Carpet is not sold by area, it is cut in drops off a roll of fixed width, and it cannot be rotated. CRI 105 requires every adjacent piece to run the same way, because carpet laid at ninety degrees reads as a different colour. So a 12 ft room off a 12 ft roll wastes nothing and a room one inch wider wastes 98%, and there is nothing in between.
Carpet by the drop
ft
ft
ft
%
no.
in
in
ft
Both layout directions are shown, because the one that uses less material is not always the one CRI 105 lets you use.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
The drops laid over the room, both directions
Worked example
An 18 ft by 13 ft room off a 12 ft roll, with a 5% trimming allowance.
The room. 18 × 13 = 234 sq ft.
Option A, drops running the 18 ft length. 13 ft across a 12 ft roll is 2 drops, 36 linear ft, 432 sq ft bought, 85% waste.
Option B, drops running the 13 ft width. 18 ft across a 12 ft roll is also 2 drops, 26 linear ft, 312 sq ft bought, 33% waste.
Cheaper on material. Option B, by 120 sq ft.
What CRI 105 5.1 prefers. Option A, because seams should run the length of the area.
They disagree. Following the standard costs 120 sq ft on this room.
That disagreement is the real content of a carpet estimate and no calculator surfaces it. Option B is a third cheaper and puts a seam across the room, running the short way, which is precisely what 5.1 asks you to avoid: main traffic crosses it rather than running along it. Whether the saving is worth a seam you will see for twenty years is a decision, and it should be made deliberately rather than by whichever number a calculator happened to print.
The formula
Rounding up, and everything follows from it:
drops = ⌈across / R⌉ linear = drops × along bought = linear × R
R
roll width, ft. A product fact, not a standard
across
the room dimension the drops sit side by side across
along
the other dimension, which each drop has to be long enough to cover
⌈ ⌉
rounded UP. This is where the cliff is
bought
always a whole number of drops times the full roll width. You cannot buy half a width
Note that the area you buy never depends on the room area at all. It is the roll width times the linear feet, and the room only enters through the rounding. That is why carpet waste is discontinuous while tile waste is smooth.
Why the waste looks like a mistake and is not
A tile estimate wastes 10%. A carpet estimate for the same room can waste 85%, and every calculator that reports it gets accused of being broken.
The difference is that tile is a small unit that can be cut and turned, and carpet is one enormous piece with a grain. CRI 105 section 5.2 makes the grain a requirement rather than a preference: adjacent pieces of the same broadloom must run the same way, because pile reflects light differently along its lay and two pieces at right angles read as two colours.
So the offcut cannot be rotated into the gap. It is scrap, and the arithmetic is a ceiling function rather than a division.
Section 5.1 then removes some of the layouts that would have saved material anyway, by asking that seams run the length of the area, that traffic runs along rather than across them, and that they do not cross doorways or sit where light rakes over them. The calculator shows both directions and tells you when the cheap one is the one the standard advises against.
A 15 ft long room off a 12 ft roll, as the width grows
Room width
Drops off a 12 ft roll
Linear carpet
Area bought
Area needed
Waste
10 ft
1
15 ft
180 sq ft
150 sq ft
20%
11 ft
1
15 ft
180 sq ft
165 sq ft
9%
11.9 ft
1
15 ft
180 sq ft
178 sq ft
1%
12 ft
1
15 ft
180 sq ft
180 sq ft
0%
12.1 ft
2
30 ft
360 sq ft
182 sq ft
98%
13 ft
2
30 ft
360 sq ft
195 sq ft
85%
15 ft
2
30 ft
360 sq ft
225 sq ft
60%
20 ft
2
30 ft
360 sq ft
300 sq ft
20%
24 ft
2
30 ft
360 sq ft
360 sq ft
0%
24.1 ft
3
45 ft
540 sq ft
362 sq ft
49%
Read 2026-09-08. Drops are the room width divided by the roll width, rounded up, which is where the cliff comes from.
