This spiral staircase calculator works out the number of steps, riser height, and tread angle from your floor-to-floor height, stair diameter, and total rotation. Enter your measurements above to plan a comfortable, code-friendly spiral.
How to use this calculator
- Enter the floor-to-floor height in inches.
- Enter the stair diameter and the total rotation in degrees.
- Read the steps, riser height, and tread angle.
How is a spiral staircase calculated?
Risers = round up (Total height ÷ Max riser) Riser height = Total height ÷ Risers Treads = Risers − 1 Tread angle = Total rotation ÷ Treads
- Max riser — typically 7.75″ by code
- Treads — one fewer than risers (top landing is not a tread)
Example calculation
For a 108″ floor height with a 270° rotation:
- Risers = ceil(108 ÷ 7.75) = 14
- Riser height = 108 ÷ 14 = 7.71″
- Treads = 13, so tread angle = 270 ÷ 13 = 20.8°
Related calculators
Frequently Asked Questions
How many steps does a spiral staircase need?
Divide the total floor-to-floor height by your target riser height and round up. A 108-inch height with 7.7-inch risers needs 14 steps.
What is a comfortable riser height?
Most codes cap risers at about 7.75 inches. Dividing the total rise into equal risers at or below that limit keeps the stair comfortable and compliant.
How do I find the tread angle?
Divide the total rotation of the staircase by the number of treads. A 270-degree spiral with 13 treads gives about 20.8 degrees per step.