Stair Calculator
Calculate a straight stair’s riser count, exact riser height, tread count, horizontal run, stair angle and geometric stringer line from the finished floor-to-floor rise. The results are planning geometry; local building requirements and the actual stringer layout still need separate verification.
Calculate Stair Rise, Run & Stringer Line
How the Stair Calculator Works
The calculator separates simple geometry from code compliance so a planning result is not presented as an approved stair design.
1. Convert total rise
Feet and inches are converted to one value in inches: total rise = feet × 12 + inches.
2. Find a whole riser count
risers = round(total rise ÷ target riser). The exact equal riser height is then total rise ÷ risers.
3. Calculate run and angle
Under the stated floor-to-floor assumption, treads = risers − 1 and total run = treads × tread run. Angle is atan(total rise ÷ total run).
4. Calculate the diagonal
The geometric slope line is √(rise² + run²). Real stringer stock and cut layout require additional construction details that this calculator does not model.
Stair Calculator Examples
Each example uses the same formulas as the calculator and keeps code approval separate from the arithmetic.
9 ft total rise
8 ft 6 in total rise
3 ft 6 in deck rise
Why the Calculator Does Not Declare a Stair “Code Compliant”
Stair requirements depend on the governing code, occupancy, jurisdiction and adopted amendments. A residential stair and an OSHA-regulated workplace stair are not governed by the same dimensional limits. For example, OSHA’s current general-industry standard for standard stairs requires a 30°–50° angle, maximum 9.5-inch riser height and minimum 9.5-inch tread depth; those numbers should not be substituted for residential IRC requirements.
The calculator therefore shows geometry and a neutral planning note rather than a green “passes code” badge. Verify the applicable adopted code and local amendments before construction.
Measurements That Matter Before Building Stairs
Finished floor-to-floor rise
Measure between finished walking surfaces. Changes in flooring thickness can change the first or last riser and create an unwanted variation.
Tread run versus board depth
Do not assume the physical tread board depth is identical to the dimension defined as tread depth/run by the applicable code. Nosing and riser geometry can affect how the dimension is measured.
Headroom, landings and rails
These are separate constraints. OSHA, for example, separately regulates vertical clearance, landings, handrails and stair-rail systems in workplace settings.
Structural stringer design
The Pythagorean diagonal is useful geometry, but it does not determine lumber size, allowable span, notches, bearing, connections or structural capacity.
What Is the 2R + G Stair Formula?
The expression 2 × riser + going is historically associated with stair comfort rather than being a universal building-code pass/fail rule. A NIST stair-safety publication discusses Blondel’s historical 2R + G = 24 in relationship and later variations. Because comfort relationships and code requirements are different concepts, this calculator does not use the formula to certify a design.
Stair Layout Mistakes to Avoid
Building the target riser
The target is only used to select a whole riser count. Build from the resulting equal riser height after all project requirements are verified.
Using the wrong code limits
Do not mix residential, commercial, workplace or local rules. The applicable jurisdiction and occupancy matter.
Treating the diagonal as cut length
The geometric hypotenuse excludes real-world end cuts, bearing, connection details and structural sizing.
Ignoring finished surfaces
Measuring rough framing instead of final floor elevations can change end-riser dimensions once finish materials are installed.
Stair Calculator FAQ
Why is the actual riser different from my target?
Why are there fewer treads than risers?
Does this calculator check residential stair code?
Is the stringer result the board length I should buy?
Can I use it for deck stairs?
What if my stair has a landing or turns?
Related Construction Calculators
Stair Calculator Sources
- International Code Council — 2021 International Residential Code, Chapter 3 / Stairways — residential stair requirements; local adoption and amendments must still be checked.
- OSHA 29 CFR 1910.25 — Stairways — current general-industry stair dimensions, angle, uniformity and related requirements.
- OSHA 29 CFR 1910.28 — Fall protection — stairway handrail/stair-rail requirements in covered workplaces.
- NIST Special Publication 1620 — Stair safety research — historical discussion of stair geometry, safety and the Blondel comfort relationship.
- CalcsDone Methodology — how formulas, assumptions, limitations and sources are documented.