Calculating Carpet for Stairs

Curved staircase featuring neutral beige carpet
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Calculating how much carpet you need to cover your stairs is one of the most difficult aspects of planning your carpet installation. The carpet on stairs is rarely installed in one single piece, and the calculations require great attention to detail.

Unless your home is custom built, stairs fall within general size standards. Keep in mind that building codes vary from region to region, so you may encounter sizes slightly different than those mentioned below. Be sure to measure carefully!

Stairs come in different shapes, sizes, and configurations. Here are the different types of stairs typically found in a home:

Box Stair

  • A box stair is straight, has no railing posts, and is closed in on all sides.
  • It is typically approximately 3' wide.
  • The usual allowance for the tread (the flat part you step on) is 10", and 8" for the riser (the back of the stair). Therefore, for every box stair, you require 3' (or the width of your stair) x 18" of carpet.

Cap Stair

  • A stair that has one open side with railing posts on it is known as a cap stair.
  • It has the same measurements for the tread and riser as a box stair (combined 18").
  • You will need approximately 4' of carpet to cover the width, to allow the carpet to flow through the posts and wrap around the outside of the stair.
  • If your stair is open with a railing on both sides of it (a double cap) you will need approximately 5' of carpet to cover the width.

Pie Stair/Winder

  • A curved stair is called a pie stair or winder.
  • The general guideline is that you will require 4' (or the width of the stair) x 30" of carpet for every pie stair -- this allows for some maneuvering of the carpet by the installer to find the best fit.

Open Stair

  • An open stair (sometimes called a Hollywood stair) is a stair with no backing.
  • Measure your stair to determine the width.
  • You will need approximately 20" of carpet to wrap completely around the stair.

Bullnose

  • A bullnose stair is often found at the bottom of a staircase.
  • It is wider than the rest of the stairs and is rounded on one or both sides.
  • The tread and riser combined have the same measurements as a box stair (18").
  • Measure the stair to determine the width -- in grand staircases, it could be quite wide.
  • You will require a piece of carpet that is the width of the stair plus a few extra inches to wrap around the outer edge of the tread, by 18".

Direction of the Pile

The carpet on stairs 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. This means that, in order to work with the size of a roll of carpet, not all of the stairs are installed in one piece.

Calculating Your Requirements

To determine how much carpet you require for your full set, you must know the width of the broadloom (usually 12' but sometimes 15') and then you can calculate based on the dimensions above, for the number of stairs you have.

Commonly, a staircase leading from the main floor to the second floor of a house will have 13 stairs, while the staircase leading from the main floor to the basement of a house will have 12 stairs. Don't just assume this, though, as higher ceiling heights could mean a higher number of stairs.

Example

For a typical set of 12 box stairs measuring 3' wide, using a carpet that is 12' wide, you would require 12' x 4'6" (in a 12' width of carpet, you get 4 box stairs -- 12' divided by 3' -- so every 12' x 18" gives you 4 stairs).

For a curved staircase that is open on one side and contains 12 cap pie stairs (without a bullnose stair), you would need 12' x 10' of carpet.

Add-On

No matter how much carpet you calculate you need, you should always buy just a bit extra, to allow for any oddities or mistakes when installing it.