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crewcalcs
Carpentry

Stair Stringer Calculator

Lay out a deck or interior stair from the total rise: risers in 1/16-inch fractions, treads, stringer length, framing-square settings, and a printable cut sheet — every dimension checked against IRC R311.7.

Finished floor to finished floor. Measure at the nose of the deck / landing.

= 9' 4" (112″)
in

Comfort target, 6–7¾″.

in

Riser-face to riser-face.

Top mount

14 treads on the stringer.

IRC R311.7 stair limits. Confirm the edition your AHJ adopted.

Riser count

Nearest is 15. Tap ±1 to trade riser height for tread count.

Advanced assumptions
in

2x = 1.5, composite ≈ 1.

in

Overhang, ¾–1¼″.

in

Added at the base.

in

At the landing.

IRC 2021 stair layout
Passes IRC 2021
7 7/16"riser
Risers
15
Riser height
7 7/16"
Treads
14
Total run
147"
Framing square
Rise 7 7/16" on the tongue · Run 10.5" on the blade
ƒShow the math
  1. 1.Number of risers
    round(totalRise ÷ target riser)
    round(112 ÷ 7.25) = round(15.45)
    = 15 risers
  2. 2.Riser heightper R311.7.5.1

    Closes on 112" exactly: cut 8 risers @ 7 7/16" and 7 @ 7 1/2" — 1/16" apart, well inside the 0.375" variation limit.

    totalRise ÷ n → nearest 1/16"
    112 ÷ 15 = 7.4667
    = 7 7/16"
  3. 3.Treads and total runper R311.7.5.2

    Flush mount: the deck/landing is the top step, so the stringer carries one fewer tread than risers.

    treads = n − 1 · run = treads × unit run
    14 × 10.5
    = 14 treads → 147" run
  4. 4.Stringer length

    Rounded up to a commercial length with a 6" cut margin.

    √(totalRise² + totalRun²)
    √(112² + 147²)
    = 184.8" → 2x12 @ 16 ft
  5. 5.Stair angle
    atan(riser ÷ unit run)
    atan(7.438 ÷ 10.5)
    = 35.3°
  6. 6.First riser cut (drop the stringer)

    Cut the bottom notch shorter by one tread thickness so every finished step rises the same amount once treads are installed.

    riser − tread thickness + finish floor (bottom)
    7.438 − 1.5
    = 5 15/16"
  7. 7.2x12 throat (remaining wood)

    Wood left behind the deepest notch on a 2x12. Under 5" is weak; under 3½" use a solid (uncut) stringer with cleats or an LSL/LVL.

    11¼" − (riser × run) ÷ √(riser² + run²)
    11.25 − (7.438 × 10.5) ÷ 12.87
    = 5.18"

Cut sheet

15 risers @ 7 7/16" · 14 treads @ 10.5″
PASSES IRC 2021
total run 147"rise112"stringer 184.8" · 2x12 @ 16 ft7 7/16"10.5"35.3°1st cut5 15/16"
Total rise112″
Risers15
Riser height7 7/16"
Treads14
Unit run (cut)10.5″
Total run147″
Stringer length184.8″
Board2x12 @ 16 ft
Stair angle35.3°
First riser cut5 15/16″
Top riser cut8 15/16″
2x12 throat5.18″
IRC R311.7 checks
  • Riser height 7 7/16" (max cut 7 1/2") · ≤ 7.75"R311.7.5.1PASS
  • Tread depth (nose-to-nose) 10.5" · ≥ 10"R311.7.5.2PASS
  • Nosing (required, tread < 11") 1" · 0.75–1.25"R311.7.5.3PASS
  • Riser variation across flight 0.063" · ≤ 0.375"R311.7.5.1INFO

crewcalcs.com · IRC 2021 · estimates for planning — verify with your AHJ before cutting.

Before you rely on this
  • To close on 112" exactly, 7 of the 15 risers are cut 1/16" taller (7 1/2" vs 7 7/16"). Keep the odd risers away from the top and bottom of the flight.
  • Not checked here: 80" minimum headroom (R311.7.2) and 36" minimum stair width (R311.7.1). Verify both on site.
  • Some jurisdictions amend these limits (a few allow up to an 8¼" riser / 9" tread). Confirm the numbers your AHJ actually enforces before you build.

How the layout works

Start with the finished floor-to-floor total rise, measured at the nose of the deck or landing. Everything else follows from it.

  • Risers. The number of risers is the rise divided by your target, rounded to a whole number; ±1 is offered so you can trade riser height for tread count. The riser height is the rise divided by that count, shown to the nearest 1/16". Because equal 1/16" risers rarely sum to the exact rise, a few risers take an extra 1/16" so the flight closes precisely — still far inside the 3/8" variation limit.
  • Treads and stringer. Total run is the tread count times the unit run. The stringer board is the diagonal, √(rise² + run²), rounded up to a stock 2x12 length with a cut margin. The throat check reports how much wood is left behind the deepest notch.
  • Cut sheet. The dimensioned drawing carries the framing-square settings, the dropped first riser, the stair angle, and each IRC R311.7 check — print it and mark the board straight from it.

Riser, tread, nosing, and variation limits come from IRC R311.7.5 for the edition you select. Headroom (R311.7.2) and stair width (R311.7.1) are reminders here, not computed — verify both on site.

Frequently asked

How many steps do I need for a given rise?
Divide the total finished floor-to-floor rise by a comfortable target riser (about 7 to 7 1/2 inches) and round to a whole number of risers. For a 112-inch rise, 112 ÷ 7.25 ≈ 15.4, so 15 risers at 7 7/16 inches. Fifteen risers gives 14 treads on a flush-mounted deck stair (the deck itself is the top step).
What is the maximum riser height and minimum tread depth?
Under IRC R311.7.5.1 the maximum riser is 7 3/4 inches, and under R311.7.5.2 the minimum tread depth is 10 inches, measured nose to nose. The largest variation between any two risers (or two treads) in a flight is 3/8 inch (R311.7.5.1/.2). Some jurisdictions amend these numbers, so confirm what your AHJ enforces.
What does "drop the stringer" mean?
Every notch on the stringer is cut to the same riser height, but once you set treads on top, each finished step rises by that height plus the tread. To keep the first step equal to the rest, the bottom of the stringer is cut shorter by one tread thickness. With a 7 3/8-inch riser and a 1 1/2-inch (2x) tread, the first riser cut is 7 3/8 − 1 1/2 = 5 7/8 inches.
Flush or standard top mount — how does it change the tread count?
On a flush mount the top riser lands on the deck or landing, so the deck is the top step and the stringer carries one fewer tread than risers (treads = risers − 1). On a standard mount a cut tread sits below the landing, so treads equal risers. It changes total run and therefore the stringer length.
What size lumber do I cut the stringer from?
Deck stringers are usually cut from 2x12. This tool sizes the board to the diagonal (√(rise² + run²)) plus a 6-inch cut margin, rounded up to a stock length. It also reports the throat — the wood left behind the deepest notch. Keep the throat above 5 inches; below 3 1/2 inches, switch to a solid (uncut) stringer with cleats or an LSL/LVL.
What are the framing square settings?
Set the riser height on the tongue (short arm) and the unit run on the blade (long arm), then step the square down the board to mark each notch. For a 7 3/8-inch riser and a 10 1/2-inch run, that is 7 3/8 on the tongue and 10 1/2 on the blade. Stair gauges (hex nuts) let you lock both settings so every notch matches.

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Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a qualified builder. See our methodology, sources, and code editions.