How do I calculate square footage for carpet?

You do not, in the way the phrase suggests. Carpet is cut in drops off a roll of fixed width, so the order is whole strips across the room, each as long as the room and as wide as the roll. The room's area only enters as the thing you compare the result against. A 13 by 18 ft room is 234 square feet and takes 432 square feet of carpet off a 12 ft roll.

How do I measure a room for flooring?

Break it into rectangles, measure each at its widest point rather than across the doorway, measure to the wall behind the trim, and add the rectangles. Keep the two dimensions separate rather than collapsing them to an area, because the roll width, the plank width, the expansion gap and the maximum floating floor size all work on one dimension at a time.

How much extra flooring should I order?

The only allowance with a document behind it that could be found from here is Bruce's, which is 5 percent for a straight lay and 10 percent for a diagonal one. That same sheet says the industry standards permit grading deficiencies up to 5 percent and calls the 5 percent a cutting and grading allowance, so on a straight lay it is the same size as the defect rate it permits. A cut-up floor needs more, and no percentage can tell you how much.

Why is carpet waste so high?

Because carpet arrives in a fixed width and cannot be rotated. CRI 105 requires adjacent pieces of the same broadloom to have the same pile direction, and a strip turned sideways can show as a different shade. That means whole strips at full roll width, rounded up, with no turning leftovers into gaps.

What width does carpet come in?

Broadloom is commonly sold at 12 ft or 15 ft, with 13 ft 2 in as the metric 4 m option. Those are manufacturing sizes, not requirements of any installation standard. Which ones your supplier stocks changes the order more than anything about the room does.

How much carpet do I need for a 13 by 18 foot room?

Between 236 and 432 square feet, depending entirely on the roll. Off a 13 ft 2 in roll with the drops running the 18 ft length it is one drop, 18 linear feet, 236 square feet. Off a 12 ft roll the same way it is two drops, 36 linear feet and 432 square feet. The room is 234 square feet in both cases.

Should planks run the long way or the short way?

Bruce recommends running parallel to the longest wall for appearance, but says the floor should run perpendicular to the floor joists unless the subfloor has been reinforced, so check the joists first. What direction does change is the arithmetic: the width you divide by the plank width decides how many rows there are and whether the last one falls under its minimum, so run the layout both ways before choosing.

Do I measure under the baseboard?

Yes where you can reach it. The floor runs under the trim and the trim is removed to fit it, so the wall face is the real dimension. The expansion gap then comes off that, a minimum quarter inch for the hardwood sheet used here (half an inch floated) and 8 mm or more for the laminate one, before any dividing starts.

How to Measure a Floor for Flooring or Carpet

Square footage is the right answer for a plank floor and the wrong one for carpet. A carpet order is decided by where the room's width falls against the width of the roll, so the same 234 square foot room takes 236 square feet of carpet off one roll and 432 off another. Nothing about the room changes.

By Lucas Pradella, builder and editor. Checked 2026-09-16.

Measure in rectangles, and keep the two dimensions apart

Break the floor into rectangles, measure each one at its widest point rather than across the doorway, and add them up. Closets, alcoves and the strip under a bay are separate rectangles, not roundings. Measure to the wall behind the trim wherever you can reach it, because the floor runs under the baseboard and the baseboard comes off.

Then write both dimensions down separately and keep them. Nearly every constraint that follows is about a single dimension rather than an area: the roll width for carpet, the plank width for a row layout, the floor width for an expansion gap, the longest connected run for a floating floor. An area is the one form of the measurement from which none of those can be recovered.

Take the width in three places while you are there. A room is rarely square, and half an inch of difference between one end and the other changes a plank layout, as the worked figures further down show.

Subtract only what is permanently in the way: an island with a toe kick, a hearth, a full height cabinet run. Furniture does not come off, and neither does anything you could walk on.

Square footage answers this for planks and misses it for carpet

Plank and tile floors are made of units small enough to cut, turn and reuse, so the order is the area plus what the cuts cost, and the cuts are a modest percentage. Carpet is one enormous piece with a grain, delivered in a fixed width, and it cannot be turned.

