Tap & Trowel Home-project calculators & repair guides

Stair stringer calculator

Enter the total rise, measured from the finished floor (or grade) at the bottom to the finished floor or deck surface at the top. You'll get the number of risers and their height, tread depth, total run, stringer length and angle, and how many stringers to cut. The results are checked against the 2021 IRC stair rules (R311.7). The defaults are a 36 in deck stair, 36 in wide.

By Jordan Hale · Updated

Units
Finished surface to finished surface. Measure at the stair, plumb.
Only if the space limits how far the stair can go. Tread depth is then worked out for you.
IRC max 7-3/4". The calculator rounds the riser count up, so the actual riser is at or below this.
Run per step, nosing to nosing. IRC min 10".
e.g. 5/4 decking is about 1"; two 2x6s are 1-1/2".
IRC min 36".
Lowest point above the stair, measured plumb from the line of the nosings. IRC min 6 ft 8 in (80").

Your stair

Risers (steps up)
–
Riser height
–
Treads to cut
–
Tread depth (run)
–
Total run
–
Stair angle
–
Stringer length (about)
–
Board to buy (2x12)
–
Stringers
–
Bottom riser cut
–
Wood left under notches (2x12)
–
IRC R311.7 check
–

    Board length includes one extra step for the bottom cut, but not trimming allowance. The stringer count follows AWC's DCA 6 deck guide: at least three cut stringers, no more than 18 in apart. Local codes vary, and many towns need a permit for new stairs.

    Illustration of three angled stair stringers standing on a concrete pad, with a short stack of wood treads, a carpenter's square, a pencil and a circular saw on the floor.
    Illustration

    How the calculator works

    1. Risers: total rise ÷ your preferred riser height, rounded up to a whole number. Then total rise ÷ that number gives the exact riser height, so every step is the same.
    2. Treads: one fewer than the risers, because the top step is the upper floor or deck. If you enter a total run, tread depth = total run ÷ treads. Otherwise your preferred tread depth is used and total run = tread depth × treads.
    3. Angle: the arctangent of riser ÷ tread.
    4. Stringer length: the diagonal of the total rise and run of the cut steps, plus one more step's diagonal for the bottom. Board length is the next standard length up (8–20 ft).
    5. Bottom riser cut: the riser height minus one tread thickness. Cutting the bottom of the stringer down by this much keeps the first step the same height as the rest once the treads are on.
    6. Wood left under notches: 11-1/4 in (an actual 2x12) minus the depth of each notch, measured square to the edge. AWC's DCA 6 commentary analyzes cut stringers with 5.1 in left, based on a 7-3/4:10 rise to run.
    7. Riser variation: the layout riser is rounded to the nearest 1/16 in, and the calculator checks that the last riser, which takes up the rounding, stays within 3/8 in of the others.

    The code limits it checks

    From the 2021 International Residential Code (IRC), section R311.7. Your town may have adopted a different edition or local amendments, so check with your building department.

    IRC stair limits (R311.7)
    RuleLimitSection
    Riser height7-3/4 in maxR311.7.5.1
    Tread depth10 in minR311.7.5.2
    Variation in a flight3/8 in max, risers and treadsR311.7.5.1–.2
    Headroom6 ft 8 in minR311.7.2
    Width36 in min above the handrailR311.7.1
    Rise of one flight12 ft 7 in maxR311.7.3
    Nosing3/4 to 1-1/4 in (not needed with 11 in+ treads)R311.7.5.3
    Handrail4 or more risers; 34–38 in highR311.7.8
    GuardWhere more than 30 in above grade; 34 in min on stairs; no 4 in sphere throughR312.1

    Next: the how to build stairs guide walks through laying out a stringer with a framing square. Setting the bottom on a pad or footing? Use the concrete calculator. For a block base, use the cinder block calculator, and for a gravel base under a landing, the gravel calculator.

    Related: board foot calculator for lumber volume.

    Sources

    How this page was made. Jordan Hale is the author and is responsible for this page. An AI assistant helped write the code, gather manufacturer data and draft the explanation. The math is covered by automated tests against the published reference values cited below. The calculator gives a planning estimate. It does not replace the product label, your supplier or an engineer. The illustration on this page is AI-generated and shows the general idea, not exact construction details.