Spiral Staircase Dimensions & Measurements

Floor-to-Floor Height

Of the standard stair dimensions, the most important measurement when planning your spiral stair design is going to be your floor to floor height. To measure your floor to floor height, take a tape measure and start at the bottom floor and measure up to the floor where your stair is going. For example, if you are installing a stair on a deck, your floor to floor measurement would be the length between ground level and the finished deck flooring. Generally, our stairs are installed in two ways: coming off a deck, loft, building, etcetera or the stairs are installed through a hole in a floor. If you are installing a stair through a hole in the floor, you can simply measure from the bottom floor to the top of the opening. Be sure to account for any concrete, underlayment, subfloor, carpeting, or tile in your measurement.

30° Treads Stair Rotation
9 Treads 270°
10 Treads 300°
11 Treads 330°
12 Treads 360°
13 Treads 390°
14 Treads 420°
15 Treads 450°

When you think of spiral stairs, you might imagine round stairs with a beautiful 360 degree spiral that has a whimsical winding shape. Each Salter Spiral Stair is unique all the way down to its rotation. As a custom shop, we can build your stair to your specification, so standard stair dimensions don’t have to be so standard. The specification that determines the rotation of your spiral stair is floor to floor height. Please refer to the table above for a reference on how to calculate the rotation of your staircase. Most, but not all of our stairs use 30 degree treads. In other words, each tread represents a 30 degree turn. Using the table above you will notice that 9 treads gets you 270 degrees of rotation (9 treads x 30 degrees = 270 degrees of rotation). A full 360 degree rotation means that you will exit the same at the same angle you entered onto the stair.

Well Opening
Stair Diameter Well Opening
3′-6″ 44″ x 44″
4′-0″ 50″ x 50″
4′-6″ 56″ x 56″
5′-0″ 62″ x 62″
5′-6″ 68″x 68″
6′-0″ 74″ x 74″

If your project includes a hole in the floor (likely a round hole for the round stairs) that calls for a spiral stair you will want to pay close attention to our well opening size dimensions. If you’re installing a stair through a hole in the floor, making sure you have the right size opening for your new spiral stair is key. We call this hole in the floor a ‘well opening’. Depending on where you’re putting the stair, your well opening size may be determined for you. If you want to utilize the space saving features of your spiral stair, and keep it in the corner of your room, walls may limit the size of your well opening. The smallest well opening that we can work with is 44” x 44”. This is because the smallest diameter we offer is 42″ x 42″ (3 foot 6 inches). A 44″ x 44″ well opening ensures that your fingers will not get pinched as you grab the handrail when you traverse up and down the stair.

Please note, that if you’re attaching your stair to the side of you loft or deck, you won’t need a well opening measurement, but you will need your diameter and footprint in the next section.

Spiral Staircase Diameter

No matter where your stair is going, you will need to keep stair diameter in mind. The simplest definition of your diameter is the width of your stair, as spiral stairs are round stairs. Much like your well opening, your diameter may be defined by your placement of the stair. Once again, walls may limit the diameter of your stair. Our designers can help you determine the best size stair for your space. The smallest size diameter we offer is 3 feet 6 inches (42″ X 42″). Standard diameters increase in six inch increments, 42″ x 42″ to 48″ x 48″ to 54″ x 54″ and so forth. You can also think of diameter as the footprint of the stair, or in other words, the space that your stair takes up. A five foot diameter stair has a footprint of 60″ x 60″. In order to determine how much space a 5″ diameter stair will take up you can simply use duct tape to mark the space off on the floor. Diameter isn’t a necessary measurement to have when you call your designer, but it can help you to determine the right size for your space. The best ways to find the right size for you is to find your footprint measurement in the chart above and call us to speak to one of our designers for an expert consultation.

Spiral Staircase Diameter & Footprint
Stair Diameter Footprint
3′-6″ 42″ x 42″
4′-0″ 48″ x 48″
4′-6″ 54″ x 54
5′-0″ 60″ x 60″
5′-6″ 66″x 66″
6′-0″ 72″ x 72″

Because spiral stairs are often used for their space saving features, many people are concerned that the stair will be too narrow for regular use. We call the actual walkable room on your tread the clear walking path, which helps you meet stair tread code. This is a major measurement as you want to be comfortable walking up and down your stair. Some spaces call for a 3’-6” diameter stair. As this measurement only gives you a 17” clear walking path, they are normally used as a secondary means of access. While this is a perfectly safe diameter and walking space size, if you have more room you may want to look at larger diameters. You gain 3 inches of walking space for every 6 inches of diameter. Our 5’ diameter stair meets code and allows for 26” of clear walking space. This probably the closest that you will get to a traditional stair walking path.


Platform Sizes

Your platform size is just as important as your clear walking path. Your platform, or landing, is an entry and exit point for your spiral stair. The size of your platform is determined by your spiral stair diameter. It is calculated to the open space of your well opening that is left after your final step. Your entry and exit point and can be determined by your platform placement in one of four quadrants in your well opening. Our designers can show you the different options during a live screen share meeting online.

Tread Dimensions

Your tread measurements are the final set of spiral staircase dimensions you need! There are several things to consider in this section depending upon your tread type. One thing that does not change is your tread radius. This measurement is the actual length of your tread from the center of the hub. As your diameter grows, so will the length of your treads. The tread radius is about 3.5” longer than your clear walking path. The true tread depth is the width of your tread 12 inches from the center of the stair. This gives you an approximate measurement of the surface you will be stepping on for each tread. Your tread width is the measurement of your tread at its widest part at the end of the stair. As your treads get longer, they will also get wider to be proportionate. The tread width varies between the steel and wood treads for the same diameter stair. Wood cover treads are typically wider as they are made to fit over the steel frame of the stair. The final tread measurement is your tread overhang. This is simply the lip of your wooden tread over your steel frame. It has a standard measurement of 7.31”.


Tread Measurements
Stair Diameter Tread Radius True Tread Depth Steel Tread Width Wood Cover Width Tread Overhang
3′-6″ 20.5″ 9.5″ 11.5″ 13.5″ 7.31″
4′-0″ 23.5″ 9.5″ 13.25″ 15.25″ 7.31″
4′-6″ 26.5″ 9.75″ 15″ 17″ 7.31″
5′-0″ 29.5″ 9.75″ 16″ 18″ 7.31″
5′-6″ 32.5″ 9.88″ 17.5″ 19.5″ 7.31″
6′-0″ 35.5″ 10″ 19.5″ 21.5″ 7.31″

Start Designing Your Spiral Staircase Today

Spiral stairs are a unique product that can be customized to meet all of your design specifications. If you have more questions about spiral staircase measurements and dimensions, give one of our consultative designers a call! They can answer any question you may have about your spiral stair design and help you find the best size for your space.

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