Skip to content
Design free

Stair stringer calculator

Enter the total rise from the landing to the top of the deck. We work out the steps, the riser and tread sizes, how many stringers and how long a board you need.

A 36 in rise takes 5 risers of 7.2 in and 4 treads of 10 in. The stair runs 3'-4" out from the deck. Cut 3 stringers from 2x12 lumber, 17.3 in on center, each about 5'-6" long, so buy 8 ft boards.

The rules this stair is checked against, from DCA 6 pages 20 to 22 and IRC R311.7. Local code may differ, so confirm with your building department.

Stair rules and your numbers
RuleLimitYoursResult
Riser height7.75 in or less7.2 inPasses
Tread depth10 in or more10 inPasses
Stair width36 in or more36 inPasses
Stringers3 or more, 18 in on center at most3 at 17.3 inPasses
Stringer span6'-0" between supports3'-4"Passes
Layout, step by step. Mark these on the stringer with a framing square.
StepTop of the step above the landingStarts out from the stair foot
10'-7 3/16"0'-0"
21'-2 3/8"0'-10"
31'-9 5/8"1'-8"
42'-4 13/16"2'-6"
5 (deck surface)3'-0"3'-4"

A stair with four or more risers needs a handrail, 34 to 38 in above the nosing. Builders' practice, not a DCA 6 rule: cut the bottom riser shorter by the thickness of the tread, so every step ends up the same height once the treads are on.

Design this deck in 3D, free

Carries over: deck height (36 in) and stair width (36 in). Not carried: tread depth, the stringer layout, and the deck width and depth, which you set in the designer. The designer lays out its own stair from the deck height.

Planning figures from the AWC DCA 6-2015 guide, not an engineer's design or a permit. Your building department has the final word, and local rules may be stricter.

Measure the total rise

Total rise is the vertical distance from the ground, or the landing pad, to the top of the deck surface. Measure it where the stair will land, after the pad is laid, and measure the deck surface and not the framing.

How the steps are worked out

Divide the total rise by the tallest riser allowed, 7.75 in, and round up. That gives the number of risers. Divide the total rise by that number to get one riser height, so every step matches. There is always one more riser than treads, because the deck surface is the top step.

Stringer count and span

Cut stringers must be 2x12 or deeper, with at least 3 for any stair and no more than 18 in apart on center. If the stair runs out more than 6'-0", DCA 6 asks for a row of posts under it so no stringer spans more than that, and the calculator tells you how many rows.

Landings

A stair that climbs more than 12'-0" needs an intermediate landing. Run the calculator on each flight.

Where the numbers come from

Every limit on this page is read from the American Wood Council, Prescriptive Residential Wood Deck Construction Guide (DCA 6), 2015 edition, a guide written for the 2015 International Residential Code. The deck designer, the builder's plan and these tools all read the same tables. The span tables assume 40 psf live load, 10 psf dead load, No. 2 grade lumber in wet service, and footings on soil that bears at least 1,500 psf.

A number that is our own choice and not in the guide, such as the 24 in joist overhang we draw by default, is never presented as a code requirement. Where a value comes from somewhere other than DCA 6, the page says so.

More free calculators

  • Deck calculator

    Joists, beam, posts, decking, hardware and concrete for a deck of any size.

  • Joist span

    The smallest joist that spans your distance, and how far it can overhang.

  • Beam span

    How far apart the posts can be for a 2-ply or 3-ply built-up beam.

  • Deck footing

    Footing size, depth and concrete bags for the whole deck.

  • Pergola span

    How far a pergola beam and rafters can span, and what limits them.

  • Paver calculator

    Pavers, gravel base, sand, edging and polymeric sand for a patio.

  • Concrete slab

    Cubic yards, 80 lb and 60 lb bags, gravel, mesh and control joints for a slab.

  • Span tables

    The joist, beam and post tables from DCA 6, by species and spacing.

Questions

Anything else? Write to us or see every calculator.

Questions people ask

For a deck stair, DCA 6 and IRC R311.7 set the maximum riser at 7.75 in and the minimum tread at 10 in. The calculator divides your total rise into the fewest equal risers that stay under 7.75 in, so every step is the same height.

3 cut stringers. DCA 6 asks for at least 3 cut stringers, no more than 18 in apart on center, and a 36 in wide stair at 17.25 in on center meets that. A 48 in wide stair also takes 4, at 15.5 in.

2x12 lumber. DCA 6 sets 2x12 as the minimum for cut stringers. The calculator also gives the length of board you need, which for a 36 in rise is about 5'-6", so one 8 ft board per stringer.

A handrail on at least one side is needed once a stair has 4 or more risers, set 34 to 38 in above the tread nosings. A 36 in rise has 5 risers, so it needs one.