1 Your measurements

Type feet and inches the way you'd write them, like 12' 6" or 30 3/8. Decimals work too.

ft
Measure along the edge where the railing goes. Leave out the stair opening, and add one post for each opening, since the runs on either side of it don't share a post.
ft
Sides join at corner posts, as around three sides of a deck.
ft
ft
AWC DCA 6 allows up to 6 ft for wood guards with 4x4 posts. For a composite or metal kit, use the span on its label.

2 What to buy

Balusters
81
2x2 wood (1-1/2 in), gaps up to 3.88 in
Posts
8
4x4, 7 sections
Railing length
40 ft
each of top rail, bottom rail and rail cap
Side 1: 12 ft2 sections with posts 6 ft on center; 12 balusters per section, 3.88 in gaps
Side 2: 16 ft3 sections with posts 5.33 ft on center; 11 balusters per section, 3.67 in gaps
Side 3: 12 ft2 sections with posts 6 ft on center; 12 balusters per section, 3.88 in gaps
Gap limitEach gap must be less than 4 in (2021 IRC R312.1.3: no 4 in sphere may pass)
Guard heightAt least 36 in above the deck surface (R312.1.2)

Corners share one post, so the sides are counted as one continuous railing. If a side ends at the house wall, the last post can be replaced by a wall attachment the manufacturer or your building department approves. Check each gap with a 4 in ball or block after installing: a twisted baluster can open a gap.

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