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DatumSpot

Stair Stringer Calculator

From a floor-to-floor rise, lay out the stringer — slope length, cut angles, the framing-square marks for each step, and whether the notched board keeps enough throat.

The finished floor-to-floor height the stair climbs.

Stringer length (slope)15' 8-1/8" · 4.778 m
Risers × treads15 × 14
Rise / run per step7-3/16" · 182.9 mm / 11" · 279.4 mm
Stair pitch33.2°
Plumb / level cut33.2° / 56.8°
Throat (2×12)5-1/4" · 132.7 mm — OK
  • Frame each step with a square: 7.20″ rise on the tongue, 11″ run on the body — stepped off 15 times.
  • Plumb cuts are vertical, level cuts horizontal; set a speed square to the 33.2° pitch to mark the plumb cut.
  • Stringer line is 15.67 ft along the slope — order the next standard length to allow for the end cuts.
  • A 11.25″ board leaves a 5.23″ throat below the notch — at or above the ~3½″ minimum net depth for cut stringers (IRC).

The geometry behind the cuts

A stringer is the sawtooth board that carries the treads and risers. Its layout is pure trigonometry off the same rise and run the stair calculator sizes: the board runs at the stair pitch (the angle whose tangent is rise ÷ run), its slope length is √(run² + rise²), and each step is marked with the rise on the tongue and run on the body of a framing square. The plumb (vertical) and level (horizontal) cuts follow directly from the pitch.

Notching a stringer removes a triangle at every step, and what's left below the notch — the throat — is what carries the load. The IRC requires a cut stringer to keep a minimum net depth (about 3½"), which is why a 2×12 is standard and a 2×10 often won't pass once it's cut. The tool flags the throat for the board you pick. Building the guard alongside it? See the guardrail & baluster calculator.

This is layout geometry, not a structural design — the riser/tread limits come from the IRC (the exact throat section number varies by edition). Verify spans, materials, and connections against the adopted code and your AHJ.

Frequently asked questions

How do you calculate stringer length?

The stringer's slope length is the hypotenuse of the total run and total rise: length = √(run² + rise²). For a 9-foot rise over a 154-inch run, that's about 188 inches (15.7 feet) — order the next standard board length to allow for the end cuts.

What are the plumb cut and level cut on a stringer?

The plumb cut is the vertical cut (parallel to the risers) at the top and bottom of the stringer; the level cut is the horizontal cut (parallel to the treads). Set a speed square to the stair pitch to mark the plumb cut — the level cut is its complement.

What size board do I need for a cut stringer?

A cut (notched) stringer must keep enough wood below the notch — the IRC requires roughly 3½ inches of net depth. A 2×12 (11¼") is the usual choice; a 2×10 (9¼") often falls short once the notch is cut. This tool shows the remaining throat for each board so you can check it.

How do I lay out stringer steps with a framing square?

Set the rise on the tongue and the run on the body of the square — for example a 7.2" rise and 11" run — then step it along the board once per riser. The tool gives the exact rise and run to use.

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