Carpet Calculator: How Much Carpet Do I Need?

How to measure a room for carpet

1. Measure the length and width wall to wall at the widest points, to the nearest inch, and convert to decimal feet (11 ft 6 in is 11.5 ft; see our square footage calculator). Replacing the baseboard too? The baseboard calculator uses the same length and width.
2. Treat closets and alcoves as separate rectangles and run each one. That overestimates a little, since an installer may cut a closet from offcuts.
3. Decide which way the seams should run (see below) and set the direction if needed.
4. Check the roll width of your style, and try both 12 and 15 ft.

Carpet for stairs: length per step

About 17.75 in of carpet per step at the International Residential Code (IRC) limits of a 10 in minimum tread and a 7 3/4 in maximum riser (City of Grinnell's stair handout, 2015 IRC). Use The Home Depot's stair method: multiply the tread depth by the number of treads and the riser height by the number of risers, add the two, and add 6 in for waste. Count the top riser too: Home Depot's example stair has 13 treads and 14 risers.

36 in is the narrowest stairway the same handout allows; for a wider stair, multiply the length by its width in feet. Over a separate pad, CRI 105 says to "extend the cushion over the stair nose," and it recommends that the carpet's machine direction run "in the length direction of the stairs," noting that most manufacturers want the pile to run down the stairs. Where the carpet wraps under the nose (cap-and-band), measure one step with a flexible tape along the path the carpet takes. The calculator covers rooms only, so add the stairs to its result yourself.

How the calculator lays out carpet

It follows the Carpet and Rug Institute's residential standard, CRI 105, and The Home Depot's installation guide:

• Full-length pieces, pile the same way. CRI 105 says: "Where two or more pieces of the same broadloom carpet are adjacent, the pile direction is required to be the same unless otherwise specified." Home Depot's guide says the same. A leftover strip can't be turned sideways to fill a gap, so every piece runs the full length.
• Trim at every wall. CRI 105 asks for "at least 3 inches along each wall for trimming," and Home Depot says to measure along the longest wall and "Add 6 inches to the measurement." Each piece is cut 6 in longer than the room, and the pieces together span the other dimension plus 6 in.
• Roll widths. Shaw Floors' catalog filters carpet by 12 ft and 15 ft widths, and some styles, such as Awaken Spirit II, come in both. Home Depot sells TrafficMaster Viking "12 ft. Wide x Cut to Length."
• Direction. It takes whichever way orders less carpet and shows the other total, so you can choose by seam placement. It also says when the other roll width needs fewer pieces, since CRI 105 says to "Keep seams to a minimum."

So you order more carpet than floor. The calculator doesn't plan fill pieces or cross seams (none of these sources give rules for them), so an installer's seam plan can use less.

Carpet by room size: 12 ft vs 15 ft roll

Square yards to order (pieces x piece length in ft) with 3 in trim at each wall, no extra, and the direction that uses less:

Past 11 ft 6 in wide, a 12 ft roll needs a second piece. At 18 x 12, one 18.5 ft piece of 15 ft carpet still beats two 12 ft pieces; at 20 x 16, both need two pieces and the 12 ft roll wins, so wider isn't always better.

Carpet roll sizes: widths and lengths

12, 13.5, or 15 ft wide, and usually about 100 ft long. The Carpet and Rug Institute says "Broadloom carpet can be 12 feet, 13.5 feet, or 15 feet in width," and Stanton's carpet glossary describes a full roll as "usually approximately 100 feet long," adding that shipping rolls "may be as short as 30 feet." The store cuts your piece off the roll, so the width is fixed and you order the length in linear feet:

The widest room leaves 3 in to trim at each wall. The calculator offers the 12 and 15 ft widths; for a 13 ft 6 in roll, use the formula above with 13.5 as the roll width. A 12 x 12 ft room spans 12.5 ft with trim, so it takes one 13.5 ft wide piece 12.5 ft long: 168.75 sq ft, or 18.75 sq yd, against 20.83 sq yd off a 15 ft roll and 33.33 off a 12 ft roll. Check which widths your style comes in before you plan the seams.

Where the seams go

Seams run the same way as the pieces, so the direction setting is also the seam direction. CRI 105 says to "Keep seams to a minimum" and, when possible, to position them 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."

The calculator can't see your doors, windows, or traffic paths, so set the direction yourself if its seam lands badly.

Square yards or square feet?

Home Depot's guide says "Carpet is measured in square yards. One square yard is 9 square feet," so divide square feet by 9. But Shaw lists carpet per square foot, and NIST Handbook 130's model method-of-sale rule has the seller state "The amount delivered (exact size shipped)" and use the square foot in U.S. customary units. The calculator shows both, plus linear feet. Laminate, vinyl plank, and wood floors are sold by the box instead; see the flooring calculator.

Square feet to square yards and linear feet chart

9 sq ft make a square yard, so divide square feet by 9; divide by the roll width for linear feet off the roll. Use the chart to convert an order or a quote between units, not to size a room: a room needs full-width pieces plus trim, as in the room-size chart above.

Home Depot's guide gives the same conversion: "90 square feet/9 = 10 square yards." For a 13 ft 6 in roll, divide square feet by 13.5.

How much carpet pad do you need?

Pad area is the floor area: 154 sq ft (17.11 sq yd) in the default room. Home Depot says "The carpet pad shouldn't be placed over the tack strips," so the full floor area runs slightly high. If the subfloor under it needs replacing, the plywood calculator counts 4 x 8 sheets for the same floor.

• Seams. Home Depot says to position the pad "so that any seams will be at right angles to the seams in the carpet." CRI 105 also allows an offset of at least 6 in.
• Lengths. CRI 105 says to install cushion "in the longest continuous lengths possible."
• Thickness. CRI 105: "Cushion thickness for Residential carpet installations should not exceed 7/16 inch (11 mm)."

How much extra carpet should you buy?

For plain carpet, none: the trim and the unused width of the last piece are already in the order. Home Depot's "Order 10 to 20 percent more carpet than you need" goes with its square-footage method, not on top of this layout. A percentage can also fall short: for a 12 x 12 ft room, floor plus 20% is 172.8 sq ft, but one 15 ft piece is 187.5 sq ft and two 12 ft pieces are 300.

CRI 105 says "Patterned carpet requires additional material for proper pattern matching," so ask how much the style's pattern repeat needs and enter it as extra. With 10%: "Order 15.95 linear ft of 12 ft wide carpet: 21.27 sq yd (191.4 sq ft)."

Tips and common mistakes

• Ordering floor area ÷ 9. That's the floor, not the order.
• A room right at 12 ft. With trim, it's wider than a 12 ft roll; try the 15 ft roll.
• Mixing inches and decimals. 11 ft 6 in is 11.5 ft, not 11.6.
• Forgetting that seams follow the pieces. Changing the direction moves them.
• Skipping the seaming diagram. CRI 105 calls correct measurements and a detailed diagram essential before an installation is planned or estimated. Ask your installer for one.