Spiral Staircase Calculator
The riser height, number of treads, tread angle and going of a spiral staircase from its floor-to-floor rise, rotation and diameter.
Result
30 °
Geometry is exact; lengths are read to the nearest sixteenth of an inch.
- Treads, not counting the landing
- 12
- Riser height
- 8 5/16 in
- Going at the walk line
- 10 1/2 in
- Going at the rim
- 15 11/16 in
- Walk line length in plan
- 10.47 ft (125.66 in)
https://calcgallery.com/lumber-carpentry/spiral-staircase-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use the Spiral Staircase Calculator
- Measure the rise from finished floor to finished floor.
- Choose the number of risers, counting the step up onto the landing, and see the riser height it gives.
- Enter how far the stair turns from the first riser to the landing edge, its diameter, and where the walk line sits.
- Read the tread angle, the number of treads and the going at the walk line and at the rim.
How it is calculated
treads = risers − 1riser height = total rise ÷ riserstread angle = rotation ÷ treadsgoing at walk = walk line radius × tread angle (angle in radians)going at rim = stair radius × tread anglewalk line = walk line radius × rotationThe going is an arc length: the distance along the circle between one nosing and the next at that radius. It grows in proportion to the distance from the centre.
Planning the opening
A spiral needs a floor opening at least as wide as the stair, and the landing must sit where the rotation brings the last riser. Headroom is measured from each tread to whatever is overhead, which on a spiral is the landing and the floor above, one turn higher. Draw the stair in plan and in section before choosing the rotation: a turn that looks fine in plan can put the stair’s top under a joist.
With your numbers
- Riser = 108 in ÷ 13 = 8.308 in
- Tread angle = 360 ° ÷ 12 treads = 30 °
Quick reference
Geometry is exact; lengths are read to the nearest sixteenth of an inch.
| Job | Tread angle |
|---|---|
| 9 ft, 13 risers, full turn | 30 ° |
| 9 ft, 12 risers, 270° | 24.55 ° |
| 10 ft, 14 risers, 450°, 6 ft | 34.62 ° |
| 8 ft, 12 risers, 5 ft | 32.73 ° |
| 2.7 m, 13 risers | 30 ° |
| 3 m, 15 risers, 1.6 m | 25.71 ° |
Common questions about the Spiral Staircase Calculator
- How many treads does a spiral staircase have?
- One fewer than the risers, because the last riser steps up onto the landing, which serves as the top tread. A 9 foot rise in 13 risers has 12 treads and risers of about 8 5/16 inches.
- What is the tread angle?
- The rotation divided by the number of treads. A full turn of 360° over 12 treads makes each tread a 30° wedge. More treads over the same turn make each wedge narrower.
- What is the walk line?
- The path a person actually follows, some way out from the centre column. The going, the depth of tread underfoot, is measured along it. Because each tread is a wedge, the going is shallow near the column and deep at the rim; the page shows both.
- Does this check my stair against building codes?
- No. Spiral stairs have their own rules for riser height, going at the walk line, clear width and headroom, and those rules vary by country and region. Check them with your building department or the stair maker before ordering.