Carpet arrives in a roll of fixed width and is cut into drops, lengths run off the roll side by side. The number of drops is the room width divided by the roll width, rounded up, and that rounding is the whole story:
A 15 ft long room off a 12 ft roll, as the width grows
Room width
Drops off a 12 ft roll
Linear carpet
Area bought
Area needed
Waste
10 ft
1
15 ft
180 sq ft
150 sq ft
20%
11 ft
1
15 ft
180 sq ft
165 sq ft
9%
11.9 ft
1
15 ft
180 sq ft
178 sq ft
1%
12 ft
1
15 ft
180 sq ft
180 sq ft
0%
12.1 ft
2
30 ft
360 sq ft
182 sq ft
98%
13 ft
2
30 ft
360 sq ft
195 sq ft
85%
15 ft
2
30 ft
360 sq ft
225 sq ft
60%
20 ft
2
30 ft
360 sq ft
300 sq ft
20%
24 ft
2
30 ft
360 sq ft
360 sq ft
0%
24.1 ft
3
45 ft
540 sq ft
362 sq ft
49%
Look at the two rows either side of 12 ft. At 12.0 the waste is zero. At 12.1, one inch wider, it is 98%: you buy two full drops and throw away most of the second.
That is not inefficiency, it is the geometry of a fixed width. And it is why the single most useful thing you can know before ordering is whether your room is under or over the roll width, which is a tape measure and thirty seconds.
It is also why a 15 ft roll is worth asking about. A 13 ft room off a 12 ft roll is two drops and 85% waste. Off a 15 ft roll it is one drop and 15%.
Why you cannot just turn the offcut round
The obvious saving is to take the leftover strip, rotate it ninety degrees and fill the gap. On tile that works. On carpet it is prohibited, and the standard says so directly:
Where two or more pieces of the same broadloom carpet are adjacent, the pile direction is required to be the same unless otherwise specified. Uniform pile direction is not required with dissimilar carpet. NOTE: Ideally, install carpet with the pile lay toward the entrance; but other factors, such as pattern, aesthetics and economic use of material may be considered.
Carpet pile has a lay, the direction the fibres lean. Light reflects off the tips one way and off the sides the other, so two pieces of identical carpet laid at right angles read as two different shades. It is not subtle, it does not settle down, and it cannot be fixed after the fact.
So every piece in a continuous area comes off the roll the same way, and an offcut that would fit if you turned it is scrap. That constraint, not carelessness, is why carpet waste runs at a level that would be scandalous in any other trade.
The note at the end of 5.2 is worth reading too: the standard's preference is pile lay toward the entrance, which is a decision about how the room looks when you walk into it, and it can conflict with the layout that uses the least material.
Measure the width first, and measure it properly
The one measurement that decides the whole order is the room dimension that sits across the roll. Everything else is linear feet.
Measure it at its widest point, including into doorways and any alcove the carpet has to reach, and add the amount that tucks under the tack strip at each side. A room that measures 11 ft 11 in wall to wall is not automatically one drop off a 12 ft roll once the tuck is included.
And measure it in more than one place. A room out of square by two inches over its length is common in an older house, and carpet has to cover the widest point, not the average.
If the number lands within a few inches over a roll width, that is the moment to ring round for a wider roll. It is the difference between one drop and two.
Fifteen foot rolls, and when to insist
Twelve feet is the common broadloom width and it is what most stock is. Fifteen exists, is widely available, and is worth asking for specifically because nobody offers it unprompted.
On a 13 ft room the difference is stark. Off a 12 ft roll that is two drops and 85% waste. Off a 15 ft roll it is one drop and 15%. The 15 ft carpet may cost more per square yard and still be far cheaper for the room, because you are buying half as much of it.
There is also 13 ft 2 in, which is four metres, and it turns up on imported goods. It covers a 13 ft room in one drop where a 12 ft roll cannot.
The question to ask a supplier is not what carpet they have. It is what widths that carpet comes in.
Stairs are a separate problem
Stair carpet is measured as run, not area: each step is a tread plus a riser, so roughly 17 to 18 inches of carpet per step, times the number of steps, at the stair width.
The complication is direction again. CRI 105 notes that pile should run down the stairs, which fixes the orientation of every piece and means stair carpet cannot be cut sideways out of the offcuts from a room. It comes off the roll the long way like everything else.
A winder or a landing makes it worse, because the pieces are no longer rectangles and each one still has to have its pile pointing the same way.
The calculator gives the linear run for straight steps and stops there, because anything with a turn in it is a template job rather than an arithmetic one.
