When I made the shift from installing to estimating flooring projects, a wise man told me 
“the cheapest insurance you can buy for a job is some extra material”. 
I think this is a very smart move, especially when figuring carpet or any other flooring materials on stairs and other intricate areas.
Carpet on stairs is rarely installed in one single piece, stairs often fall within general size so I often use 12″ for the tread and 9″ down the riser. With this I just need the width to finish the size to order per step. 12″+9″(21″) X width = Area of one step X how many steps.
Keep in mind that stairs come in different sizes and shapes. So you may encounter sizes different than those mentioned above. 
Here are a few different types of stairs you might come across:

Box Stair

  • A box stair is closed in on both sides by the walls.
  • It is often approximately 3′ to 4′ wide.
  • My usual allowance for the tread (the flat part you step on) is 12″, and 9″ for the riser (the back of the stair). Therefore, for every box stair, you require 3′ (or the width of your stair) x 21″ of carpet.

Open Sided Stair

  • This stair that has one open side typically with railing posts on the open side.
  • It has the same measurements for the tread and riser as a box stair (combined 21″).
  • You will need to add more carpet to cover the width, to allow the carpet to flow through the posts and wrap around the outside of the stair railing.
  • If your stair is open with a railing on both sides of it you will need to add width to the carpet to cover the overhang on both sides. Here is where it pays to have extra material for the installer.

Pie Shape Stair

  • The pie shape step will require 3′ (or the width of the stair) x 33″ or the measurement of the longest part of the tread and down the riser. You will have some extra waste here as you must figure the longest side only.

    Open or Floating Stair

    • This stair is a stair with no riser. The carpet will wrap all the way around the tread.
    • Measure your stair to determine the width. Then measure all the way around the step and add 3″. You may need approximately 23″ of carpet to wrap completely around the step.


    • A bullnose stair is often found at the bottom of a staircase and might be wider or wrapped on one or both sides.
    • Measure the stair width, including extra to cover the edges. Here is where having some extra material can really make a difference. Add 6″ to the overall width you figure after covering the edges.

    Direction of the Pile

    The carpet must run in the same direction on every stair, and the direction of the pile must go with the flow of traffic (from top to bottom, not from side to side).
    For a typical set of 16 box stairs measuring 3′ wide, using a carpet that is 12′ wide, you would require 12′ x 7′ (in a 12′ width of carpet, you get 4 widths of box stairs, (12′ divided by 3′) so every 12′ x 21″ gives you 4 stairs side by side on the roll).
    You should add another 12″ to the roll of carpet you order to allow for any oddities or mistakes when installing it.

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