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Stair Railing Calculator

The stair in; the balusters, the posts and the rail out.

Counts the parts; their strength and fastening are yours and your building department's to check.

3' · 36 in · 3 ft · 0.9144 m

4' · 48 in · 4 ft · 1.2192 m

11" · 11 in · 0.9167 ft · 0.2794 m

7 3/4" · 7.75 in · 0.6458 ft · 0.1968 m

5 risers

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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Balusters

16 2 a tread

4 treads of 11" over 3' 8" of run, at 33.21°.

How many risers 5 risers of 7 3/16"
Each riser 7 3/16"
Total run 3' 8"
Stringer length, top tread a riser below the floor 4' 4 9/16"
Stringers, 12" apart at most 5 stringers
More numbers
How many treads 4 treads of 11"
Angle 33.21 °
Hung flush instead 5' 5 3/4"
Each stringer board, at least 4' 7 11/16" cut from a 8 ft board
Stringer left under the notches 5 1/4"
Twice the riser plus the tread 25 3/8"
Cost of what is ticked $139.28
Rail, nosing to nosing 4' 4 9/16"
Posts 4
DrawingThe project drawn to scale. Stair 1: 7 3/16" × 11" × 3' × 3' 8".7 3/16"11"3'3' 8"Stair 1

Breakdown

Flight Measurements Qty Risers Share
Stair 1 3' × 4' × 11" × 7 3/4" Add ×1 5 risers

What to buy

Shopping list

5 boards $109.90 Reference price $21.98. Type your store's.
4 pieces $99.92 Reference price $24.98. Type your store's.
5 pieces $64.90 Reference price $12.98. Type your store's.
2 pieces $47.68 Reference price $2.98. Type your store's.
16 pieces $39.68 Reference price $2.48. Type your store's.
4 pieces $51.92 Reference price $12.98. Type your store's.
Ticked items $139.28

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 3/16", 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 36", within the 151" the code allows between floors or landings.

Meets IRC 2021 · R311.7.1

48" 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 5 risers, and the code asks for one from 4.

Not checked IRC 2021 · R312.1.1

Required along any open side: the stair climbs 36", more than 30" above the floor below.

How this was worked out

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

Stair 1: risers 3' ÷ 7 3/4" = 4.645, rounded up 5
Stair 1: each riser 3' ÷ 5 7 3/16"
Stair 1: total run 4 × 11" 3' 8"
Stair 1: angle atan(7 3/16" ÷ 11") 33.21°
Stair 1: stringer, marked along its edge 4 steps of 13 1/8", the square set at 7 3/16" and 11" 4' 4 9/16"
Stair 1: bottom riser, as cut 7 3/16" − 1 1/2" tread + 0" flooring to come 5 11/16"

Change:Tread thicknessFlooring still to go down, at the bottom

Stair 1: top tread's cut, below the upper floor 7 3/16" + 1 1/2" tread − 0" flooring to come 8 11/16"

Change:Tread thicknessFlooring still to go down, at the top

Stair 1: every riser cut the same instead every riser 7 3/16": first step 8 11/16", last 5 11/16" 3" apart; a flight may differ 3/8"
Stair 1: board for each stringer 4' 4 9/16" + 5 11/16" × 7 3/16" ÷ 13 1/8" 4' 7 11/16" → 8 ft
Stair 1: stringer under the notches 11 1/4" − 7 3/16" × 11" ÷ 13 1/8" 5 1/4"

Change:Dressed, 8 in and wider

Stair 1: stringers 48" ÷ 12" = 4, rounded up, + 1; at least 4 5
Stair 1: the rail 4 steps along the nosings × 13 1/8" 4' 4 9/16" → 1 × 8 ft
Stair 1: balusters 11" tread ÷ (4 3/8" opening + 1 1/2" baluster), one more than fit: 2 a tread × 4 treads 8 a side

Change:Widest opening in a stair's guardDressed, 2 in up to 8 in

Stair 1: posts 3' 8" of run ÷ 72", rounded up, + 1 2 a side

Change:Most between guard posts

5 risers of 7 3/16" 2 × 7 3/16" + 11" = 25 3/8", against 25" 3' 8" of run

Change:Comfortable step, 2 × riser + tread

6 risers of 6" 2 × 6" + 11" = 23", against 25" 4' 7" of run

Change:Comfortable step, 2 × riser + tread

7 risers of 5 1/8" 2 × 5 1/8" + 11" = 21 5/16", against 25" 5' 6" of run

Change:Comfortable step, 2 × riser + tread

Cost 2 × 8 ft × $2.98 + 16 × $2.48 + 4 × $12.98 $139.28

Assumptions

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

Assumption Value Confidence
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
Widest opening in a stair's guardIRC 2021 R312.1.3 exception 2 (R321.1.3 in IRC 2024): a guard on the open side of a stair has no opening a 4⅜ in sphere passes through - against 4 in on a floor or a deck. in From a data sheet or an agency document
Dressed, 2 in up to 8 inNIST PS 20-20, Table 3: from 2 in nominal and under 8 in wide, lumber is dressed dry ½ in under it - 2 to 1½, 4 to 3½, 6 to 5½. in From a data sheet or an agency document
Most between guard postsAWC DCA 6-15, Figure 30 (Stair Guard Requirements): 6 ft 0 in at most between the posts of a deck stair's guard. Inside a house a rail often runs from newel to newel with no post between; set your own figure. in From a data sheet or an agency document
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

How many balusters a stair railing takes

Count them by the tread. The IRC lets no 4⅜ inch sphere through the guard on a stair's open side (2021 R312.1.3, 2024 R321.1.3), so the gap between two 2×2 balusters — 1½ inches wide once dressed — has to stay under 4⅜ inches. On an 11 inch deck-stair tread one baluster leaves far too much and two leave 4 inches: 11 ÷ (4⅜ + 1½) = 1.87, so 2 a tread. A deck stair 36 inches high has 5 risers of 7.2 inches and 4 treads, so it takes 8 balusters a side, 16 for both sides. Where two would leave a gap of exactly 4⅜ inches, a sphere that size fits, so the page adds one: an 11¾ inch tread takes 3.

Posts

The AWC's deck guide, DCA 6, puts the posts of a deck stair's guard no more than 6 feet apart along the stair. The 4 treads of that stair run 44 inches, so a post at each end is enough: 2 a side, 4 in all. Inside a house a rail often runs from newel to newel with nothing between; type your own spacing under Assumptions and the count follows.

The rail

The rail follows the line of the nosings, from the first tread to the floor above: 4 steps of √(7.2² + 11²) = 13⅛ inches is 4 feet 4 9/16 inches, one 8 foot length a side with room to cut it to the posts. A rail longer than the longest board sold is bought in pieces of that board and joined over a post. The stair's own stringers, treads and risers are on the list too, unticked: tick them if you are building the stair as well.

How high

What IRC 2021 IRC 2024 Height
Guard on a stair's open side, from the nosingsR312.1.2R321.1.234 in at least
Same guard, its top used as the handrailR312.1.2R321.1.234 to 38 in
Handrail, from the nosingsR311.7.8.1R320.234 to 38 in
Guard on a landing or a deckR312.1.2R321.1.236 in at least

A guard is required where the open side drops more than 30 inches; a stair of 4 risers or more needs a handrail on at least one side. The triangle under a guard's bottom rail, between the riser and the tread, may not pass a 6 inch sphere.

What the page does not check

It counts the parts; it cannot see them. Whether the posts hold the load the code asks of a guard, how they are bolted to the stringer, and the heights as built are yours and your building department's to check. The prices are estimates: type your store's. We are not connected with any store or maker.