Stair Calculator — Free Online Tool
Stairs live or die by the riser height — uneven steps are a trip hazard and a code violation. This stair calculator divides your total rise into equal, code-friendly risers and gives you the stringer length to cut.
What is the Stair Calculator?
Building codes (IRC) cap riser height at 7-3/4 inches and require treads at least 10 inches deep — the classic comfortable target is a 7-inch rise with an 11-inch tread. The calculator rounds your total rise to the nearest whole number of risers, then divides evenly so every step is identical. Uneven risers are the #1 stair defect inspectors flag.
The stringer length comes from the Pythagorean theorem on total rise and total run — cut it from a 2×12, and always cut a test stringer from cheap lumber first. This stair calculator gives you the geometry; check local code for handrail, headroom (6'8" minimum), and landing requirements before you build.
How to use this calculator
- Measure the total rise floor-to-floor in inches.
- Optionally enter the total run available (else 10" treads are assumed).
- Click Calculate Stairs for risers, riser height, and stringer length.
- Verify against local code, then cut a test stringer from cheap lumber.
Formula
Frequently asked questions
What is the maximum stair riser height?
7-3/4 inches under the IRC — and all risers must be equal within 3/8 inch. This stair calculator keeps you under the max.
What is the ideal stair rise and run?
About 7-inch rise with an 11-inch tread — comfortable for most people and well within code.
How many stringers do stairs need?
Two for stairs up to 36 inches wide; three or more for wider stairs.
What is the minimum stair headroom?
6 feet 8 inches measured vertically from the tread nosing — check before framing.