separate orders. Seam sealer is required on every seam and transition, without exception.
The rest of the room
If the floor is going hard rather than soft, the
tile calculator counts tiles with
a waste factor that follows the lay pattern and the
thinset calculator reads the
mortar off the manufacturer's trowel chart. Overhead, the
ceiling tile calculator
counts the grid as well as the panels, and around the edges the
crown molding calculator
turns a perimeter into whole lengths of stock. For everything in one
room at once, the
room materials calculator
takes a single set of dimensions and gives you the whole list.
Common questions
How do you calculate carpet?
Not by dividing the area by nine. Carpet comes off a roll of fixed width — 12 feet on most broadloom — so you cover a room in full-width strips called drops, and what you buy is a length of roll. A 20 by 14 foot room takes 2 drops of 14.25 feet, which is 28.5 linear feet or 38 square yards — noticeably more than the 31.11 square yards of floor.
Does it matter which way the carpet runs?
Often by a lot. On the worked example, running the drops along the 20 foot dimension takes 40.5 linear feet and running them the other way takes 28.5 — 144 square feet of difference for the same room, the same roll and the same carpet. This calculator works both and leads with the cheaper. It is the single biggest saving available on a carpet order and no area-based estimate can see it.
Why is there so much waste?
Because a room is almost never a whole number of roll widths wide. The leftover strip down one side is bought whether it is used or not: on the worked example the offcut is 62 square feet, 22 per cent on top of the room. That is not sloppy measuring, it is the shape of the problem — and it is why a room 12 feet wide or less is dramatically cheaper to carpet per square foot than one 13 feet wide.
How wide is a roll of carpet?
Twelve feet is the usual broadloom width, and the manufacturer whose installation guide this page cites publishes 12 ft and 12 ft 6 in. Fifteen foot rolls are sold in some residential markets and six foot rolls in some commercial ones. Because widths genuinely vary, the field here is editable rather than a fixed list — enter what your roll actually is, since the whole calculation hangs off it.
Can I lay the drops in different directions to save carpet?
No, and it is the mistake that ruins a job that was otherwise fine. Carpet has a pile direction: two drops laid against each other reflect light differently and look like two different colours in the same room, which is obvious in daylight and impossible to fix. Every drop runs the same way. That constraint is exactly why the layout matters and why the saving comes from choosing one orientation for the whole room rather than mixing.
What does a pattern repeat do to the quantity?
It rounds every drop up to a whole repeat, because two drops seamed together have to show a full pattern — the manufacturer's instruction is to always cut on the pattern. On a large repeat that can add a foot or more to every single drop, which is most of why patterned carpet costs more per square yard installed than its price per yard suggests. Enter the repeat and the tool accounts for it.
Where should the seams go?
Out of the main traffic path and, if you can, running away from the main light source rather than across it — a seam lit along its length is far more visible. This calculator tells you how many you will have (1 on the worked example) but not where to put them, because that depends on the door, the window and where the sofa goes. What is not optional is seam sealer: the guide cited here requires it on every seam and transition, and says so with the words "No exceptions".
What about closets, doorways and stairs?
Not included, and they should be measured as their own rectangles. A doorway needs the carpet to run through the opening; a closet is a small room with its own drop. Stairs are a different calculation entirely — each step takes the tread depth plus the riser height plus a nosing allowance, and a flight can eat several square yards more than its plan area suggests. Add them separately rather than hoping the offcut covers them.
Sources
Only the roll widths one manufacturer publishes are stored. Fifteen foot broadloom is sold in some residential markets and six foot in some commercial ones, so the width is an editable field rather than a closed list — the point of the calculator is what a fixed roll width does to the cutting layout, not which widths exist. No waste factor is stored: carpet waste is not a percentage, it is whatever falls off the end of the drops once the room is laid out against the roll, and computing it is the job the page exists to do.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