And where the seams are allowed to go
Section 5.1 constrains the layout further:
Keep seams to a minimum. When possible, position seams so that: Seams should run the length of the area. Main traffic should run parallel to, rather than across, the seam. Natural light does not strike across the seam. Seams are away from areas subjected to pivoting traffic. Seams are not perpendicular to doorway openings.
Read them as a list of layouts you cannot use. Seams run the length of the area, so turning the drops to run the short way, which would often save material, is out. Not perpendicular to doorway openings, because a seam crossing a threshold is walked over at right angles by everyone who uses the room, which is exactly how a seam fails.
Natural light does not strike across the seam is the one people find surprising, and it is the same physics as pile direction: raking light across a seam shows it, and a seam under a window will be visible in a way the same seam elsewhere is not.
The calculator works out drops both ways and shows you both, because the cheaper direction is not always a direction you are allowed to use.
The standard
Read 2026-09-08. CRI 105, Standard for Installation of Residential Carpet, The Carpet and Rug Institute, 2019. Sections 5.1 and 5.2, quoted in full on the page. Published free by the trade association that writes it, which is why it can be cited precisely rather than paraphrased.
Roll width is a product fact, not a standard. Twelve feet is the common broadloom width, fifteen is widely available and worth asking for, and 13 ft 2 in exists because it is four metres. None of those are in CRI 105 and the calculator lets you type whatever your supplier actually has, because that is the number that decides the order.
Pad, tack strip, transitions and stair work are not quantified here. Stairs in particular are their own problem: the standard notes that pile should run down the stairs, which fixes the direction and therefore the cutting.
Frequently asked questions
How much carpet do I need for a 13x18 room?
Off a 12 ft roll, two drops. Running them the 18 ft length gives 36 linear ft and 432 sq ft for a 234 sq ft room, which is 85% waste. Running them the 13 ft way gives 312 sq ft and 33% waste, but puts the seam across the room, which CRI 105 5.1 advises against. Off a 15 ft roll it is one drop and about 270 sq ft.
Why is there so much carpet waste?
Because carpet cannot be rotated. CRI 105 section 5.2 requires adjacent pieces of the same broadloom to have the same pile direction, since pile reflects light along its lay and two pieces at right angles read as different colours. So an offcut that would fit if you turned it is scrap, and the arithmetic rounds up to whole drops of the full roll width.
What width does carpet come in?
Twelve feet is the common broadloom width. Fifteen feet is widely available and worth asking for specifically. Thirteen feet two inches also exists, because it is four metres. Roll width is a product fact rather than a standard, so ask your supplier what widths the carpet you want actually comes in, since it decides the order more than the price per yard does.
Which way should carpet seams run?
Along the length of the area, per CRI 105 5.1, with main traffic running parallel to the seam rather than across it. The standard also asks that seams are not perpendicular to doorway openings, are away from pivoting traffic, and are not where natural light strikes across them, because raking light shows a seam that would otherwise be invisible.
Can I use the leftover carpet in another part of the room?
Only if it runs the same way. Uniform pile direction is required between adjacent pieces of the same carpet, so a strip turned ninety degrees will read as a different shade no matter how well it is seamed. Leftovers can be used in a separate room, where nothing adjacent shows the difference.
How much carpet do I need for stairs?
About a tread plus a riser per step, so 17 to 18 inches each for a typical stair, times the number of steps, at the stair width. It has to come off the roll with the pile running down the stairs, so it cannot be cut sideways from room offcuts.
Is a wider roll always better?
Only if your room needs it. A 15 ft roll in an 11 ft room wastes more than a 12 ft roll would. The question is which roll width your specific room fits under, and the answer flips completely at the roll width rather than gradually.
Check these numbers yourself
CRI 105, Standard for Installation of Residential CarpetThe Carpet and Rug Institute, 2019. Sections 5.1 and 5.2, quoted in full on the page. Published free by the trade association that writes it, which is why it can be cited precisely rather than paraphrased.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Roll width decides everything and it is not a standard. Ask your supplier what widths the carpet you want comes in. CRI 105 was read on 8 September 2026. Pad, tack strip, transitions and any stair with a turn in it are not quantified here, and a patterned carpet needs a pattern repeat allowance on top of everything on this page.