That last point is a standard rather than a habit. CRI 105, the Carpet and Rug Institute's residential installation standard, requires the pile direction of adjacent pieces of the same broadloom to be the same unless otherwise specified. The standard states the rule rather than the reason. The practical effect is that a piece turned a quarter turn can show as a different shade beside its neighbor. A leftover strip that fits the gap only when turned is therefore not usable there.

What follows from that is a rounding up rather than a division. Each full-length strip unrolled and cut for the room is a drop, the strips sit edge to edge, and the number of them is the room dimension lying across the roll divided by the roll width, rounded up. You buy whole drops at the full roll width. The area of the room never enters the calculation at all.

Which roll it comes off decides the waste, not how big the room is

Broadloom is commonly made in 12 ft and 15 ft widths, and in 13 ft 2 in, the metric 4 m roll. The standard sets no width. They are product facts, and they are what your supplier happens to stock. Here is one 18 ft long room at seven different widths, ordered off each.

Our arithmetic: strips needed across the room (width over roll, always rounded to the next whole strip), each 18 ft long and a full roll wide. Waste is measured against the room's own area
Room width, 18 ft long	Off a 12 ft roll	Off a 13 ft 2 in roll	Off a 15 ft roll
10 ft, 180 sq ft	216 sq ft, 20% waste	236 sq ft, 31%	270 sq ft, 50%
11 ft, 198 sq ft	216 sq ft, 9%	236 sq ft, 19%	270 sq ft, 36%
12 ft, 216 sq ft	216 sq ft, 0%	236 sq ft, 9%	270 sq ft, 25%
13 ft, 234 sq ft	432 sq ft, 85%	236 sq ft, 1%	270 sq ft, 15%
14 ft, 252 sq ft	432 sq ft, 71%	472 sq ft, 88%	270 sq ft, 7%
15 ft, 270 sq ft	432 sq ft, 60%	472 sq ft, 75%	270 sq ft, 0%
16 ft, 288 sq ft	432 sq ft, 50%	472 sq ft, 64%	540 sq ft, 88%

Two things fall out of that grid, and neither appears on any carpet page found from here.

The waste belongs to the pairing, not to the room. A 13 ft room wastes 85 percent off a 12 ft roll and 1 percent off a 13 ft 2 in roll. Nothing about the room changed. It is not a large room or an awkward one, it is a room that happens to sit one foot past a roll width.

And a wider roll is not automatically the better roll. Read the 14 ft row across. The 13 ft 2 in roll wastes 88 percent there and the narrower 12 ft roll wastes 71. Just past a roll width is the worst place a room can be, and the wider the roll, the more that overshoot costs. Which is why the first question to a supplier is what widths they stock, not what the carpet costs per square foot.

Which way the drops run is a decision, and the standard has an opinion

A rectangular room can be carpeted in two orientations and they are rarely the same price. Take an 18 by 13 ft room off a 12 ft roll. Run the drops along the 18 ft length and 13 ft across needs two drops, 36 linear feet, 432 square feet. Run them along the 13 ft width instead and 18 ft across also needs two drops, but only 26 linear feet, 312 square feet. The second way is 120 square feet cheaper on identical carpet in an identical room.

Section 5.1 of CRI 105 asks that, when possible, seams run the length of the area, that main traffic run parallel to a seam rather than across it, that natural light not strike across it, and that seams not sit perpendicular to doorway openings. The cheaper orientation above puts its seam across the room, which is the arrangement those clauses steer away from.

That is a trade between material and appearance, and only the owner of the room can make it. Measure both directions so the choice is visible before the order goes in.

The same room, ordered six ways

Set the carpet orders beside a plank order for the identical floor.

