Stair Calculator

1. Measure total rise. Total rise is the vertical distance from the finished floor at the bottom to the finished floor at the top. Measure in inches for accuracy. A standard single-story floor-to-floor height is 96 to 120 inches (8 to 10 feet). Include any thickness added by finish flooring like hardwood or carpet at top and bottom.
2. Set preferred rise per step. The default of 7 inches is a comfortable, code-compliant riser height. The IRC (International Residential Code) caps residential risers at 7-3/4 inches. Lower rise values (6 to 6.5 inches) create gentler stairs but require more steps and a longer horizontal run. Higher values near the 7.75-inch limit feel steep.
3. Set preferred run per step. Run is the horizontal depth of each tread, measured from nosing to nosing. The IRC requires a minimum of 10 inches. Most comfortable stairs use 10 to 12 inches. A 7-inch rise paired with an 11-inch run satisfies the common "7-11 rule" used by builders for comfortable stair geometry.
4. Read the outputs. The calculator shows the exact number of steps needed, the actual rise per step (which may differ slightly from your preferred value), total horizontal run, stringer length, and stair angle. The stringer length tells you the minimum board length needed for the cut stringers.

Example: a 108-inch total rise at 7 inches preferred rise needs 16 steps (108 / 7 = 15.43, rounded up). The actual rise is 108 / 16 = 6.75 inches per step. Total run at 11-inch treads is 176 inches (14.67 ft). Stringer length is sqrt(108² + 176²) = 207 inches (17.25 ft).

Stair design uses four key calculations: number of steps, actual rise, total run, and stringer length. All dimensions interact, so changing one forces adjustments to the others.

The "7-11 rule" is a builder shorthand: riser + tread = 17 to 18 inches for a comfortable stair. A 7-inch rise plus 11-inch tread = 18, which is ideal. A 6.75-inch rise plus 10.5-inch tread = 17.25, also acceptable. Stairs steeper than 40 degrees feel uncomfortable. Most residential stairs fall between 30 and 37 degrees. Exterior deck stairs often use a shallower 5 to 6-inch rise for elderly users.