Stair Calculator
Calculate a straight stair layout from total rise and tread depth. Get riser count, exact riser height, tread count, total run, stair angle and theoretical stringer length in metric or imperial units.
Straight-stair inputs
Keep the first three fields simple. Use Advanced settings only when your project needs a fit or planning check.
Advanced settings: step count, fit, width, stringers & reference checks
Project geometry
Rise, run & stringer results
How to calculate stairs, formulas, examples & FAQ
How to use the stair calculator
- Measure total rise from lower finished floor to upper finished floor.
- Enter a target riser height and the tread depth for each step.
- The calculator chooses a whole number of risers and divides the rise evenly so every riser is equal.
- Open Advanced settings for a fixed riser count, available-run fit check, stair width, stringer count, waste or optional reference limits.
Stair calculator formulas
What the inputs mean
Total rise is the vertical distance between finished floor surfaces. Riser height is the vertical height of one step. Tread depth (or run) is the horizontal depth of one tread. For a straight stair flight, the upper floor or landing is the final walking level, so there is normally one fewer physical tread than risers.
Worked example
Risers = ceil(108 ÷ 7.5) = 15.
Actual riser = 108 ÷ 15 = 7.2 in.
Treads = 15 − 1 = 14.
Total run = 14 × 10 = 140 in.
Stringer length = √(108² + 140²) ≈ 176.82 in (14.73 ft).
Angle = arctan(7.2 ÷ 10) ≈ 35.75°.
Comfort check 2R + T = 24.4 in, within the common 24–25 in planning range.
Accuracy and limitations
The arithmetic is performed with full floating-point precision and rounded only for display. The calculator is limited to straight-flight geometry. It does not calculate framing strength, stringer throat depth, cut layout, headroom clearance geometry, landings, handrails, guards, winders, spiral stairs, structural connections or permit approval.
A theoretical stringer length is the diagonal of the total-rise/total-run triangle. Actual lumber length and cutting layout depend on the stair construction method, top and bottom cuts, tread thickness, attachment details and local practice. Do not treat the diagonal as a final cut list.
Reference checks are not code approval
The optional 2021 IRC residential reference is included because stair dimensions are often discussed in relation to IRC R311.7.5.1 and R311.7.5.2. The official ICC code text states a maximum riser of 7¾ inches; tread depth, width, headroom, landings and other requirements are separate checks. Codes are adopted and amended locally, so use the code edition actually adopted for the project.
Reference: 2021 International Residential Code — ICC Digital Codes.
Frequently asked questions
How many risers should a staircase have?
Choose a practical target riser height, divide total rise by that target and round up to a whole number. Then divide total rise by the chosen riser count so every riser is equal. A local code may impose a maximum riser height.
Why are there usually one fewer treads than risers?
In a straight flight, the upper floor or landing is the final walking level. That means the number of physical treads is normally one less than the number of risers.
How do I calculate stair stringer length?
For straight-stair geometry, use the Pythagorean theorem: stringer length = √(total rise² + total run²). The result is a theoretical diagonal, not a finished cut length.
What is a comfortable stair ratio?
A common planning rule checks 2 × riser + tread depth against about 24–25 inches. This is a comfort guideline, not a universal code requirement.
Does this calculator guarantee code compliance?
No. It provides arithmetic and optional reference checks only. Stair rules vary by jurisdiction, building type, code edition and amendments.