Our arithmetic on published roll widths, the CRI 105 pile rule and the allowance printed in Bruce document 210049, for one 13 by 18 ft room of 234 sq ft
How the same room is ordered	What you buy	Against 234 sq ft
Carpet, 12 ft roll, drops along the 18 ft length	2 drops, 36 linear ft, 432 sq ft	plus 85%
Carpet, 12 ft roll, drops along the 13 ft width	2 drops, 26 linear ft, 312 sq ft	plus 33%
Carpet, 15 ft roll, drops along the 18 ft length	1 drop, 18 linear ft, 270 sq ft	plus 15%
Carpet, 13 ft 2 in roll, drops along the 18 ft length	1 drop, 18 linear ft, 236 sq ft	plus 1%
Engineered hardwood, straight lay	246 sq ft at the published 5%	plus 5%
Engineered hardwood, diagonal lay	257 sq ft at the published 10%	plus 10%

The four carpet rows span 196 square feet on one room, and every one of them is a correct answer to the question of how much carpet a 13 by 18 ft room needs. The two plank rows barely move, because a plank floor's waste is cutting and a carpet order's waste is rounding. That is the whole difference between the two trades in one table.

It also explains why a carpet quote and a carpet order are built on different areas. A published installed rate is per square foot of floor, and the roll you pay for is whatever the drops came to.

Plank floors: the gap comes off before anything is divided

Measuring for a plank floor starts by taking the expansion gap off both dimensions, and the gap is not one number across the trade.

Bruce's engineered hardwood instructions, document 210049, ask for a minimum 1/4 in (6 mm) expansion around all vertical obstructions, and 1/2 in (13 mm) for a floating floor. So the gap is measured off every fixed thing the floor meets, and on a tape that means noting where the casings, pipes and cabinet ends are, not only where the walls are.

Pergo's laminate guide publishes the same quantity as a rate instead: 1 mm per meter of floor width with a minimum of 8 mm, lowered to 5 mm in very low humidity regions and raised to 10 mm in high ones. At 8 m, about 26 feet, the rate and the minimum meet. A narrower floor gets 8 mm; a 40 ft wide open plan works out at just over 12 mm, and that is a reason to measure the full connected width rather than room by room.

The last row has a minimum width, and half an inch of room decides it

The usable width divided by the plank face width gives whole rows plus a leftover strip, and the manufacturers set a minimum for that strip. For a floating installation, Bruce asks you to plan the layout so the last row is never ripped narrower than 1 in (2.5 cm), and says plainly that this may require ripping the first row as well. Pergo sets the same rule for laminate at 5 cm.

Here is what that does to a real measurement. A 13 ft room, 5 in engineered planks, floated, so half an inch of gap each side: 155 in of usable width, 31 rows exactly, a full 5 in last row, nothing ripped. Now measure the same room at 13 ft and half an inch. Usable width 155.5, which is 31 whole rows with 0.5 in left over, which is less than the 1 in allowed. The sheet says this may require ripping the first row. Splitting the shortfall evenly across the two, which is our arithmetic rather than the sheet's, cuts the first and last rows each to 2.75 in and the count becomes 32.

Half an inch of room width, one extra row and two rips. That is why the width is worth measuring in three places and writing down to the nearest eighth, and it is the single strongest argument for measuring the room yourself rather than accepting a figure off a plan.

Whether the offcut starts the next row is published too

A plank cut to finish one row leaves a piece, and reusing that piece to open the next row is where a plank floor saves material. The end joint stagger decides if it is allowed, and the two manufacturers here set it at very different distances.

Bruce asks for end joints of adjacent rows to be staggered 4 in to 6 in. Pergo requires at least 30 cm, which is 11.8 in, and separately that the first and last panel of a row be more than 20 cm. Put those two Pergo numbers against a 48 in board, beside a row that starts with a full board, and the leftover can open a row only if it is at least 7.9 in long and no more than 36.2 in, the second figure being 48 less the 11.8 in stagger. That reading of the two rules as one test is this site's arithmetic, not Pergo's, and a different neighboring row moves the upper figure.

The Bruce sheet has a stagger but no shortest starting piece, so it sets only one end of that range. Check which instructions came with the floor before counting leftovers as savings.

