Deck Stair Calculator

Size an outdoor stair from the deck down to the landing, with stringers and tread boards.

Deck height

Deck surface down to the ground or landing. 6 to 240 inches.

Maximum riser

4 to 9 inches. The default is 7 3/4.

Tread depth

8 to 14 inches. Two 5 1/2 inch boards plus a 1/4 inch gap is 11 1/4.

Top of the stair

The risers, stringer count, and tread boards appear when the deck height is in range.

Tread thickness

1 inch for 5/4 boards, or 1 1/2 inches for 2x6.

Nosing

0 to 1 1/2 inches. Leave 0 when the tread is at least 11 inches.

Stringer stock

Stair width

12 to 120 inches. Three stringers is the minimum.

Tread material

Stringer spacing

6 to 18 inches. Wood 2x / 5/4 allows 18" at most.

Board width

1 to 12 inches. A 5/4x6 or 2x6 board is 5 1/2 inches.

Gap between boards

0 to 1 inch. The default gap is 1/4 inch.

Riser count

Auto (1)

Auto uses the fewest risers that stay under the maximum.

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Code reference

Riser height

7 3/4 in maximum

IRC R311.7.5.1 (2024: R318.7.5.1)

Residential stair riser. The IRC does not set a minimum.
Tread depth

10 in minimum

IRC R311.7.5.2

Measured horizontally from nosing to nosing.
Nosing

3/4 to 1 1/4 in

IRC R311.7.5.3 (2024: R318.7.5.3)

Not required when the tread is at least 11 inches.
Stringer throat

5 in minimum

AWC DCA 6 Figure 28

Outdoor stringers. A thinner throat fails the check.
Bottom landing

36 in deep

IRC R311.7.6, R311.7.7 (2024: R318.7.6)

At least as wide as the stair, in the direction of travel.
Guard

36 in high

IRC R312.1.1

Open sides when the stair rises 30 inches or more.

Code limits cited on this calculator
Check	Limit	Reference	Note
Riser height	7 3/4 in maximum	IRC R311.7.5.1 (2024: R318.7.5.1)	Residential stair riser. The IRC does not set a minimum.
Tread depth	10 in minimum	IRC R311.7.5.2	Measured horizontally from nosing to nosing.
Nosing	3/4 to 1 1/4 in	IRC R311.7.5.3 (2024: R318.7.5.3)	Not required when the tread is at least 11 inches.
Stringer throat	5 in minimum	AWC DCA 6 Figure 28	Outdoor stringers. A thinner throat fails the check.
Bottom landing	36 in deep	IRC R311.7.6, R311.7.7 (2024: R318.7.6)	At least as wide as the stair, in the direction of travel.
Guard	36 in high	IRC R312.1.1	Open sides when the stair rises 30 inches or more.

Code checks reference the 2021 International Residential Code (section numbers for the 2024 edition shown in parentheses). States and cities adopt different editions and often amend them, so limits in your area may differ.

Based on the IRC model code. Your local jurisdiction may adopt a different edition or amendments — confirm with your building department.

How to measure

Deck height is the vertical distance from the finished deck surface down to the finished landing the stair will meet. Set the tape at the deck edge and hold it plumb. Do not follow the slope the stringer will take, and do not stop the tape on grass, gravel, or a loose paver. If the landing is not built yet, measure to the grade where it will sit and add the thickness of the pad you plan to pour. Measure both ends of the deck edge. When they differ, enter the taller number so every riser still reaches the high side. Stair width is the clear distance between the outside faces of the outer stringers, or between the guards if those guards are already set. Tread depth for two deck boards is 11.25 inches: each board is 5.5 inches and the gap between them is 0.25 inch. Measure your own board if the product is narrower, and use the gap the manufacturer allows. The landing in front of the first riser needs 36 inches of level depth in the direction of travel. If the ground drops across that 36 inches, add the drop to the deck height, because this page does not solve a sloping grade. Write the deck height, the board width, and the gap before you change the riser count.

Formula

The riser count is the deck height divided by the maximum riser, rounded up. Riser height is the deck height divided by that count. With the top tread below the deck, the tread count is one less than the riser count, and the run is that count times the tread depth. Stringers are at least three. The page divides the stair width by the spacing the material allows, rounds up, and adds one. Boards on one tread are the tread depth plus the nosing, divided by the board width plus the gap, rounded up. Total boards are that count times the number of treads. Each board is as long as the stair is wide. A 36 inch stair with wood spacing of 18 inches uses three stringers.

A 40.5 inch deck with an 11.25 inch tread and no nosing needs 6 risers at 6.75 inches, 5 treads, 3 stringers, and 10 boards at 2 per tread.

Common mistakes

Measuring along the stringer slope instead of straight down from the deck surface.
Leaving a 0.75 inch nosing on an 11.25 inch tread, which orders three boards per tread.
Keeping 18 inch spacing after switching to a composite that allows less.
Building the bottom pad shorter than 36 inches in the direction of travel.
Forgetting to add the landing thickness when the pad is not built yet.

FAQ

How do I measure deck height?

Measure from the finished deck surface straight down to the finished landing, or to the ground where that landing will sit. Hold the tape vertical at the edge of the deck, not along the slope of the future stair. If the landing is not poured yet, add the thickness of the landing you plan to build. Use the taller side when the deck edge is not level. The number is the whole flight, not one step.

Why is the tread 11.25 inches?

Two common deck boards are 5.5 inches wide. A quarter inch gap between them adds 0.25 inch. Two boards plus one gap equal 11.25 inches, so the default tread is exactly two boards. The page rounds the board count up after dividing the tread plus nosing by the board width plus the gap. Leave the nosing at zero with this tread so the count stays at two boards. A 0.75 inch nosing pushes the same tread to three boards.

How many stringers do I need?

The page uses at least three stringers. It divides the stair width by the spacing the tread material allows, rounds up, and adds one. Wood 2x and 5/4 decking allow 18 inches, so a 36 inch stair uses three stringers. Trex Enhance allows 9 inches, and the same width uses five. Picking a composite and leaving the spacing at 18 inches fails the spacing check. The stringers are 2x12 by default.

What is the 36 inch landing?

The bottom of the stair needs a landing at least as wide as the stair and at least 36 inches deep in the direction you walk. The drawing shows that 36 inch depth in front of the first riser. A deck stair that is not a required exit and rises three risers or fewer can use a 36 inch wide landing under the 2024 exception. Stringers bear on a footing or on that landing.

When do I add a nosing?

A tread of 11 inches or more does not need a nosing under the model code, which is why the default nosing is zero. Shorten the tread below 11 inches and the nosing check fails until you enter a nosing from 0.75 to 1.25 inches. Adding nosing also increases the board count, because the covered length is the tread plus the nosing. Keep the tread at 11.25 inches when you want two boards and no nosing.

Does a passing check mean the stair meets my local code?

A passing row means the number meets the model-code limit printed beside it. Riser, tread, and nosing follow the 2021 IRC, with the 2024 section in parentheses. Throat, span, stock, and spacing follow DCA 6 for decks. Your city may adopt another edition or amend a number. A pass is not a permit. Confirm the adopted code and the deck-board installation guide before you order stringers.

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By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026