ConstructionCalc

Deck Stair Calculator: Risers, Treads & Stringer Length

Risers, treads and stringers for your deck from its height, kept within the residential code's step limits.

By Marko VisicBSc Physics, University of Ljubljana

5risers

of 7 3/16 in, 4 treads of 11 in

4 ft 8 7/8 in stringer

36 in44 in runlanding7 3/16 in11 inthroat 5 1/4 in
A side view at true proportion: the cut stringer, its treads, the landing and the deck's rim. The red tick is the wood left under a notch.
Riser height
7 3/16 in
Treads
4 × 11 in
Total run
3 ft 8 in
Stringers
3 cut 2x12s
First rise on the stringer
6 3/16 in
Tread boards
8 pieces, 24 ft

Within the limits: risers at most 7¾ in, treads at least 10 in, cut stringers no more than 18 in apart with at least 5 in of wood under each notch (here 5 1/4 in). With four or more risers the stairs need a handrail, and open sides more than 30 in above grade need guards.

5 risers of 7 3/16 in, 4 treads of 11 in, 3 stringers 18 in apart at most

Divide the total rise by 7¾ in and round up for the risers. A 36 in high deck takes 5 risers of 7 3/16 in and 4 treads; at 11 in treads the stairs run 44 in, on stringers 4 ft 8 7/8 in long along the steps.

The riser count fixes everything else, so get the rise right first: measure from the top of the decking to the top of the landing where the stairs will end.

How many steps for my deck

The 2021 IRC caps a riser at 7¾ in and sets a tread at no less than 10 in. Its stair section applies to stairs serving a building, porch or deck, so deck stairs follow it like any other.

Risers = total rise ÷ 7¾, rounded up; riser height = total rise ÷ risers

Treads = risers − 1; total run = treads × tread depth

Example: 36 ÷ 7¾ = 4.65, so 5 risers of 36 ÷ 5 = 7 3/16 in. The deck is the top step, so there are 4 treads; at 11 in each the stairs run 44 in, and the stringer measures 4 ft 8 7/8 in along the nosings.

Every riser in a flight has to come out within 3/8 in of the others, and every tread within 3/8 in. That is why the calculator divides the rise evenly instead of using full 7¾ in steps with a short one at the bottom.

Deck stair chart by height

Risers, treads, run and stringer length for common deck heights, with 11 in treads. The last column checks the cut-stringer span.

Deck stairs by height with eleven-inch treads, risers split evenly under the code limit.
Deck heightRisersRiser heightTreadsTotal runStringer lengthCut-stringer span
18 in36 in222 in28 7/16 inwithin the limit
24 in46 in333 in40 13/16 inwithin the limit
30 in47 1/2 in333 in44 5/8 inwithin the limit
36 in57 3/16 in444 in56 7/8 inwithin the limit
48 in76 7/8 in666 in81 5/8 inwithin the limit
60 in87 1/2 in777 in97 5/8 inneeds a post, landing or solid stringers
72 in107 3/16 in999 in122 7/16 inneeds a post, landing or solid stringers
96 in137 3/8 in12132 in163 3/16 inneeds a post, landing or solid stringers

Stairs rising more than 12 ft 7 in need a landing partway; Wisconsin draws that line at 12 feet.

How to calculate stair stringers for a deck

Wisconsin’s prescriptive deck rules, which its safety department says are mainly based on the American Wood Council’s DCA 6 guide, give the stringer numbers: sawn or solid 2x12s, cut stringers no more than 18 in on center, spanning no more than 6 feet level between bearings, with at least 5 inches of wood left under each notch, the throat.

A dry 2x12 is 11¼ in deep, so a notch can go 6 1/4 in into it at most. The notch depth, square to the stringer, is the riser times the tread divided by the diagonal of the two. The example’s 7 3/16 by 11 in steps cut 6 in and leave 5 1/4 in. A full 7¾ in riser with a 12 in tread leaves only 4 3/4 in, too little for a cut stringer.

