How to Measure Stairs for Carpet: Treads, Risers and Square Yards

Measure one tread’s depth, one riser’s height and the stair width. Each step takes tread plus riser plus a 1 to 2 in nose allowance. Multiply by the steps and the width, add about 10%, and divide square feet by 9.

A straight flight with 14 risers and 13 treads, a 10 in tread, a 7.5 in riser and a 36 in width needs 261 in of carpet, or 65.25 sq ft (7.25 sq yd). With 10% added that is 71.78 sq ft, so an order of 8 sq yd.

How do you measure stairs for carpet?

1. Measure the tread depth from the front edge (the nosing) back to the riser behind it, and the riser height from one tread to the next. Flooring Clarity says not to assume every step is the same, so measure each step if they look different.
2. Measure the stair width across the tread, in inches.
3. Count the risers and the treads. On a straight stair where the upper floor is the last step, a flight of n risers has n − 1 treads, so the carpet covers n risers and n − 1 treads. If a separate top tread is built, it has n treads. Count them on your stair. The stair rise and run guide shows how the counts relate.
4. Pick a nose allowance, the extra carpet that wraps over the front edge of each tread. Bob Vila suggests “an extra inch or two” per stair, and this guide uses 2 in. That is an editorial figure, so adjust it to your carpet and pad.
5. Add the length, multiply by the width, and convert. Treat each landing as a separate rectangle. The carpet measuring guide covers room and roll-width math.

What is the formula for stair carpet?

carpet length (in) = treads × tread depth + risers × riser height + treads × nose allowance
area (sq ft)       = carpet length ÷ 12 × stair width ÷ 12
square yards       = area ÷ 9
order              = area × (1 + allowance) ÷ 9      (allowance about 10%, a convention)

Think of it as one long strip that is as wide as the stair and as long as the carpet needs to be, running up the flight like a ribbon over the treads and risers. The strip is cut along the roll, so it uses its own length of roll whatever the roll width. The 10% allowance is the same convention as for rooms, and the result is rounded up to the next quarter or half yard per the supplier’s rules.

Worked example: a 14-riser straight stair, 36 in wide

• Counts: 14 risers and 13 treads. Tread depth 10 in, riser height 7.5 in, nose allowance 2 in, width 36 in. These are example dimensions, not requirements.
• Carpet length: 13 × 10 + 14 × 7.5 + 13 × 2 = 130 + 105 + 26 = 261 in, which is 261 ÷ 12 = 21.75 ft.
• Area: 21.75 × 3 ft = 65.25 sq ft, which is 65.25 ÷ 9 = 7.25 sq yd.
• With 10%: 65.25 × 1.10 = 71.775 sq ft, which is 71.78 sq ft and 71.775 ÷ 9 = 7.98 sq yd. The order is 8 sq yd.

A shortcut that takes 13 identical steps of (10 + 7.5 + 2) = 19.5 in gives 253.5 in, or 63.375 sq ft. It is 1.875 sq ft short, because it forgets the 14th riser (7.5 in × 3 ft = 1.875 sq ft). On a different stair the numbers change, but the counting does not: use one more riser than treads when the top floor is the last step.

A second stair shows the rounding. 12 risers and 11 treads, an 11 in tread, a 7.75 in riser, a 1 in nose allowance and a 42 in width: 11 × 11 + 12 × 7.75 + 11 × 1 = 121 + 93 + 11 = 225 in, which is 18.75 ft. Area: 18.75 × 3.5 ft = 65.625 sq ft. With 10%: 72.1875 sq ft, which is 8.02 sq yd, so 8.25 sq yd at a quarter-yard step.

How much carpet do you need for stairs by step count?

Same example dimensions as above: 10 in tread, 7.5 in riser, 2 in nose allowance, 36 in width, treads = risers − 1. This is an illustration. Your stair dimensions replace these.

Dividing square feet by three, which some guides do, is wrong: a square yard is 3 ft × 3 ft, which is 9 sq ft. The table divides by 9.

What about a stair runner?

A runner is a narrower strip down the middle of the treads, bought by length, not by area. Measure one tread and one riser, multiply by the number of steps, and add the two, the method This Old House describes, and include an extra 2 feet. For the 14-riser example the strip is 21.75 ft long, so with the extra 2 ft you order 23.75 ft. The width of the runner is a separate choice.

For wall-to-wall carpet, it is worth asking how the store prices the cut. The 21.75 ft strip is 36 in wide, but it is cut from a roll that may be 12 ft wide. If a store charged for the full roll width, the same strip would be 21.75 × 12 = 261 sq ft, which is 29 sq yd. Whether it does depends on the store, so ask.

How do landings, bullnose steps and turns change it?

• Landings. Measure each as its own rectangle, width × length, and add it to the stair area. Bob Vila handles the landing separately from the flight.
• Bullnose. A wider, rounded bottom step needs extra carpet (Bob Vila). The sources used here give no figure, so the installer measures it.
• Waterfall stairs. Beaulieu notes that a waterfall stair is tacked on the riser too, and that most manufacturers run the pile down the stairs, with the machine direction along the length of the stairs. Follow the maker’s instructions for the carpet you choose.
• Winders and turns. Stairs that turn need pattern and seam planning, and they are not covered here.

Assumptions, common errors and limits

• The 2 in nose allowance and the 10% waste are conventions, not standards. Adjust them for your carpet, pad and installer.
• Counting treads and risers the same. With the upper floor as the last step, there is one more riser than tread.
• Dividing by 3. Square yards are square feet ÷ 9.
• Using one step for the whole flight when the steps differ. Measure each step if they look different.
• Forgetting the width. A strip as wide as the stair, not as wide as the roll.
• Carpet and pad take up room. Building codes measure stair dimensions without the covering: a City of Boise handout on IRC R311.7.5 says dimensions “shall be exclusive of carpets, rugs or runners”. Thick carpet and pad reduce the tread depth you are left with. The guide on stair rise and run explains the limits.
• This is a quantity estimate. It does not check stair dimensions, handrails or fastening. Ask your installer to measure and lay out the flight.

Common questions