Understanding Stair Formulas & Building Code
Cutting stair stringers is often considered one of the most mathematically demanding tasks in rough carpentry. If your risers (the vertical part of the step) are not identical, the stairs will be a tripping hazard. This calculator ensures every step is uniform while adhering to building codes.
The 7-11 Rule (Standard Code)
Most modern building codes, including the International Residential Code (IRC), dictate that a residential stair riser cannot exceed 7 ¾ inches, and the tread depth (the part you step on) must be at least 10 inches. The "ideal" comfortable stair hits close to a 7-inch rise and an 11-inch run. Our calculator targets a highly comfortable 7.25-inch riser height by default.
Calculating the Total Run and Stringer Length
Because the top step typically ties into the deck or floor joist, there is always one fewer tread than there are risers. The Total Run is simply the number of treads multiplied by your tread depth. This tells you exactly how far out into the yard or room the stairs will extend. Using the Pythagorean theorem (A² + B² = C²), we then calculate the diagonal length required for your 2x12 lumber to cut the stringers.
Prep Your Landing
Stringers must rest on a solid footing to prevent sinking and rot over time. If you are building exterior deck stairs, you should pour a concrete landing pad at the base. Use our Concrete Slab Calculator to determine exactly how many bags of ready-mix concrete you need to pour the bottom landing.