Deck Stair Calculator
Enter the height of your deck for the number of steps, riser height, run and stringer length. Then check them against the stair code and AWC’s deck guide: stringer span, the wood left under each notch, how many stringers, handrails and what goes at the bottom. For planning — your building department approves the stair.
Last updated · Code: 2021 & 2024 IRC, AWC DCA 6-15
The short answer
Divide the total rise by 7¾ inches and round up for the number of risers, then divide the rise by that number for each riser’s height. You need one fewer tread than risers, each at least 10 inches deep. A 36-inch-high deck takes 5 risers of 7 3/16 in and 4 treads: 3 ft 4 in of run on 3 cut 2x12 stringers.
Deck stair calculator
Steps
5 risers × 7 3/16 in
Stringers
3 × 2x12, 8 ft boards
Passes: Riser 7 3/16 in
IRC maximum 7 3/4 in. All 5 risers come out the same height, inside the ⅜ in variation limit — keep the bottom one equal when you set the landing.
IRC R318.7.5.1 (2024) / R311.7.5.1 (2021)
Passes: Tread 10 in deep
IRC minimum 10 in, measured nosing to nosing. Under 11 in, the treads need a ¾–1¼ in nosing.
IRC R318.7.5.2–R318.7.5.3 (2024) / R311.7.5.2–.3 (2021)
Passes: Width 36 in
At least 36 in clear above the handrail; a bottom landing as wide as the stair.
IRC R318.7.1, R318.7.6 (2024) / R311.7.1, R311.7.6 (2021)
Note: Handrail needed
5 risers: a graspable handrail on at least one side, 34–38 in above the nosings, running from over the top riser to over the bottom one.
IRC R318.7.8, R320.2 (2024) / R311.7.8 (2021)
Note: Guards on open sides
The top of the stair is more than 30 in above grade, so the open sides of the treads that high need a guard at least 34 in above the nosings, with gaps that stop a 4⅜ in sphere. Open risers there must stop a 4 in sphere.
IRC R321.1, R318.7.5.1 (2024) / R312.1, R311.7.5.1 (2021)
Passes: Throat 5 7/16 in
Wood left under each notch of a 2x12; DCA 6 asks for at least 5 in.
AWC DCA 6-15 Fig. 28
Passes: Stringer span 3 ft 4 in
Measured level. Cut 2x12 stringers may span 6 ft under DCA 6.
AWC DCA 6-15 Stair Requirements, Fig. 28
Note: 3 stringers
About 18 in apart across 36 in. DCA 6: cut stringers no more than 18 in apart, and at least 3.
AWC DCA 6-15 Fig. 29
Note: Bottom landing and footing
A landing at least 36 in deep and as wide as the stair. Stringers bear on posts or a pad on footings at least 12 in below undisturbed ground or below the frost line, whichever is deeper — not on a patio slab that doesn't reach that depth.
IRC R318.7.6 (2024); AWC DCA 6-15 Stair Footing Requirements, Fig. 34
Your stringer cut layout
For planning: the IRC and AWC DCA 6 limits as published. Your building department and any local amendments decide what passes.
How to mark and cut deck stair stringers
- Measure the rise where the stairs land. Run a level out from the deck surface to the landing spot and measure down. On sloping ground the bottom lands lower than you think — the calculator’s slope option accounts for it.
- Set a framing square with stair gauges at the riser height on one arm and the run on the other, and step it along a straight 2x12, crown up.
- Drop the bottom. Cut the first riser shorter by one tread thickness, so the bottom step is the same height as the rest once the treads are on.
- Check the throat — the wood left between the inside corner of each notch and the bottom edge. DCA 6 wants at least 5 in on a 2x12.
- Cut one, test it, then trace it for the rest, and finish notch corners with a handsaw rather than over-cutting with a circular saw.
Deck stair code at a glance
| Item | Requirement | 2024 IRC / 2021 IRC |
|---|---|---|
| Riser height | 7 3/4 in max; ⅜ in max difference in a flight | R318.7.5.1 / R311.7.5.1 |
| Tread depth | 10 in min, nosing to nosing; ⅜ in max difference | R318.7.5.2 / R311.7.5.2 |
| Nosing | ¾–1¼ in, unless treads are 11 in or deeper | R318.7.5.3 / R311.7.5.3 |
| Open risers | Stop a 4 in sphere where more than 30 in above grade | R318.7.5.1 / R311.7.5.1 |
| Width | 36 in clear above the handrail | R318.7.1 / R311.7.1 |
| Flight height | 12 ft 7 in max between landings (DCA 6: landing above 12 ft) | R318.7.3 / R311.7.3 |
| Landings | Top and bottom, as wide as the stair, 36 in deep on a straight run | R318.7.6 / R311.7.6 |
| Handrail | 4+ risers: one side, 34–38 in above the nosings | R318.7.8, R320.2 / R311.7.8 |
| Guards | Open sides more than 30 in above grade: 34 in min on stairs, gaps under 4⅜ in | R321.1 / R312.1 |
The 2024 IRC renumbered the stair sections (R311.7 became R318.7) without changing these numbers. The 2024 edition also lets a deck stair of three or fewer risers that isn’t the required way out to grade have a bottom landing 36 in wide. DCA 6 adds stair lighting at the top, switched from inside or on a motion sensor.
