Stair stringer calculator
Calculate riser height, tread layout, stringer length, and stair angle with a real-time stringer profile diagram.
How Stair Calculations Work
Building stairs starts with one measurement: the total rise — the vertical distance from the finished lower floor to the finished upper floor. Every other dimension follows from this number and the building code constraints that govern safe stair design.
The calculator divides your total rise by a target riser height (typically 7" to 7-3/4") to find the number of risers. It then calculates the actual riser height by dividing the total rise evenly. The number of treads is always one less than the number of risers for standard mount stairs — the upper floor itself serves as the final step.
Standard vs. Flush Mount
Standard mountis the most common for interior stairs. The stringer bears on a ledger plate attached to the floor framing. The last step up brings you onto the finish floor — there's no tread board at the top, just the floor itself.
Flush mount is typical for deck stairs and exterior applications. The stringer is face-mounted (bolted) to the rim joist. The top tread is flush with the finish floor or decking surface. This means one additional tread on the stringer compared to standard mount, and the total run is one tread depth longer.
IRC Code Requirements
- Maximum riser height: 7-3/4" (196mm) per IRC R311.7.5.1
- Minimum tread depth: 10" (254mm) per IRC R311.7.5.2
- Minimum headroom: 6'-8" (80") per IRC R311.7.2
- Rise + run comfort rule: riser height + tread depth should total 17" to 18"
FAQ
What is the maximum riser height allowed by code?
The IRC limits risers to 7-3/4" (196mm) maximum. Most carpenters target 7" to 7-1/2" for comfort.
How do I measure total rise?
Measure from the surface of the finished lower floor to the surface of the finished upper floor. Include any flooring material (hardwood, tile, carpet pad) that will be installed.
What size stringer board do I need?
Most stairs use 2x12 stringer boards. After cutting the notches, you need at least 3-1/2" of solid wood remaining at the narrowest point for structural integrity.