Stair Calculator — Rise, Run & Stringer Layout

How the stair calculator works (rise, run, stringer)

A stair calculator turns one hard number — the total rise — into everything you need to cut a staircase: how many risers, how tall each riser is, how deep each tread is, the total run, the stringer length, the slope angle, and whether the layout passes IRC 2024 and IBC. This free stair rise and run calculator does it instantly in your browser, in inches or millimetres, with a scaled side-profile diagram and a material cut list.

Quick answer: divide the total rise by 7.75 in (the maximum riser) and round up to get the riser count N. Riser height is R = total rise / N. Going is T = N − 1 for a standard mount (T = N for flush). Total run is T × tread depth. Stringer length is √(rise² + run²). The comfort check is 2R + T, aiming for 24–25.5 in.

Stair calculation formulas

Every number on the results panel comes from these formulas (lengths in inches internally, converted for metric display):

• Riser count: N = ⌈total rise ÷ 7.75 in⌉ (never fewer than 2). A target riser below 7.75 in is used first, then capped by code.
• Riser height: R = total rise ÷ N
• Going (treads): T = N − 1 (standard) or T = N (flush — top step level with the floor)
• Total run: Run = T × D, where D is the tread depth
• Stringer length: S = √(R² + Run²) — the hypotenuse you mark on the 2×12
• Slope angle: θ = arctan(R ÷ D) × 180 ÷ π
• Comfort formula: C = 2R + D — keep it between about 24 and 25.5 in (610–648 mm)
• Bottom riser cut: R − tread thickness (the first step sits on the floor, so you subtract the tread stock)
• Headroom: total rise − subfloor thickness − R (clear height from the first nosing to the underside of the upper floor)

IRC 2024 and IBC stair code limits

The badge row under the inputs checks your layout against the common residential and commercial limits. IRC R311.7.5 covers risers, treads and nosings for one- and two-family homes; IBC 1011.5 is the commercial baseline and is stricter on going.

Check	IRC 2024	IBC
Max riser height	7.75 in (196 mm)	7.75 in (196 mm)
Min tread / going	10 in (254 mm)	11 in (279 mm)
Riser variation	≤ 0.375 in (9.5 mm)	≤ 0.375 in (9.5 mm)
Min headroom	80 in (2032 mm)	80 in (2032 mm)
Nosing projection	0.75–1.25 in typical	0.5–1.25 in typical

Because every step in this calculator uses the same riser and tread, the computed riser variation is 0 — the 0.375 in rule matters when you cut stringers by hand and drift between steps. Always dry-fit the first stringer before ripping the rest.

How to lay out a stair stringer with these numbers

How to calculate stair stringers starts on the shop floor: mark the total rise and total run on the framing square, step off N risers and T treads along the 2×12, and cut the notches. The calculator gives you the exact loop:

• 1. Mark the total rise on the stringer stock and square off the top cut so it bears on the upper floor / header.
• 2. Step the riser and tread — set the square to R on the rise leg and D on the run leg, and walk it down N times (T going steps plus the final riser).
• 3. Cut the bottom riser short by the tread thickness (R − tread thickness) so the first tread lands at the same height as the rest.
• 4. Check the length — your stringer diagonal should match the calculated stringer length; if it is longer than your stock, the cut list tells you how many 2×12s to glue / splice.
• 5. Verify headroom — clear height from the first nosing to the soffit above must be at least the minimum headroom you set (80 in default).

Standard vs flush top mount

Standard mount: the top step finishes one riser below the upper floor — going T = N − 1, total run = (N − 1) × D. This is the default for most interior stairs where the floor finish eats the last rise.

Flush mount: the top step lands level with the upper floor — going T = N, total run = N × D. Use it when the framing and finish build-up let the last tread finish flush with the finished floor. Flush adds one tread of run, so the angle gets slightly shallower.

The comfort formula (2R + T)

The stair comfort formula — twice the riser plus the tread — approximates a natural stride. Land between 24 and 25.5 in and the flight walks well. Under 24 in it feels steep and choppy; over 25.5 in it feels flat and slow. The calculator flags anything outside that band as a comfort warning (orange badge), separate from the hard code checks.

Example: a 7.0 in riser with an 11 in tread gives 2(7) + 11 = 25 in — right in the sweet spot, which is why those are the defaults.

Frequently Asked Questions