States amend stair rules. North Carolina’s 2024 residential code (R311.7.5.1–.2) allows risers up to 8.25 in and treads down to 9 in: a 33 in rise there takes 4 risers of 8 1/4 in instead of the IRC’s 5 of 6 5/8 in. Tick “my code amends the riser or tread limit” in the calculator to use yours.
When the stringers need help
AWC’s DCA 6 caps a cut 2x12 stringer at 6 ft measured level, and a solid one at 13 ft 3 in. With 10 in treads, a cut stringer covers about 62 in of rise; with 11 in treads, about 54 1/4 in. Taller stairs need one of three fixes:
- A 4x4 post under the stringers partway down, notched and through-bolted, on a 12 in round or 10 in square footing 6 in thick.
- A landing, built as a small free-standing deck — size its joists with the deck joist span chart.
- Solid stringers with the treads on 2x4 ledgers, which span further but need stouter treads (DCA 6 Table 6).
The other limit is the throat. Every notch removes wood from the 2x12, more as the steps get bigger: at the maximum 7¾ in riser, 10 in treads leave 5 1/8 in, but 11 in treads leave only 4 15/16 in — under DCA 6’s 5 in. Lower the riser a little or use solid stringers.
How many stringers?
| Stair width | Cut stringers (≤ 18 in apart) | Solid stringers (≤ 36 in apart) |
|---|---|---|
| 36 in | 3 | 2 |
| 42 in | 4 | 3 |
| 48 in | 4 | 3 |
| 60 in | 5 | 3 |
Wood treads: 5/4 or 2x boards on cut stringers no more than 18 in apart, at least three per stair. Composite and PVC treads carry their own stair span, usually closer than wood’s — follow the tread’s installation instructions and enter that spacing in the calculator.
What goes at the bottom of deck stairs
The stair needs a footing like the rest of the deck. DCA 6 has the stringers bear on a 2x4 block fastened to posts on footings at least 12 in below undisturbed ground or below the frost line, whichever is deeper. It says not to bear them on a concrete patio or pad that isn’t founded that deep. The IRC adds a landing at least 36 in deep and as wide as the stair. Find your depth with the deck footing depth lookup.
Stairs on sloping ground
Each tread moves the bottom step further from the deck. If the ground falls away, it lands lower, which can need another riser, which pushes it further out again. Measure 30 in at the deck on ground falling 1 in per ft and the flat answer is 4 risers; on the slope it settles at 5 risers of 6 11/16 in over 2 ft 9 5/16 in of rise. The calculator repeats that until the numbers stop changing.
What this calculator doesn’t tell you
- Your local amendments, beyond the riser and tread limits you enter.
- How to attach the stringers to the rim (hangers or a ledger strip) and anchor guard posts — follow the hardware maker and DCA 6.
- L-shaped and winder stairs, spiral stairs, or stairs over 12 ft high.
- Tread spans for composite products, which come from their makers.
Local rules vary. Planning figures from the IRC and AWC DCA 6 as published — not an engineered design. Your building department approves the stair and its adopted code decides.
Sources and method
Stair, handrail and guard rules: 2024 IRC R318.7, R320 and R321.1, and the 2021 IRC R311.7 and R312.1, read on the International Code Council’s free view on October 23, 2026. North Carolina: 2024 NC Residential Code R311.7.5. Stringer span, spacing, throat and stair footing: AWC’s Prescriptive Residential Wood Deck Construction Guide (DCA 6-15), the current edition, based on the 2015 IRC. Numbers are transcribed and credited; the diagrams are our own.
The stringer’s level span is taken as the total run; its length is the straight line from the rim to the landing, and the board length adds one step for the top and bottom cuts, rounded up to stock lengths. The throat assumes a 2x12 at its actual 11¼ in.
Frequently asked questions
- How many steps do I need for a 3 ft deck?
- 36 in of rise: 5 risers of 7 3/16 in and 4 treads (3 ft 4 in of run at 10 in). 24 in: 4 × 6 in. 48 in: 7 × 6 7/8 in.
- What is the maximum riser height for deck stairs?
- 7¾ in (IRC), with treads at least 10 in and no more than ⅜ in variation. Some states amend it — North Carolina allows 8¼ in risers and 9 in treads.
- How many stringers do I need for deck stairs?
- Cut stringers no more than 18 in apart and at least three (DCA 6): 3 for 36 in, 4 for 48 in, 5 for 60 in. Composite treads usually need more — check their instructions.
- How far can a deck stair stringer span?
- 6 ft measured level for a cut 2x12, 13 ft 3 in for a solid one (DCA 6). With 10 in treads that covers about 62 in of rise; above that, add a post on a footing or a landing.
- Do deck stairs need a handrail?
- At 4 or more risers, on at least one side, 34–38 in above the nosings. Open sides more than 30 in above grade also need a guard.
- What should deck stairs sit on?
- Footings at least 12 in deep or below the frost line, whichever is deeper, under posts or a pad, with a 36 in landing — not a patio slab that isn’t founded that deep. Frost depth by zip