Deck Stair Calculator

The deck's height in; the steps and the stringers out.

Follow your tread guide and building department; our guide may be an older edition.

Units

Total rise, floor to floor

7' · 84 in · 7 ft · 2.1336 m

Width

3' · 36 in · 3 ft · 0.9144 m

Tread depth, nosing to nosing

11" · 11 in · 0.9167 ft · 0.2794 m

Tallest riser you allow

7" · 7 in · 0.5833 ft · 0.1778 m

Quantity

Add or subtract

12 risers

How the stringers hang

Stringer board

Stringer spacing

Waste (%) ≈

Nothing is added unless you add it: boards are bought whole and long enough. Waste is added once, to every board and piece on the list.

This page recalculates as you type. There is no Calculate button.

Answer

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Stringers, 12" apart at most

4 stringers

11 treads of 11" over 10' 1" of run, at 32.47°.

SafetyDCA 6-15 Figure 28 — A cut stringer spanning 121", past the 72" the deck guide allows: a 4×4 post on a footing under it, or a landing, shortens it.

How many risers 12 risers of 7"

Each riser 7"

Total run 10' 1"

Stringer length, top tread a riser below the floor 11' 11 7/16"

More numbers

How many treads 11 treads of 11"

Angle 32.47 °

Hung flush instead 13' 7/16"

Each stringer board, at least 12' 2 3/8" cut from a 14 ft board

Stringer left under the notches 5 3/8"

Twice the riser plus the tread 25"

Cost of what is ticked $582.46

Breakdown

Flight	Measurements	Qty	Risers	Share
Stair 1	7' × 3' × 11" × 7"	Add ×1	12 risers	100%

What to buy

Shopping list

2×12 stringer boards, 14 ft 4 boards $151.92 Price per board Reference price $37.98. Type your store's.

Treads, 12" deep × 36" long 11 pieces $274.78 Price per tread Reference price $24.98. Type your store's.

Riser boards, 7" × 36" 12 pieces $155.76 Price per riser board Reference price $12.98. Type your store's.

Ticked items $582.46

Untick what you have, and the riser boards for open risers.

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Standards

We hold the criteria and the clause, not the wording of the standard.

Meets IRC 2021 · R311.7.5.1

Every riser is 7", within the 7 3/4" the code allows.

Meets IRC 2021 · R311.7.5.1

The finished risers differ by 0", within 3/8": the bottom riser is cut for the tread and the flooring.

Meets IRC 2021 · R311.7.5.2

Treads of 11" are at least the 10" the code asks.

Meets IRC 2021 · R311.7.5.3

Not required: every tread is at least 11" deep.

Meets IRC 2021 · R311.7.3

The flight rises 84", within the 151" the code allows between floors or landings.

Meets IRC 2021 · R311.7.1

36" wide, at least the 36" the code asks above the handrail.

Not checked IRC 2021 · R311.7.2

The code asks 80" of headroom over the nosings; measure the flight as a stair under an opening to check it.

Not checked IRC 2021 · R311.7.8

Required: a handrail on at least one side. The flight has 12 risers, and the code asks for one from 4.

Not checked IRC 2021 · R312.1.1

Required along any open side: the stair climbs 84", more than 30" above the floor below.

Meets DCA 6-15 · p. 20

12" stringers, at least the 12" the deck guide asks.

Does not meet DCA 6-15 · Figure 28

A cut stringer spanning 121", past the 72" the deck guide allows: a 4×4 post on a footing under it, or a landing, shortens it.

Meets DCA 6-15 · p. 20

84" of rise, within the 144" the deck guide allows without a landing.

How this was worked out

Every number below is yours. Each constant links to the row where you can change it.