The published allowance is 5 percent, and it covers grading as well as cutting

The Bruce sheet gives its overage figure and explains it in one breath. The floors are made to standards which permit grading deficiencies not to exceed 5%, and 5% must be added to the actual square footage needed for cutting and grading allowance, 10% for diagonal installations.

The two figures are the same number, and that is the point. Up to one board in twenty may be below grade, and the order allowance, which the sheet calls a cutting and grading allowance, is one board in twenty. If every board the grading permits is set aside, a straight lay's 5% has little or nothing left for cuts. That reading is this site's, not Bruce's.

For measuring, that turns into a practical rule. A simple rectangle laid straight can be ordered at the published figure. Every bay, doorway, hearth and stair nosing on your sketch is cutting the percentage was never sized for, and the flooring calculator here works that out from the room instead. A flat ten percent for a straight lay has no document behind it that could be found from here.

One measurement decides whether the floor can be laid in one piece

A floating floor is a single connected sheet that expands and contracts as a unit, and there is a published maximum size for it. Pergo's rigid click instructions give 13 m by 13 m for standard residential use at 15 to 35 C, 8 m by 8 m where the range runs 5 to 65 C, and 30 m by 30 m with an HVAC system. Thirteen meters is 42.7 feet.

An open kitchen and family room can run past that. What counts is the continuous floor, measured through every opening it runs under without a break. Pergo's laminate guide, which gives the same 13 by 13 m maximum, makes intermediate T-profiles mandatory for irregular L, U and T shaped rooms, where heating and cooling zones are separate, where the maximum installation size is exceeded, and in every doorway.

So the longest continuous run is a figure to take while the tape is already out, even though nothing on a flooring order asks for it.

What to write down before you go to the yard

Each rectangle's length and width, measured at the widest point, to the nearest eighth of an inch.
The total area, which is what a plank or tile order and every published price are built on.
Each dimension on its own, because a carpet order turns on whichever one lies across the roll.
The longest connected run through every doorway, for a floating floor.
Which way you want the planks or the pile to run, since it changes the order and not only the look.
The roll widths your supplier actually stocks, before deciding what carpeting the room costs.
The plank width and length off the carton, which is the face width that covers floor rather than the nominal size on the label.

What this guide does not decide

Stairs. Treads and risers are measured and ordered separately from any floor, and are not covered here.

The subfloor. Nothing here says whether what you are measuring is flat enough, dry enough or sound enough to receive a floor.

Which document governs your floor. Two manufacturers appear on this page and they disagree about the gap, the stagger and whether the first piece of a row has a minimum length. The sheet in your box wins every time.

The price. This site publishes none of its own. This Old House puts installed carpet at $3 to $11 per square foot, with labor alone at $0.50 to $1, padding at $0.75 to $2 and removal of the old carpet at $2 to $3, all per square foot of floor. Last price check: 2026-09-08. Its general flooring guide gives solid hardwood at $3 to $10, laminate at $4 to $20 and vinyl at $7 to $22 per square foot. Last price check: 2026-09-09.

