Steel Stair Stringer Calculator

Total rise divided by target riser height gives the riser count, exactly as with wood. Actual riser height, total run, stair angle, and stringer length all follow the same formulas: length = √(rise² + run²), angle = arctan(rise ÷ run).

For a 120 in rise with a 7.5 in target riser: 120 ÷ 7.5 = 16 risers exactly, 15 treads, and at 11 in per tread that's a 165 in total run. Length = √(120² + 165²) ≈ 204 in (17 ft), at an angle of about 36°.

What changes is how that triangle gets turned into a physical part — steel doesn't get notched with a circular saw, it gets cut at an angle and has brackets or plates attached.

A channel stringer (C-channel steel running the full diagonal length) gets both ends cut at the stair angle so the flat top and bottom faces land parallel to the floor and the header. The bottom cut angle equals the stair angle from horizontal; the top cut is the same angle, mirrored.

A plate stringer — a solid steel plate, sometimes laser-cut with a zigzag top edge to support individual tread brackets — uses the same overall angle for its end cuts but often has the tread-support profile cut directly into the top edge rather than adding separate brackets.

Either way, the two angles that matter for the cut list are the stair angle itself (about 36° in the example above) and its complement, 90° minus that angle (about 54°) — used depending on which face of the material you're referencing the cut from.

A mono stringer runs a single heavy steel beam (often a rectangular or I-beam section) down the center line of the stair, with treads cantilevered off brackets welded to either side. It carries the full load of the stair through one member instead of splitting it across two or three.

Because there's no outer stringer catching the eye, the tread brackets and their weld points become the visible detail — bracket angle has to match the stair angle exactly, or treads sit visibly out of level with each other.

Mono stringers see much higher point loads per weld than a pair of side stringers carrying the same stair, which is one of several reasons floating and mono-stringer stairs are a poor candidate for a DIY calculation alone.

These angles are what you'd set a chop saw or plasma cutter guide to when cutting the ends of a channel or plate stringer square to the floor and header planes.

Wood carpentry tolerates layout error of an eighth of an inch or so without much visible consequence. Welded steel stringers are far less forgiving — a bracket welded a couple degrees off angle shows up immediately as a tread that's visibly out of level with its neighbors.

Fabricators commonly work to tolerances of about ±1 mm (roughly 1/32 in) on bracket placement and ±0.5° on cut angles for finish-grade stairs, tighter than typical wood-framing tolerances by an order of magnitude. Confirm the tolerance your specific fabrication shop or engineer specifies rather than assuming a number.

Any stair carrying its full load through a single central stringer, or any stair with open risers and no visible support along one or both sides, concentrates load and deflection in ways that a geometry calculator alone can't verify as structurally sound.

This calculator gives you the rise, run, angle, and length numbers to start a design conversation — it does not size the steel section, verify deflection under load, or confirm the design meets structural code. For any mono-stringer, floating, or cantilevered stair, get a structural engineer to size and stamp the design before fabrication.

For wood stringer throat depth and board sizing instead, see the 2x12 stair stringer calculator.

If you're just after the raw angle and slope numbers without the stringer fabrication detail, the stair angle calculator and stair slope calculator cover that directly.