Spiral stair calculator for iPhone.

Calculate after changing dimensions and inspect the new result. A valid number range does not by itself make the stair comfortable or suitable for its intended use.

Total rise H

Finished floor-to-floor height, greater than zero.

Target rise

The rise you would like. Pro² rounds H ÷ target rise to choose the riser count, then divides H by that whole count to obtain the actual rise.

Inside radius r

Distance from the centre to the inner tread edge, not a diameter. Zero is accepted, but an inner radius of zero produces a degenerate inner edge.

Tread width w

The radial span from inner edge to outer edge. It is not the going measured around the stair. Outside radius = r + w.

Total rotation

The accumulated turn in degrees. The supported input range is 1–720°. It is not the angle of one tread.

Units and count adjustment

Length inputs use metric units or feet and inches. The − and + control beside Treads changes the tread count and linked riser count. Changing the height or target rise returns the count to automatic selection.

Enter a total rise of 3000 mm, target rise 190 mm, inside radius 60 mm, tread width 900 mm and rotation 360°. Use the automatic count for this example.

3000 ÷ 190 ≈ 15.79, so the count is 16 risers. The actual rise is 187.5 mm and there are 15 treads. The rotation is shared across those 15 treads: 360 ÷ 15 = 24° each.

A tread’s 900 mm radial width reaches from radius 60 mm to radius 960 mm. The outside diameter is twice the outside radius. These straight radial sizes do not tell you the going around the curve.

For a 24° tread, the arc is the curved distance around an edge. The chord is the straight line connecting that arc’s endpoints. With θ in radians, arc = radius × θ and chord = 2 × radius × sin(θ ÷ 2). A chord is shorter than its arc.

The rise is the same at both edges, but the horizontal distance around each edge differs. Pro² uses the inner arc for the inside pitch and the outer arc for the outside pitch. In this example they are about 82.37° and 25.00°; 24° is a third quantity, the plan angle of one tread.

For each edge, the stringer calculation unfolds the curved travel into a horizontal arc length and vertical rise. The length per tread is √(rise² + arc²); the reported total covers 15 such increments. It is not a straight board cut between the bottom and top floors.

Thickness, connections, support conditions and bending or fabrication methods are not included. The result describes geometric lengths, not a purchase or fabrication specification.

The app labels this output “Headroom”. The calculation is a vertical separation based on the rise and tread rotation. At 360°, it is 15 × 187.5 = 2812.5 mm. It is not the full 3000 mm floor-to-floor height.

For a turn greater than 360°, the value uses the number of tread intervals in one full revolution. For a turn smaller than 360°, it is capped at the available tread intervals rather than extrapolated to a complete revolution.

Tread thickness, an overhead floor, beams and a headroom envelope are not subtracted or tested. Treat this as a geometric separation, not proof of usable clear headroom. The final 187.5 mm rise to the upper floor is distinct from the 15 tread intervals in the table.

Each row accumulates distance from the same starting reference. Inside and outside stringer columns follow their respective sloping edge lengths. Inside plumb is the accumulated vertical rise; inside level is the accumulated inner arc length, not a straight radius or chord.

The table stops at tread 15: its vertical measurement is 2812.5 mm. The last rise to the floor brings the total to 3000 mm. Do not add one row to the previous row again.

Scroll the table horizontally, or focus it and use the arrow keys.

• The calculator supports 2–101 risers and therefore 1–100 treads; a target rise that implies a count outside that range is rejected.
• An extremely large per-tread rotation or tiny inner edge can be mathematically valid but unsuitable for a build. Inspect the geometry and intended use.
• There is no input for material thickness, a tread nosing, a selected walking-line offset, loads, price or regional rules.
• The diagram and example do not replace project-specific checks of headroom, guarding, supports, usable tread depth and construction details.

Is tread width the same as going?

No. Here width runs radially from the inner to the outer edge. Going around the curve varies with radius; the app reports arcs and chords at the two edges.

Why is the actual rise not 190 mm?

190 mm is the target. The chosen whole count is 16, so 3000 ÷ 16 gives 187.5 mm.

Does 360° mean each step turns a full circle?

No. It is the total rotation. In this example each of 15 tread sectors spans 24°.

Can I substitute a chord length for an arc length?

No. A chord is the straight distance between endpoints; an arc follows the circle. Use the named result and the matching inner or outer radius.