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How To Measure Your Stairs For Carpet

We don’t recommend trying to carpet stairs yourself — there are a lot of finicky details that are better left to the professionals — but that doesn’t mean you can’t measure! When you start shopping for carpet, you’re going to see prices by the square foot, which means you’ll want to know how many square feet of carpet you’ll actually need to cover your stairs. Here’s a hint: it’s more than you might think.

Knowing Your Stairs

First, let’s talk about a little terminology. The tread is the part that you stand on. In most houses, it’s a standard 10 inches deep, but it’s best to measure just in case your stairs aren’t exactly that size. The riser is the flat part at the back of the stair — the part your foot bumps up against if you trip. That’s usually 8 inches tall.

A box stair is the simplest version. It simply has a tread and a riser, for a combined 18 inches of surface per stair. Multiply that by the width of your stairs — usually a standard 36 inches — and then by the number of stairs you need to cover, and you’ll have your final number. Remember that there will be one more riser than there are treads — the top of the stairs is simply the floor of your upstairs hallway, so you don’t need to account for that separately.

A cap stair has one open side with railings on it. It probably has the same combined length of 18 inches as a box stair, but you’ll need an extra foot of width on your carpet to wrap around the posts and over the edge of the stair. You also have the option of stopping the carpet before the posts, in which case you won’t need the extra width. The same goes for a double cap stair — add a foot for each side.

A pie stair is a triangle-shaped stair that wraps around a corner, also known as a winder. You’ll want the grain of the carpet to line up with the carpet on the rest of the stairs, so these steps usually aren’t measured to their exact shape. Instead, plan on a 48” x 30” piece for each winder so that the installer can maneuver the carpet for the best fit.

An open stair or Hollywood stair is a stair with no backing. Often, these types of stairs aren’t carpeted at all, but if you do want to cover them, you’ll need at least 20 inches of carpet, plus two times the height of the step, to wrap all the way around.

A bullnose stair is usually found at the bottom of the staircase. It’s wider than the rest of the stairs, and it’s usually rounded on one or both sides. The tread and riser should be the same size as your box stairs, but the width might be substantially wider. You’ll need a piece of carpet the width of the widest stair, plus a few extra inches to wrap around the outer edge.

Finally, if your stairs have a nose on the front of the tread that overhangs the riser, you’ll need to add another three inches to the 18” of tread plus riser.

A Few Examples

Let’s start with a simple box stair. For a typical set of 12 box stairs, you’ll multiply the 36” width by the 18” tread and riser, then multiply by 12 stairs. The total is 7776 square inches of carpet, which equates to 54 square feet. Simple enough, right?

Now let’s take a more complex case. You have a flight of 15 stairs with noses, open to a railing on one side. The stairs take a 90-degree turn at a corner, so three of the stairs are pie stairs. Here’s how the math would work out.

  • Pie stairs: three pieces, each 48” x 30” for a total of 4320 square inches.
  • Cap stairs: 12 stairs, each 48” wide, with a rise of 8” and a tread of 10”, plus 3” for the nose. The total is 12,096 square inches.
  • Total square inches: 16,416
  • Total square feet: 114

You can see how this staircase, even though it’s just a foot wider and a few stairs taller, requires more than twice as much carpet to cover! That’s why measuring before you shop is so important — you don’t want to come up short, and you don’t want to be surprised by how much it costs.

Now that you have your measurements, add 10% for safety. It’s impossible to install carpet, especially on stairs, without having to trim and adjust here and there, so it’s good to have a little more than you need.

With these techniques, you can determine exactly how much carpet you need to cover your stairs, so you can come into our warehouse equipped with the exact numbers you need to shop and compare. And with our network of professional installers, we’ll have your stairs carpeted and looking fresh in no time!