The span limit is the one tall decks hit. The example runs 44 in, inside the 6 feet; a 96 in deck runs 132 in and needs a 4x4 post under the stringers, an intermediate landing, or two solid stringers, which may span 13 feet 3 inches on a 36 in wide stair. At the bottom, stringers bear on a solid surface at least 3½ in thick and 8 inches across.

When you mark the stringer, cut the bottom rise shorter by one tread thickness: the treads add the same thickness to every step above, and the first step would come out tall without it. The calculator gives that first rise.

Deck stair code: the IRC limits

Stairway limits in the 2021 International Residential Code, Section R311.7.
RuleLimitIRC section
Riser heightat most 7¾ inR311.7.5.1
Tread depthat least 10 inR311.7.5.2
Nosing3/4 to 1¼ inR311.7.5.3
Stair widthat least 36 inR311.7.1
Headroom6 feet 8 inchesR311.7.2
Rise of one flight12 ft 7 inR311.7.3
Landing depthat least 36 inR311.7.6
Tread and landing slope2 percentR311.7.7

A nosing is not required where the tread is at least 11 in deep, which is why the calculator starts there. Flights with four or more risers need a handrail, and open sides more than 30 in above grade need guards; heights and openings are in the deck railing height guide. Local codes can differ, so check with your building department before you cut.

Deck stairs cost

The calculator counts the wood: the stringers, and the tread boards as pieces and linear feet at your stair width. Enter your yard’s price for a 2x12 and for a foot of tread board and it adds them up. Prices vary too much by region and material for a national figure, so the page does not quote one; hardware, footings and railings are extra.

Deck stair calculator FAQ

How many steps do I need for a three-foot deck?

Five risers and four treads. A 36 in rise divided by the 7¾ in limit is 4.65, so it rounds up to 5 risers of 7 3/16 in; the deck itself is the top step, so there is one tread fewer than risers.

What is the maximum riser height for deck stairs?

7¾ in under the 2021 IRC, which covers stairs serving a deck. The tallest riser in a flight may beat the shortest by no more than 3/8 in, so the bottom and top steps have to match the rest.

How deep should deck stair treads be?

At least 10 in, measured nosing to nosing. Under 11 in the code also wants a nosing of 3/4 in to 1¼ in; at 11 in or more none is required.

How many stringers do deck stairs need?

Cut stringers go no more than 18 in on center in Wisconsin’s prescriptive deck rules, which follow the American Wood Council’s DCA 6 guide. A 36 in stair takes 3.

How long can a deck stair stringer span?

A cut 2x12 stringer may span 6 feet, measured level between its bearings. Past that, add a post or a landing, or frame a 36 in stair with two solid stringers, good to 13 feet 3 inches.

Do deck stairs need a landing at the bottom?

Yes: the IRC wants a floor or landing at the top and bottom of every stairway, at least as wide as the stairs and 36 in deep for a straight run. The deck is the top landing.

Sources & method

Risers are the fewest that keep each step at or under the riser limit, split evenly; treads are one fewer, since the deck is the top step. Limits are from the 2021 International Residential Code, Section R311.7, read on the ICC’s site. Stringer spacing, span, throat and bearing are from Wisconsin’s prescriptive deck standards, SPS 325 Appendix B, and the 2x12’s depth from the American Softwood Lumber Standard, PS 20.

  1. International Code Council. 2021 IRC Chapter 3 Building Planning, Section R311.7 Stairways. 2021 IRC
  2. International Code Council. 2021 IRC Chapter 3 Building Planning (Sections R301.5, R311.7.8, R312). 2021 IRC
  3. Wisconsin Department of Safety and Professional Services (published by the Legislative Reference Bureau). Wisconsin Administrative Code, chs. SPS 320–325 Appendix B, Prescriptive Deck Standards, Section 14: Stairs. Register December 2015 No. 720
  4. Wisconsin Department of Safety and Professional Services. Uniform Dwelling Code Appendix B with commentary (2016). 2016
  5. National Institute of Standards and Technology (U.S. Department of Commerce). Voluntary Product Standard PS 20-20, American Softwood Lumber Standard. January 2020