Stair Riser, Tread & Stringer Geometry Calculator
Calculate uniform stair rise, run, stringer throat depth, and incline angle complying with international building codes. This layout engine divides total floor-to-floor vertical rise into identical, code-compliant steps adhering to the classic Blondel 25-inch rule, giving you exact layout cuts for 2×12 framing stringers.
What Is the Ideal Riser Height and Tread Depth for Stairs?
Under the International Residential Code (IRC Section R311.7.5), the maximum unit riser height is 7-3/4 inches (197 mm) and the minimum tread run is 10 inches (254 mm). To achieve natural human ergonomic comfort, carpenters follow the historic Blondel rule: (2 × Riser Height) + Tread Run = 24 to 25 inches. For a standard 9-foot (108″) vertical floor rise, fifteen risers of exactly 7.20 inches (7-3/16″) paired with a 10.5-inch run deliver a 24.9-inch stride value, passing all residential inspection codes.
Stair Riser & Tread Calculator
Calculate uniform unit rise, run, stringer throat depth, and incline angles adhering to residential building codes.
How to Lay Out and Cut a 2×12 Stair Stringer with Framing Gauges
Follow these four critical carpentry steps to scribe, notch, and install code-compliant stair stringers:
Lock Brass Stair Gauges on Your Framing Square
Attach brass stair gauges onto your steel framing square: clamp one gauge on the tongue at the exact unit rise (e.g. 7-3/16″) and the other gauge on the body at the unit run (e.g. 10-1/2″). This locks the ratio permanently to prevent pencil creep.
Step Off Risers and Treads Down the 2×12 Plank
Select a straight 2×12 framing plank with minimal crown. Register the brass gauges against the top crown edge. Scribe the first tread and riser, slide the square down until the run line intersects the previous mark, and repeat for all steps.
Cut the Tread Thickness Off the Stringer Foot (Crucial)
If you do not trim the bottom of the stringer, adding your tread material will make the bottom step too tall and the top step too short. Scribe a line parallel to the bottom foot cut and saw off exactly one full tread thickness (e.g. 1.0″).
Saw to Corners with a Handsaw (Never Overcut)
Use a circular saw to cut along the tread and riser lines, stopping exactly 1/2″ before the inside corner. Finish each inside corner cut cleanly with a hand handsaw or jigsaw. Overcutting with a circular saw blade notches the stringer spine and ruins its structural strength.
Why Strict Step Uniformity Prevents Dangerous Stair Falls
Human locomotion relies on subconscious muscle memory when descending stairs. After taking two steps on a staircase, the human brain automatically calculates foot landing elevation. If any step varies in height by as little as 3/8 of an inch, the walker will stumble or pitch forward.
The 3/8″ Maximum Variance Code
Under IRC R311.7.5.3, the greatest riser height within any flight of stairs must not exceed the smallest by more than 3/8″ (9.5 mm). Unequal bottom steps due to unadjusted stringers are the #1 cause of failed framing inspections.
The 5.0″ Stringer Throat Rule
The throat is the uncut solid wood backbone behind the notched steps. A notched 2×12 stringer must maintain a minimum throat depth of 5.0 inches (or 3.5″ in rare jurisdictions) to carry 40 lb/sq ft live loads without fracturing.
Stringer Spacing Limits
Cut stringers must be spaced no farther than 16 inches on-center for standard 5/4 wood decking treads, or 12 inches on-center for composite decking treads (Trex / TimberTech) to prevent spongy, sagging step treads under load.