Last reviewed: July 16, 2026 · Prepared by the Starlight Tools Editorial Team using the calculation method and primary code sections cited below; not professionally reviewed or a substitute for local approval.
Three viable whole-riser layouts nearest the chosen solution. “Comfort” uses the separate 24–25 in (610–635 mm) Blondel heuristic; “code” uses the selected dimensional preset.
| Option | Risers / treads | Riser | Tread run | Total run | Angle | 2R+T comfort | Available-run fit | Preset status |
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N = whole-number riser count; automatic mode chooses the compliant count closest to total rise ÷ target rise.R = H ÷ N, where H is total rise and R is exact unit rise.n = N − 1 for a standard mount because the upper floor is the final tread; flush top-tread mode uses n = N.L = T × n, where T is effective tread run and L is total horizontal run. The upper landing and nosing overhang are not added.2R + T is Blondel’s comfort heuristic; 24–25 in (610–635 mm) is treated as comfortable, separately from code.angle = atan(R ÷ T).stringer = √(H² + L²); per-step diagonal is √(R² + T²). Add 12 in (300 mm) for the displayed minimum stock length.A standard mount terminates against the upper landing, which supplies the last walking surface. Flush top-tread mode models a separate tread at upper-floor elevation and therefore adds one tread run.
The presets are screening checks for straight, rectangular stairs—not a permit determination. Local adoption, amendments, occupancy, accessibility, nosing, width, handrail, guard, landing, and structural rules can change what applies. Verify with the authority having jurisdiction.
With a 7 1/2 in target under the 2021 IRC preset, 108 ÷ 7.5 = 14.4, so the calculator evaluates whole counts and selects 15 risers. Exact rise is 108 ÷ 15 = 7.2 in (about 7 3/16 in). A standard mount has 14 treads because the upper floor is the fifteenth walking level.
Using a 10 1/2 in tread run gives 14 × 10.5 = 147 in (12 ft 3 in) total run. The angle is atan(7.2 ÷ 10.5) = 34.44°. Geometric stringer length is √(108² + 147²) = 182.41 in, about 15 ft 2 7/16 in. Add the displayed 12 in cutting allowance when selecting stock.
At a 7 1/2 in target, 96 in typically uses 13 risers at about 7.385 in (roughly 7 3/8 in), while 108 in uses 15 risers at 7.2 in (roughly 7 3/16 in). A standard flight has 12 and 14 treads respectively. The selected limits can change the recommendation.
Divide the finished-floor rise by a whole riser count for exact unit rise. Multiply effective tread run by the tread count for total horizontal run, then check both dimensions against the selected preset.
The upper floor or landing acts as the final walking surface. A standard flight with 15 vertical rises therefore needs 14 constructed treads. The optional flush top-tread model includes a separate tread at the upper level.
Model-code tread depth is generally measured between the foremost projections of adjacent treads. The calculator calls this effective tread run. For material planning it also reports physical board depth as effective run plus nosing.
Subtract tread thickness from the bottom riser cut so the finished first rise matches the remaining risers. Recheck any finish flooring at both ends before cutting.
Roughly 30°–37° is a common planning range, and 2R+T between 24 and 25 in is a separate stride heuristic. Neither replaces dimensional code checks.
Select “Fit available run.” The tool compares whole riser counts and shows nearby options. More risers lower each rise but generally create another tread, so the shortest option is not always the most comfortable or code-compliant.