Skip to content
Stairs Calc

Plumb cut & seat cut

The vertical and horizontal end cuts that let a stringer meet the floors.

The plumb cut is the vertical cut at each end of a stringer, and the seat cut (or level cut) is the horizontal one — together they let the angled board sit flat on the lower floor and against the upper framing. Every notch in a cut stringer is also a plumb-and-seat pair: the plumb face becomes the riser line and the seat face the tread line. Example: marked with a framing square set to the riser height and run, the plumb cut runs vertical when the stair is installed and the seat cut runs dead level. Getting the bottom plumb cut wrong by the thickness of one tread is the classic stringer mount error. These are layout geometry, not code, but they decide whether every step lands level.

Try the calculator Stair Stringer Calculator

Related terms

Stringer The notched board that carries the treads and risers; usually a 2×12. Stringer mount (standard vs flush) Whether the top tread sits one riser below, or level with, the upper floor. Riser (board) The vertical board that closes the gap between two treads on a closed-riser stair. Tread The horizontal board you step on; its depth equals the run plus the nosing.

Stair calculators

Use this term in a calculator

Stair Stringer Calculator Get the exact stringer length, throat depth, recommended board size, and a step‑by‑step cut layout with framing‑square settings — then print a full‑scale template.
All stair terms

Written by the Stairs Calc editorial team. Methodology and code references: see our methodology.

Built and maintained by builders, drafters and engineers who plan stairs for a living — every code limit is transcribed from the published standard and cited to its exact section.

Last reviewed 2026-06-20 against IRC 2021/2024

Stairs Calc gives accurate geometry and checks it against published building-code limits, but results are estimates for planning. Codes are adopted and amended locally and change over time. Always confirm dimensions against your local adopted code and a licensed professional before you build.