Key takeaways
- Stringer length = √(total rise² + total run²), so a 108-inch rise over a 130-inch run needs about 169 inches.
- DCA 6 calls for 2x12 stringers at minimum, and at least three where all of them are cut stringers.
- DCA 6 requires an intermediate landing once the stair's total height exceeds 12 feet.
Enter the total rise, the number of risers, your tread depth, the stair width and the stringer type. The tool gives the stringer length, the board length to buy and a stringer count, then compares your stair with figures from the American Wood Council's deck guide, DCA 6. It doesn't design the stair or tell you it meets your local code.
If you don't have risers and tread depth yet, start with the stair calculator, which works them out from the total rise.
How the calculator works
- Treads: risers − 1, for a top that lands flush with the deck
- Total run: treads × tread depth
- Stringer length = √(total rise² + total run²)
- Board to buy: the stringer length rounded up to the next even foot (check which lengths your lumber yard stocks)
- Stringer count = the larger of 3 (cut stringers) and width ÷ 18, rounded up, + 1
Then it raises three flags:
- Total rise over 12 feet: DCA 6 calls for an intermediate landing.
- Total run compared with 13 feet 3 inches: the solid-stringer span figure in DCA 6's commentary.
- Stringer size: DCA 6 says all stringers shall be at least 2x12.
The length is the straight line from the bottom of the stair to the top. Your layout adds a little at each end for the top connection and the bottom cut, so if the result lands just under an even length, think about the next size up.
Worked example
A deck 108 inches above the landing spot, with 14 risers and 10-inch treads, 36 inches wide, cut stringers:
- Treads: 14 − 1 = 13
- Total run: 13 × 10 = 130 inches (10 feet 10 inches)
- Stringer length: √(108² + 130²) = √(11,664 + 16,900) = √28,564 = 169.0 inches, or 14 feet 1 inch
- Board to buy: 14.08 feet rounds up to 16 feet
- Count: 36 ÷ 18 = 2, plus 1 = 3, and the cut-stringer minimum is also 3, so 3 stringers
The flags: 108 inches is 9 feet, under the 12-foot landing figure. The 130-inch run is under 13 feet 3 inches (159 inches). Three 2x12 boards at 16 feet each is the order.
Stringer length by total rise
Using 10-inch treads and the riser counts from the stair calculator:
| Total rise | Risers | Total run | Stringer length | Board to buy |
|---|---|---|---|---|
| 30 in | 4 | 30 in | 42.4 in | 4 ft |
| 48 in | 7 | 60 in | 76.8 in | 8 ft |
| 96 in | 13 | 120 in | 153.7 in | 14 ft |
| 108 in | 14 | 130 in | 169.0 in | 16 ft |
| 120 in | 16 | 150 in | 192.1 in | 18 ft |
The last row shows why rounding matters. At 192.1 inches the line is a hair over 16 feet, so a 16-foot board leaves nothing for the end cuts.
How many stringers by width
| Stair width | Width ÷ 18, rounded up, + 1 | Count with cut stringers |
|---|---|---|
| 36 in | 3 | 3 |
| 42 in | 4 | 4 |
| 48 in | 4 | 4 |
| 60 in | 5 | 5 |
What DCA 6 says about stairs
DCA 6 is the American Wood Council's Prescriptive Residential Wood Deck Construction Guide. The edition we read is based on the 2015 International Residential Code (IRC). The sentences that matter here:
- "All stringers shall be a minimum of 2x12."
- "If only cut stringers are used, a minimum of three are required."
- For stairs wider than 36 inches, cut and solid stringers "shall be placed at a maximum spacing of 18" on center".
- "If the total vertical height of a stairway exceeds 12'-0", then an intermediate landing shall be required."
A cut stringer has the steps sawn into it. A solid stringer is left whole, with treads hung on cleats or hangers.
Span. DCA 6 limits how far a stringer can span, using dimensions in its Figure 28, and says a post on a footing or an intermediate landing can shorten the span. Its commentary mentions "the maximum span of 13'-3" permitted for solid stringers", measured as horizontal span, not along the board. We only show that solid-stringer figure. The cut-stringer limit is in the drawing, which we haven't read in a form we can quote, so check Figure 28 yourself before you rely on cut stringers over a long run.
Treads. DCA 6's Table 6 lists minimum tread sizes by species. Its Southern pine row is 2x4 or 5/4 boards on cut stringers and 2x8 on solid stringers. Check your species in the guide.
Where the stair meets the ground
DCA 6 says stair footings "shall be placed at least 12 inches below the undisturbed ground surface or below the frost line, whichever is deeper". It also says stringers shall not bear on new or existing concrete pads or patios that are not founded below this depth. So a stair landing on an old patio slab is a question for your building department. For the concrete itself, see the concrete steps calculator and the deck footing and joist span tables.
What the IRC does and doesn't say
The 2021 IRC sets the rise, tread, headroom and landing figures in Section R311.7, which the stair calculator compares with. We searched the deck section of the 2021 IRC, Chapter 5, and found no stair stringer provision. That's why DCA 6 is the usual prescriptive reference for deck stringers. Your jurisdiction may have its own deck guide or local changes, and it decides what applies.
Note the two different height figures. The IRC's maximum rise for one flight is 12 feet 7 inches. DCA 6 asks for an intermediate landing above 12 feet.
Common mistakes
- Measuring to the dirt you'll dig out. Total rise runs from the deck surface to the finished landing, so measure after you've decided what the stair lands on.
- Counting a tread for every riser. With a flush top, there's one fewer tread than risers, and that changes the run and the length.
- Buying boards at the exact length. The line length doesn't include the end cuts, so a board that only just covers it is too short.
- Using 2x10s for stringers. DCA 6's figure is 2x12, and a cut stringer loses a lot of depth to the notches.
- Landing the stair on an existing slab. DCA 6 doesn't allow stringers to bear on a pad that isn't founded below its footing depth.
Frequently asked questions
How long should a stair stringer be?
Square the total rise and the total run, add them, and take the square root. A 108-inch rise and 130-inch run gives about 169 inches, so buy 16-foot boards.
How many stringers do I need for 36-inch-wide stairs?
DCA 6 calls for at least three if they're all cut stringers. The calculator's 18-inch spacing count also gives three at 36 inches.
What size lumber do stair stringers need?
DCA 6 says all stringers shall be a minimum of 2x12. Your jurisdiction may say more, so check with your building department.
How far can a stair stringer span?
DCA 6's commentary gives 13 feet 3 inches of horizontal span for solid stringers. Cut stringers have a shorter limit in the guide's Figure 28, which you should check directly.
When does a deck stair need a landing?
DCA 6 requires an intermediate landing when the total height of the stair exceeds 12 feet. Under the guide, the landing is built as a non-ledger deck, one that isn't attached to the house.
Does the IRC say how many stringers a stair needs?
We found no stringer section in the 2021 IRC's deck provisions. DCA 6 sets the count and spacing for decks built to it.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- American Wood Council: DCA 6, Prescriptive Residential Wood Deck Construction Guide
- 2021 International Residential Code, Chapter 5, Section R507
- 2021 International Residential Code, Chapter 3, Section R311.7