Stair geometry starts with one controlling measurement: the vertical distance between finished floor surfaces. Every riser must then divide that rise consistently while the resulting flight still fits the available run.
Measure finished floor to finished floor
Measure vertically from the top of the lower finished floor to the top of the upper finished floor. Include planned flooring, underlayment, and finish thicknesses. Measuring framing before finishes are resolved can make the first or last riser different from the others.
Do not measure along the slope: The total rise is a plumb vertical measurement, not the diagonal stringer line.
Divide the rise into equal risers
Choose a riser count that keeps every unit rise within the applicable project criteria. Divide the total rise by that whole count; do not round individual risers and then multiply because accumulated rounding changes the landing elevation.
Count treads and calculate the run
A straight flight between floor levels normally has one fewer tread than risers because the upper floor is the final walking surface. Tread depth is measured horizontally from nosing to nosing and is not automatically the same as the physical board depth.
Check the space around the flight
Measure available horizontal run, clear width, headroom above the nosing line, landing depth, doors, and circulation at both ends. The geometry may divide cleanly while the real stair still conflicts with the room.
- Available run before the landing
- Clear width at handrails and walls
- Lowest headroom over the nosing line
- Landing and door swing conflicts
- Guard, handrail, lighting, and structural requirements
Verify before cutting
Recheck the total rise after finish materials are selected and lay out the entire stringer on full-size stock before cutting. Stringer size, remaining throat, bearing, connections, species, grade, and loads are separate structural decisions.
Safety-critical work: A geometry result is not a permit, structural design, or complete code review.
Sources and review
Last reviewed July 26, 2026. The associated calculator includes tested fixtures, explicit assumptions, and input validation.
Field conditions, product instructions, approved plans, and applicable local requirements control the actual project. Review the full measurement disclaimer.