Stair Stringer Calculator
IRC R311.7.5-compliant riser count, riser height, total run, and stringer length for a straight-run stair, from total floor-to-floor rise.
How this is calculated
IRC R311.7.5.1 caps riser height at 7.75in, and requires every riser in a flight to be within 3/8in of every other riser — you can't mix a few tall risers with a few short ones to average out. Since floor-to-floor rise is almost never an exact multiple of 7.75in, the calculator finds the smallest number of risers that keeps each one at or under the code max, then divides the total rise evenly across them:
Risers = ceil(total rise ÷ 7.75)
Riser height = total rise ÷ risers
Treads = risers − 1 (the top riser lands on the upper floor)
Total run = treads × tread depth
Stringer length = √(rise² + run²)
IRC R311.7.5.2 sets a 10in minimum tread depth, also measured nose to nose, with the same 3/8in flight-to-flight consistency rule as risers.
How this calculator works
IRC R311.7.5.1 (maximum riser 7-3/4") and R311.7.5.2 (minimum tread 10"), with the rise split evenly so every riser is identical. The metric mode uses the user's own maximum riser (limits differ by country) and checks the step-length rule 2h + g ≈ 63 cm (comfortable at about 60–65 cm).
Worked example
A total rise of 108" floor to floor with 10" treads.
Risers = ceil(108 ÷ 7.75) = 14, so each riser is 108 ÷ 14 = 7.71". Treads = 13, total run = 13 × 10 = 130", and the stringer length is √(108² + 130²) = about 169".
That stair is code-compliant: risers under 7-3/4" and treads at the 10" minimum.
Frequently asked questions
What is the maximum riser height?
IRC R311.7.5.1 limits risers to 7-3/4", and all risers in a flight must be within 3/8" of each other.
What is the minimum tread depth?
IRC R311.7.5.2 requires at least 10", measured nose to nose.
Why is there one fewer tread than risers?
The top riser lands on the upper floor, which acts as the last "tread", so a straight stair has one less tread than risers.
How long a board do I need for the stringer?
At least the calculated stringer length plus some extra for the plumb and level cuts at the top and bottom. A 169" stringer usually means a 16 ft board.
Common mistakes
- Measuring rise to the subfloor. Measure the total rise from finished floor to finished floor, including flooring thickness, or the first or last step will be off.
- Uneven risers. The IRC limits the difference between the tallest and shortest riser in a flight to 3/8 inch; the calculator splits the rise evenly for that reason.
- Forgetting headroom. Stairs need at least 6 ft 8 in of headroom measured from the nosing line (IRC R311.7.2).
What it does not cover
Headroom, handrails, guards, winders and landings.
Sources
Sources, checks and limits for this calculator →
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