Stair 1: risers 7' ÷ 7" = 12, rounded up 12

Stair 1: each riser 7' ÷ 12 7"

Stair 1: total run 11 × 11" 10' 1"

Stair 1: angle atan(7" ÷ 11") 32.47°

Stair 1: stringer, marked along its edge 11 steps of 13 1/16", the square set at 7" and 11" 11' 11 7/16"

Stair 1: bottom riser, as cut 7" − 1 1/2" tread + 0" flooring to come 5 1/2"

Change:Tread thickness Flooring still to go down, at the bottom

Stair 1: top tread's cut, below the upper floor 7" + 1 1/2" tread − 0" flooring to come 8 1/2"

Change:Tread thickness Flooring still to go down, at the top

Stair 1: every riser cut the same instead every riser 7": first step 8 1/2", last 5 1/2" 3" apart; a flight may differ 3/8"

Stair 1: board for each stringer 11' 11 7/16" + 5 1/2" × 7" ÷ 13 1/16" 12' 2 3/8" → 14 ft

Stair 1: stringer under the notches 11 1/4" − 7" × 11" ÷ 13 1/16" 5 3/8"

Change:Dressed, 8 in and wider

Stair 1: stringers 36" ÷ 12" = 3, rounded up, + 1; at least 4 4

12 risers of 7" 2 × 7" + 11" = 25", against 25" 10' 1" of run

Change:Comfortable step, 2 × riser + tread

13 risers of 6 7/16" 2 × 6 7/16" + 11" = 23 15/16", against 25" 11' of run

Change:Comfortable step, 2 × riser + tread

14 risers of 6" 2 × 6" + 11" = 23", against 25" 11' 11" of run

Change:Comfortable step, 2 × riser + tread

Cost 4 × $37.98 + 11 × $24.98 + 12 × $12.98 $582.46

Assumptions

Every one of these is editable. Change one and the answer changes with it.

Assumption	Value	Confidence
Code edition
Built as
Dressed, 8 in and widerNIST PS 20-20, Table 3: 8 in nominal and wider is dressed dry ¾ in under it - 8 to 7¼, 10 to 9¼, 12 to 11¼.	in	From a data sheet or an agency document
Tread thicknessMyCarpentry's default tread, a 2× board 1½ in thick; a 5/4 tread is 1 in. The bottom riser is cut shorter by it.	≈ in	Trade convention
Nosing1 in, the middle of the ¾–1¼ in nosing the code allows.	≈ in	Trade convention
Flooring still to go down, at the bottomFlooring still to go down at the foot of the stair; 0 when that floor is finished. The stringer stands under it, so its bottom riser is cut taller by it.	≈ in	Trade convention
Flooring still to go down, at the topFlooring still to go down at the top of the stair; 0 when that floor is finished. It moves where the stringer hangs, not how it is cut.	≈ in	Trade convention
Stringer spacing, composite treadsTrex installation guide 2026, p. 36: Signature, Transcend and Select 1×6 and Transcend 1×4 treads, stringers no more than 12 in apart.	in	From a data sheet or an agency document
Fewest stringers, composite treadsTrex installation guide 2026: composite treads fastened across at least four stringers; applied here to every composite tread.	stringers	From a data sheet or an agency document
Least stringer to keep under the notchesAWC DCA 6-15, Figure 28 (p. 21): a cut stringer keeps at least 5 in below its notches. MyCarpentry and Blocklayer say the same.	in	From a data sheet or an agency document
Comfortable step, 2 × riser + treadTwice the riser plus the tread. Riser counts are ranked by how near they come to 25 in, inside the 60–65 cm Omni's calculator calls comfortable.	≈ in	Trade convention
Lowest riser before a warningBlocklayer's stair calculator warns below a 4 in riser.	≈ in	Trade convention
Deepest tread before a warningBlocklayer's stair calculator warns above a 14 in tread.	≈ in	Trade convention
Tallest riser the code allowsIRC 2021 R311.7.5.1 (R318.7.5.1 in IRC 2024): a riser is not more than 7¾ in, measured between the leading edges of the treads.	in	From a data sheet or an agency document
Most two risers of a flight may differIRC 2021 R311.7.5.1 (R318.7.5.1 in IRC 2024): the tallest riser of a flight is no more than ⅜ in taller than the shortest.	in	From a data sheet or an agency document
Shallowest tread the code allowsIRC 2021 R311.7.5.2 (R318.7.5.2 in IRC 2024): a tread is at least 10 in deep, measured nosing to nosing.	in	From a data sheet or an agency document
Least nosing, where one is requiredIRC 2021 R311.7.5.3 (R318.7.5.3 in IRC 2024): a nosing projects at least ¾ in.	in	From a data sheet or an agency document
Most nosingIRC 2021 R311.7.5.3 (R318.7.5.3 in IRC 2024): a nosing projects no more than 1¼ in.	in	From a data sheet or an agency document
A tread this deep needs no nosingIRC 2021 R311.7.5.3 (R318.7.5.3 in IRC 2024): no nosing is required where the tread is at least 11 in deep.	in	From a data sheet or an agency document
Least headroomIRC 2021 R311.7.2 (R318.7.2 in IRC 2024): at least 6 ft 8 in of headroom, measured up from the sloped line of the nosings.	in	From a data sheet or an agency document
Narrowest stairIRC 2021 R311.7.1 (R318.7.1 in IRC 2024): at least 36 in of clear width above the handrail.	in	From a data sheet or an agency document
Tallest flight between floors or landingsIRC 2021 R311.7.3 (R318.7.3 in IRC 2024): a flight rises no more than 12 ft 7 in between floors or landings.	in	From a data sheet or an agency document
Risers that call for a handrailIRC 2021 R311.7.8 (R318.7.8 in IRC 2024): a handrail on at least one side of a flight with four or more risers.	risers	From a data sheet or an agency document
Open-side drop that needs a guardIRC 2021 R312.1.1 (R321.1.1 in IRC 2024): a guard along an open side more than 30 in above the floor or grade below.	in	From a data sheet or an agency document
Shallowest deck stair stringerAWC DCA 6-15, Stair Requirements (p. 20): all stringers shall be a minimum of 2x12. A deck guide's rule: read on a deck stair's page.	in nominal	From a data sheet or an agency document
Longest span of a cut stringerAWC DCA 6-15, Figure 28: a cut stringer spans 6 ft 0 in at most, measured level; past that a 4×4 post on a footing, or a landing, shortens it. A solid stringer may span 13 ft 3 in.	in	From a data sheet or an agency document
Tallest deck stair without a landingAWC DCA 6-15, Stair Requirements (p. 20): a stairway taller than 12 ft 0 in needs an intermediate landing.	in	From a data sheet or an agency document

