How to Build Stairs: Calculations and Construction

Stair Calculations

Building stairs that are comfortable and code-compliant requires accurate calculations. The two critical measurements are the unit rise (the height of each step) and the unit run (the depth of each tread). Building codes generally require a maximum unit rise of 7-3/4 inches and a minimum unit run of 10 inches. The relationship between rise and run is governed by the formula: 2R + T should equal 25-26 inches, where R is rise and T is tread depth.

To calculate the number of risers, divide the total rise (the vertical distance from floor to floor) by the desired unit rise. Round to the nearest whole number. Then divide the total rise by the number of risers to get the actual unit rise. The total run equals the number of treads (one less than the number of risers) multiplied by the unit run. Use our Stair Calculator to perform these calculations quickly and verify that your dimensions meet code requirements.

Cutting the Stringers

The stringer is the structural support that holds the treads. Most residential stairs use cut stringers (notched 2x12 lumber) or housed stringers (grooved to receive treads). Cut stringers are more common for DIY projects because they require only basic carpentry skills. Start by selecting a straight, defect-free 2x12 of sufficient length. Mark the unit rise and unit run on a framing square using stair gauges clamped to the tongue and blade.

Starting at the bottom of the stringer board, step off each rise-run combination. Verify the total number of steps and the overall length before cutting. Mark the bottom cut to account for the finished floor thickness (typically 3/4 inch). Cut carefully with a circular saw, finishing the inside corners with a handsaw or jigsaw (the circular saw cannot reach the corner of the notch). Use the first stringer as a template to trace and cut the remaining stringers, ensuring they are identical.

Installation

Stairs with more than three risers need a handrail by code. The handrail must be 34-38 inches above the stair nosing (the front edge of the tread) and must be graspable (between 1.25 and 2 inches in cross-section). Install the stringers first, attaching the top to the header or framing and the bottom to a framing anchor or kickboard. Use a minimum of three stringers for stairs up to 36 inches wide, and four stringers for wider stairs.

Cut and install the treads (the horizontal surfaces you step on) using 1x or 2x lumber. For hardwood stairs, use 5/4x6 or wider boards. Leave a 1/4 to 3/8 inch nosing overhang on the front of each tread. Cut risers (the vertical boards between treads) from 3/4-inch plywood or matching lumber. Install risers first, then treads. This way the tread sits on top of the riser, creating a clean joint and preventing squeaks.

Common Code Requirements

Stair width must be at least 36 inches. Headroom above the stairs must be at least 6 feet 8 inches. The maximum vertical rise for a single flight of stairs without a landing is 12 feet. Landings must be at least as wide as the stairs and at least 36 inches deep in the direction of travel. Open risers (stairs without riser boards) are not allowed if the opening would allow a 4-inch sphere to pass through. Always check your local building code, as requirements vary by jurisdiction.