Run the numbers

Frequently asked questions

How do I calculate square footage for carpet?: You do not, in the way the phrase suggests. Carpet is cut in drops off a roll of fixed width, so the order is whole strips across the room, each as long as the room and as wide as the roll. The room's area only enters as the thing you compare the result against. A 13 by 18 ft room is 234 square feet and takes 432 square feet of carpet off a 12 ft roll.
How do I measure a room for flooring?: Break it into rectangles, measure each at its widest point rather than across the doorway, measure to the wall behind the trim, and add the rectangles. Keep the two dimensions separate rather than collapsing them to an area, because the roll width, the plank width, the expansion gap and the maximum floating floor size all work on one dimension at a time.
How much extra flooring should I order?: The only allowance with a document behind it that could be found from here is Bruce's, which is 5 percent for a straight lay and 10 percent for a diagonal one. That same sheet says the industry standards permit grading deficiencies up to 5 percent and calls the 5 percent a cutting and grading allowance, so on a straight lay it is the same size as the defect rate it permits. A cut-up floor needs more, and no percentage can tell you how much.
Why is carpet waste so high?: Because carpet arrives in a fixed width and cannot be rotated. CRI 105 requires adjacent pieces of the same broadloom to have the same pile direction, and a strip turned sideways can show as a different shade. That means whole strips at full roll width, rounded up, with no turning leftovers into gaps.
What width does carpet come in?: Broadloom is commonly sold at 12 ft or 15 ft, with 13 ft 2 in as the metric 4 m option. Those are manufacturing sizes, not requirements of any installation standard. Which ones your supplier stocks changes the order more than anything about the room does.
How much carpet do I need for a 13 by 18 foot room?: Between 236 and 432 square feet, depending entirely on the roll. Off a 13 ft 2 in roll with the drops running the 18 ft length it is one drop, 18 linear feet, 236 square feet. Off a 12 ft roll the same way it is two drops, 36 linear feet and 432 square feet. The room is 234 square feet in both cases.
Should planks run the long way or the short way?: Bruce recommends running parallel to the longest wall for appearance, but says the floor should run perpendicular to the floor joists unless the subfloor has been reinforced, so check the joists first. What direction does change is the arithmetic: the width you divide by the plank width decides how many rows there are and whether the last one falls under its minimum, so run the layout both ways before choosing.
Do I measure under the baseboard?: Yes where you can reach it. The floor runs under the trim and the trim is removed to fit it, so the wall face is the real dimension. The expansion gap then comes off that, a minimum quarter inch for the hardwood sheet used here (half an inch floated) and 8 mm or more for the laminate one, before any dividing starts.

Check these numbers yourself

CRI 105, Standard for Installation of Residential CarpetThe Carpet and Rug Institute, 2019. Section 5.2 requires uniform pile direction between adjacent pieces of the same broadloom, which is what makes an offcut unusable when rotated. Section 5.1 sets where seams should go when possible. Published free by the trade association that writes it, so it can be cited precisely rather than paraphrased. Read 2026-09-08.
Bruce (AHF Products), Engineered Hardwood installation instructionsDocument 210049. Source of the 5% grading and cutting allowance and the sentence explaining what the 5% is for, the 1 in minimum last row for floating floors and the instruction to rip the first row with it, the 4 to 6 in end joint stagger, the 1/4 in minimum expansion space around all vertical obstructions (1/2 in for floating floors), and the direction recommendation relative to the longest wall and the joists. Read 2026-09-09.
Pergo, Installation Guide LAMINATE FLOORINGVersion 07.2026. Source of the 1 mm per meter expansion formula and its 8 mm minimum, the 5 cm minimum first and last row, the 20 cm minimum first and last panel, the 30 cm minimum end joint stagger the 13 by 13 m maximum floating installation size and the T-profile conditions. Read 2026-09-09.
Pergo, Installation instructions VINYL PLANKS & TILES (Rigid Click)Version 11.2025. Source of the maximum installation size that varies with temperature range: 13 by 13 m for standard residential use, 8 by 8 m at 5 to 65 C, 30 by 30 m with an HVAC system. Read 2026-09-09.
This Old House, carpet installation costWritten by Stephanie Minasian-Koncewicz, published 8 June 2026. Source of the carpet figures on this page. Priced per square foot of floor, which is a different area from the one a carpet order is built on. Read 2026-09-08.
This Old House, flooring costWritten by Stephanie Minasian-Koncewicz, updated 5 June 2026. Source of the per square foot ranges for hardwood, laminate and vinyl quoted at the end of this page. Read 2026-09-09.

Figures here last checked against the source documents on 2026-09-16. Codes are amended locally; confirm against the edition your jurisdiction enforces.

About the author

Lucas Pradella builds and maintains Nails & Numbers. He is not a tradesperson: every figure on the site is traced to a named, dated document, and the page links it so you can check it yourself. More about how these pages are made.

Every guide on this site: Guides.