Stair stringer calculator
Enter the total rise from the ground to the top of the deck. You get the number of steps, exact riser height, stringer length, a cut diagram, code checks and a materials list.
Materials list
How to lay out the stringer
Planning estimate only. Based on IRC 2021 R311.7 and AWC DCA 6; your city or county may amend them. Confirm with your local building department before cutting.
How the stair math works
- Number of risers = total rise ÷ maximum riser (7¾" under IRC 2021), rounded up.
- Riser height = total rise ÷ number of risers. Every riser must be within ⅜" of the others.
- Treads = risers − 1, because the deck surface is the top step.
- Total run = treads × run per step.
- Stringer length = √(total rise² + (total run + one run)²), plus a few inches for the end cuts.
Worked example: 36" rise
36 ÷ 7.75 = 4.65, so 5 risers. 36 ÷ 5 = 7.2" per riser (about 7 3/16"). That's 4 treads; at 10" each, 40" of total run. The stringer runs √(36² + 50²) ≈ 61.6", so an 8 ft 2x12 is enough.
Deck stair code requirements (IRC 2021)
- Maximum riser height: 7¾" (R311.7.5.1)
- Minimum tread depth: 10" (R311.7.5.2)
- Largest riser and tread may not exceed the smallest by more than ⅜"
- Minimum width: 36" (R311.7.1)
- Handrail required with 4 or more risers, 34–38" above the nosings (R311.7.8)
- Maximum 151" (12' 7") of rise in one flight before a landing (R311.7.3)
- Cut 2x12 stringers spaced no more than 18" on center, with at least 5" of throat (AWC DCA 6)
Local codes can be stricter. Check with your building department, especially for stairs serving a deck more than 30" above grade. The full checklist, including guards, nosings and landings, is in our deck stair code requirements guide.
Solved layouts by total rise
- 18" rise3 risers of 6"
- 24" rise4 risers of 6"
- 30" rise4 risers of 7 1/2"
- 36" rise5 risers of 7 3/16"
- 42" rise6 risers of 7"
- 48" rise7 risers of 6 7/8"
- 54" rise7 risers of 7 11/16"
- 60" rise8 risers of 7 1/2"
- 66" rise9 risers of 7 5/16"
- 72" rise10 risers of 7 3/16"
- 78" rise11 risers of 7 1/16"
- 84" rise11 risers of 7 5/8"
- 90" rise12 risers of 7 1/2"
- 96" rise13 risers of 7 3/8"
- 102" rise14 risers of 7 5/16"
- 108" rise14 risers of 7 11/16"
Stair building guides
- How to Cut Stair Stringers (Step by Step)Measure the rise, lay out a 2x12 with a framing square, drop the stringer and cut clean notches. A complete walkthrough for deck stairs.
- Deck Stair Code Requirements (IRC 2021)Riser height, tread depth, width, handrails, guards and landings for deck stairs under the 2021 International Residential Code, in one checklist.
- How to Calculate Stair Rise and RunThe formulas for riser height, step count, tread depth and total run, plus the comfort rules builders use and how to handle sloped ground.
- How Many Stringers Do I Need for Deck Stairs?Stringer count by stair width and spacing, why composite treads need more, and when cut stringers need extra support.
Frequently asked questions
What size lumber for stair stringers?
Cut stringers should be 2x12 pressure-treated lumber for outdoor stairs. A 2x10 doesn't leave enough wood below the notches (the throat) for a 7"+ riser.
How many stringers do I need?
Space stringers no more than 18" apart. A 36" wide stair needs 3; a 48" stair needs 4. Composite treads often need closer spacing. Check the decking manufacturer's stair span. See stringer count by width.
What does "dropping the stringer" mean?
The bottom riser is cut shorter by the thickness of one tread. Every other step gets a tread on top, so without the drop the first step would be taller than the rest. Our step-by-step stringer cutting guide covers the full layout.