Stairs for a deck

A deck 36 in above the ground, with risers no taller than the IRC's 7¾ in, takes 36 ÷ 7¾ = 4.645, so 5 risers of 7 3/16 in. With 11 in treads — two 5½ in boards — there are 4 treads and 44 in of run, and the stringer is 4 ft 4 9/16 in along its notches, cut from an 8 ft 2×12.

How many stringers

Under wood treads, stringers stand no more than 18 in apart (AWC's DCA 6), and there are at least three. A stair 48 in wide takes ⌈48 ÷ 18⌉ + 1 = 4.

Composite treads span less, and their makers ask for stringers closer together. Pick the spacing in your tread's installation guide; the figures below are the ones we read, by guide and page:

Treads	Apart at most	Guide
Trex Transcend, Select 2×6	16 in	Trex 2026, p. 36
Trex Signature, Transcend, Select 1×6	12 in	Trex 2026, p. 36
TimberTech Legacy, Reserve, Terrain+, Prime, Prime+	10 in	REV 09/23, p. 8
Trex Enhance; TimberTech Premier	9 in	Trex 2026; REV 09/23
TimberTech Terrain	8 in	REV 09/23, p. 8

At 12 in apart the 48 in stair takes ⌈48 ÷ 12⌉ + 1 = 5 stringers, and Trex asks that a tread be fastened across at least four. We have no tie to Trex or TimberTech; guides change between editions — Trex's 2025 guide gave Select 1×6 9 in — so check the one for the boards you buy.

What the code asks of a deck stair

A stair that climbs 36 in needs a guard along any open side, since it is more than 30 in above the ground, and a handrail on at least one side, since it has more than three risers. Measure the rise from the deck to where the stair will land.