Spiral Staircase Calculator
Size a spiral staircase from the floor-to-floor height, overall diameter and number of treads. Get the riser height, rotation per tread, tread depth at the walkline, headroom per full turn, clear walking width and estimated cost — with a pass/fail check against the US IRC spiral-stair rules, a by-tread-count table, a by-diameter table, and a downloadable PDF.
| Treads | Riser | Rot / tread | Tread @ wl | Code |
|---|---|---|---|---|
| Enter a stair to compare | ||||
| Diameter | Radius | Clear width | Outer arc |
|---|---|---|---|
| Enter a stair to compare | |||
How to use the spiral staircase calculator
Enter the floor-to-floor height, the overall diameter, the center-column size, the number of treads and the total rotation, and get the riser height, rotation per tread, tread depth at the walkline, headroom per full turn, clear width and cost — with a pass/fail IRC check, a by-tread-count table, a by-diameter table and a downloadable PDF.
Enter height & diameter
Floor-to-floor rise, the overall diameter the stair occupies, and the center-column size.
Set treads & rotation
The number of rotating steps and the total turn — ¾, full or 1¼ turn, or a custom angle.
Confirm code & cost
Leave the 9½″ max riser and 12″ walkline for US code, and add a price per tread if you want.
Read the result & save a PDF
See every dimension and a pass/fail check, compare tread counts and diameters, and download it.
How a spiral staircase is calculated
A spiral is a straight stair wrapped around a pole: the height sets the risers, the rotation sets the turn per step, and the walkline is where the wedge-shaped tread depth is measured.
Worked example — 108″ rise, 60″ diameter, 12 treads, 360° turn, 4″ column
Risers: 12 + 1 = 13
Riser height: 108 ÷ 13 = 8.31 in
Rotation per tread: 360 ÷ 12 = 30°
Tread at walkline: (2 + 12) × 30° in radians = 7.33 in
Headroom per turn: 8.31 × (360 ÷ 30) = 99.7 in
A 9-foot rise on a 60-inch spiral with 12 treads gives an 8.31-inch riser and a 7.33-inch tread at the walkline — both inside the IRC limits, with 99.7 inches of headroom per turn.
Spiral staircase reference charts
The numbers that come up most — tread count by ceiling height, how diameter sets the clear width and tread depth, and the IRC spiral-stair rules.
| Floor-to-floor | Risers | Treads | Riser height |
|---|---|---|---|
| 96 in (8 ft) | 11 | 10 | 8.73 in |
| 102 in (8.5 ft) | 12 | 11 | 8.50 in |
| 108 in (9 ft) | 13 | 12 | 8.31 in |
| 120 in (10 ft) | 13 | 12 | 9.23 in |
| 126 in (10.5 ft) | 14 | 13 | 9.00 in |
Treads are chosen so the riser stays at or under 9.5 inches, and there’s always one more riser than treads. Taller ceilings need more steps; the calculator picks the balance and checks it.
| Diameter | Radius | Clear width | Tread @ walkline |
|---|---|---|---|
| 48 in | 24 in | 22.0 in | 7.33 in |
| 60 in | 30 in | 28.0 in | 7.33 in |
| 64 in | 32 in | 30.0 in | 7.33 in |
| 72 in | 36 in | 34.0 in | 7.33 in |
Tread depth at the walkline depends on the rotation and column, not the diameter — but the diameter sets the clear walking width, which must be at least 26 inches. A 48-inch kit is right at the edge, so size up where you can.
| Requirement | Limit | Measured |
|---|---|---|
| Riser height | Max 9-1/2 in | Floor-to-floor ÷ risers |
| Tread depth | Min 6-3/4 in | At the walkline, 12 in from the narrow edge |
| Headroom | Min 78 in | Vertically from the walkline |
| Clear walking width | Min 26 in | Outer radius − column radius |
These are the residential spiral allowances — looser than a straight stair because the layout is compact. All treads must be identical, and local codes can differ, so always confirm with your building department.
A spiral is set by four things: the height divides into risers, the diameter is the footprint, the rotation sets how far each tread turns, and the walkline 12 inches in is where tread depth is judged.
- More treads, shorter riser — adding steps lowers the riser but shrinks the turn and tread depth.
- Diameter sets clear width — a bigger circle gives more walking room, which must be at least 26 inches.
- Tread is judged at the walkline — 12 inches from the column, where the wedge is at its usable depth.
Everything the calculator works out
One set of inputs gives you every working dimension — riser, rotation per tread, tread depth, headroom and clear width — plus a pass/fail code check, a cost estimate, two comparison tables and a report to save.
