Deck Stair Stringer Calculator

Background.

Every riser on a deck stair has to match its neighbours to within 3/8 inch — the tolerance the American Wood Council's deck guide, DCA6, carries over from the International Residential Code. The only reliable way to hit it is to work down from the total height: measure from the finished landing to the finished deck surface, divide by the riser you would like, round up to a whole number of steps, and let that number set the exact height of every one. Starting from a favourite riser and stepping down until the deck runs out is how a stair ends with a short or tall step at the bottom.

From the total rise and a target riser, you get the riser count, the actual riser height, the number of treads, how far the stair reaches out from the deck, and the diagonal layout length of the stringer. A tread-thickness entry lowers the bottom cut so the first step comes out the same height as the rest once treads are on, and the stair width and maximum spacing give the number of stringers and how far apart they sit.

The results compare your actual riser and tread with limits you enter, preset to the 7¾-inch maximum riser and 10-inch minimum tread in DCA6. Other limits are not tested, and some bite on very ordinary decks. DCA6 lets a notched 2x12 stringer span no more than 6 feet horizontally, and the 54-inch deck in the worked example below needs a run of 73.5 inches — just past that limit. Use the layout numbers to plan and mark the cut, then check span, throat depth, guards, handrails and footings against DCA6 or the rules your building department applies before you buy lumber.

What is deck stair stringer calculator?

A stair stringer is the sloped board that carries the treads. A cut stringer is notched into a sawtooth, with each tread resting on a level cut and each riser on a plumb cut; a solid stringer stays whole and the treads sit on ledgers or hangers fixed to its face. Laying one out means turning a single measurement, the total rise, into a whole number of equal risers, a matching set of treads, and a total run. The stringer layout length is the diagonal of the right triangle formed by the total rise and the total run: the line through the corners of the notches from the landing up to deck height.

How to use this calculator.

1. 01Measure the total rise in inches from the finished landing surface to the top of the decking. If the ground slopes, measure at the spot where the bottom step will land, roughly one total run out from the deck, using a straight board and a level, and rerun the numbers if the riser count changes.
2. 02Enter a target riser height (4 to 8.25 inches). It acts as a ceiling: the actual riser comes out equal to it or lower, never higher.
3. 03Enter the tread depth as the horizontal step from one nosing to the next, which is the length of the level cut on the stringer. The tread boards will overhang that by the nosing, which DCA6 sets at 3/4 to 1¼ inches.
4. 04Enter the stair width and the maximum stringer spacing your treads allow; DCA6 permits 18 inches on center for cut stringers under 2x or 5/4 wood treads, and composite tread makers often require less.
5. 05Enter the actual tread thickness, about 1 inch for 5/4 decking or 1½ inches for 2x lumber. Optionally enter the length of the board you plan to use; the board-length line then says whether the stringer layout length fits it, and leaving the box blank or 0 skips that check.
6. 06Enter the maximum riser and minimum tread your jurisdiction enforces, or 0 in either box to skip that comparison, then read the check beneath the results.

The formula.

The layout runs in a fixed order. The riser count is N = ⌈H ÷ r_target⌉, the total rise H divided by your target riser and rounded up to a whole number; rounding up is what keeps every actual riser at or below the target. The actual riser is r = H ÷ N. Because the deck surface itself is the top step, there is one tread fewer than risers, N − 1, and the total run is (N − 1) × T for a tread depth T. The stringer layout length follows from Pythagoras, L = √(H² + run²).

The bottom riser cut is r − t, where t is the tread thickness. On a cut stringer every tread sits on top of a level cut, so the first tread would stand a full thickness taller than the other risers if the stringer bottom were left at r. Trimming t off the bottom drops the whole stringer by that amount; every step above still rises r, and the top step lands r below the deck surface.

Stringer count uses the stair width W and your maximum spacing s: the number of gaps is ⌈W ÷ s⌉, the stringers are one more than the gaps, and the actual spacing is W divided by the number of gaps. The width is treated as the distance between the outer stringers.

Two things the formula does not tell you are worth working out by hand. Each notch cuts r × T ÷ √(r² + T²) deep into the board, measured square to its edge, and what remains of a 2x12's actual 11¼-inch depth is the throat that carries the load. And L is the length of the sawtooth line, not of the board: the stringer extends beyond it where it is cut plumb against the rim joist and level on its bearing at the bottom, so the lumber has to be longer than L.

A worked example.

Frequently asked questions.

References& sources.

1. [1]American Wood Council (2018). DCA6 Prescriptive Residential Wood Deck Construction Guide Based on the 2015 International Residential Code. AWC.
2. [2]Simpson Strong-Tie (2026). Deck Connection and Fastening Guide. Simpson Strong-Tie.
3. [3]International Code Council (2024). 2024 International Residential Code, Chapter 3 Building Planning, Section R311.7 Stairways. ICC Digital Codes.
4. [4]International Code Council (2018). 2018 International Residential Code, Section R311.7.5.1 Risers. ICC Digital Codes.
5. [5]International Code Council (2018). 2018 International Residential Code, Section R311.7.6 Landings for Stairways. ICC Digital Codes.
6. [6]American Wood Council (n.d.). Is Your Deck Safely Connected to Your House? AWC.