Calculate riser height, tread depth, stringer length, and angle for building code-compliant stairs. Get a complete materials list for your stair project.
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Total rise is the vertical distance from the finished floor at the bottom to the finished floor at the top. Measure from the actual floor surfaces, not the subfloor. If flooring hasn't been installed yet, add the flooring thickness to your measurement. For basement stairs, measure from the concrete floor to the top of the finished floor above.
The bottom of the stringer sits on the floor, so the first riser is measured from the floor to the top of the first tread. Since the tread adds height, you must cut the bottom of the stringer shorter by the tread thickness to keep all risers equal. This is called "dropping the stringer." Without this adjustment, your first step would be taller than the others.
2x12 lumber is the standard for stair stringers. After cutting the notches for treads and risers, you need at least 3.5" of solid wood remaining for strength. A 2x12 (which is actually 11.25" wide) provides enough material for typical 7" to 8" risers and 10" to 11" treads. For extra-long spans or heavy loads, consider LVL (laminated veneer lumber) stringers.
For stairs up to 36" wide with 1" thick treads, three stringers are standard (one on each side and one in the center). For wider stairs or thinner treads, add more stringers. A general rule is no more than 16" between stringers for 1" treads, or 12" for 3/4" plywood treads. Using more stringers prevents bounce and creates a more solid feel.