The figures behind every spiral
Built for anyone planning a spiral stair
A loft, a deck, a compact second floor, a mezzanine — a spiral saves space, but only if the riser, tread and headroom all land inside code. Getting the layout right on paper avoids an expensive custom order that won’t pass.
Adding a space-saving spiral to a loft, deck or basement and wanting to know the size and whether it passes.
- Start from floor-to-floor height
- Pick a diameter that fits the opening
- Check all four code limits
Fitting a stair into a tight footprint and dialing in tread count, diameter and rotation before ordering a kit.
- Compare tread counts for the riser
- Size diameter to the opening
- Save the PDF for the order
Specifying a spiral for a mezzanine or feature stair where headroom, clear width and the turn all have to work.
- Check headroom per turn
- Confirm the walkline tread
- Verify with code & supplier
7 tips for sizing a spiral staircase
The things that quietly fail a spiral layout — and the checks that get the size right before you order.
Spiral staircase FAQ
The sizing, code and comfort questions people ask most when planning a spiral staircase.
Start with the floor-to-floor height and divide it by the number of risers to get the riser height — and there’s always one more riser than treads, because the top step lands on the floor above. Then divide the total rotation by the number of treads to get the angle each step turns.
The tread depth is measured along the walkline, 12 inches in from the center column: tread = (column radius + 12) × the per-tread angle in radians. The calculator works all of this out and checks it against the US code limits.
Under the US IRC (R311.7.10.1), spiral stairs may use a riser up to 9.5 inches, must have a tread depth of at least 6.75 inches measured at the walkline (12 inches from the narrow edge), at least 78 inches of headroom, and a clear walking width of at least 26 inches.
These are looser than a straight stair precisely because spirals are compact. The calculator checks all four and flags any that fall short, but your local building department has the final say — always confirm before building.
It depends on the opening and the climb. Common residential spiral kits run 48 to 72 inches in diameter; bigger diameters give a deeper tread and more comfortable clear width, but take more floor space.
Work from your floor-to-floor height to set the riser and tread count, then pick the largest diameter that fits your opening — the clear walking width must be at least 26 inches (diameter minus the center column, halved). The by-diameter table shows how the clear width changes as you size up or down.
Enough that the riser stays at or under 9.5 inches. Divide the floor-to-floor height by 9.5 and round up to find the minimum number of risers, then subtract one for the tread count.
For a typical 108-inch (9-foot) rise, that’s about 13 risers and 12 treads. More treads make a gentler climb with a shorter riser, but each tread then turns through a smaller angle and gets shallower — the calculator balances both and checks the result.
The walkline is the path your feet actually follow around the spiral, set by code at 12 inches in from the narrower (inner) edge of the treads — near the center column. Tread depth is measured there because that’s the tightest usable part of the step.
The IRC requires at least 6.75 inches of tread depth at this line. Closer to the column the wedge-shaped tread is much narrower, and further out it’s wider, so the walkline is the fair place to judge whether the steps are deep enough to walk safely.
Because the last step up isn’t a tread — it’s the floor above. You climb onto each tread in turn, and the final riser lifts you onto the upper landing, so the number of risers is always the tread count plus one.
That’s why the calculator divides the floor-to-floor height by treads + 1 to get the riser height. Forgetting this is the most common spiral-stair mistake, and it makes every riser come out slightly too tall.
At least 78 inches under the IRC — measured vertically from the walkline of a tread up to the underside of the structure directly above it, typically where the stair passes under the floor opening after a full turn.
On a spiral, headroom depends on how much height the stair gains in one 360-degree turn, which is the riser height times the number of steps per full turn. A tighter turn or taller riser climbs faster and clears the opening sooner; the calculator reports the headroom per full turn so you can check it.
They take more care than a straight stair because every tread is wedge-shaped and you turn as you climb, so the usable depth is narrower toward the center. That’s why code measures tread depth at the walkline and allows a taller riser.
A larger diameter, more treads and a moderate rotation all make a spiral easier to use. They’re a great space-saving option, but for a primary everyday stair, favor a comfortable layout over the tightest one that just passes code.
Estimate Only — Confirm With Your Stair Supplier & Code: This calculator sizes a spiral stair from the cylinder geometry — risers = treads + 1, riser = height ÷ risers, rotation per tread = total rotation ÷ treads, and tread depth = (column radius + walkline) × the per-tread angle — and checks it against the US IRC spiral-stair allowances (R311.7.10.1): riser up to 9.5 in, tread at least 6.75 in at the walkline, headroom at least 78 in, and clear width at least 26 in. It assumes identical treads and a single center column, and does not cover handrails, landings, structural loads or the manufacturer’s engineering. Codes and amendments vary by jurisdiction. For reference and estimating only; verify the final design with your stair supplier and local building department before ordering or building.