Railing picket spacing calculator
Put in the run, the posts and the picket size and get the pickets per bay and the real clear gap, checked against the 4" sphere rule in the 2024 IRC or IBC. Then the steel to order: feet and pounds of picket, rail and post stock.
Updated November 2026
Quick answer
The 2024 IRC limits the clear gap between pickets to 4", so a 4" sphere can't pass. On the open side of a stair it allows 4-3/8", and the IBC holds stairs to 4". A 10 foot run with 3 posts of 2" × 2" × 1/8" square tube and 1/2" square bar pickets takes 12 pickets per bay, 24 in all, at a 3-15/16" gap. Steel weight: 115.4 lb.
How picket spacing works
Posts split the run into equal bays. In each bay, n pickets make n + 1 equal gaps, one at each post. Use the fewest pickets that bring every gap to the limit or under, then space them evenly.
Worked example: a 10 foot guard with 1/2" square bar pickets
- Bays: 3 posts make 2. Opening: (120 − 3 × 2) ÷ 2 = 57".
- Pickets: ⌈(57 − 4) ÷ (0.5 + 4)⌉ = ⌈11.78⌉ = 12 per bay, 24 in all.
- Gap: (57 − 12 × 0.5) ÷ 13 = 3.923", which reads 3-15/16" rounded up on the tape.
- Layout: pickets 4-7/16" on center, the first one centered 4-3/16" off the post face.
- Steel, from the Metal Weight Calculator's lb/ft:
- 24 pickets × 34" = 68.0 ft at 0.85 lb/ft = 57.9 lb
- 2 rails × 10'-0" = 20.0 ft at 1.28 lb/ft = 25.5 lb
- 3 posts × 42" = 10.5 ft at 3.05 lb/ft = 32.1 lb
The weights come from the same math as the square bar weight calculator, round bar weight chart, flat bar weight chart and square tube weight chart. The takeoff is full lengths, so add your drops and cut waste before you order.
Building the stair too? The stair stringer calculator lays it out and tells you when the open side needs a guard. Coping round rail to round posts? The pipe coping template prints the cut line full size to wrap on the rail.
Guard code: IRC vs IBC (2024)
The IRC covers one- and two-family houses and townhouses. The IBC covers commercial, industrial and multifamily common areas. These are the model codes, with section numbers from the 2024 editions.
The fine print on these rules
- IRC 2024 R321.1.1: More than 30" down to the floor or grade below.
- IRC 2024 R321.1.1: Checked at any point within 36" horizontally of the open edge.
- IRC 2024 R321.1.2 Exception 1: From the line through the nosings. Where the guard top is also the handrail, 34" to 38" (Exception 2).
- IBC 2024 1015.2: More than 30" down to the floor or grade below.
- IBC 2024 1015.2: Checked at any point within 36" horizontally of the open edge.
- IBC 2024 1015.3: From the line through the nosings. Only in R-3 and inside R-2 dwelling units: 34" on the open sides of stairs (1015.3, exception for open sides of stairs), and 34" to 38" where the top of the guard is also the handrail (1015.3, exception for guards that serve as handrails).
- IBC 2024 1015.4: Inside R-2 and R-3 dwelling and sleeping units, 4-3/8" on the open side of stairs (1015.4, exception for stairs in dwelling units).
Questions welders keep asking about railings
How far apart should balusters be?
Close enough that the clear gap between them is 4" or less (2024 IRC R321.1.3, IBC 1015.4). Measure the gap, not center to center: with 1/2" square bar pickets a 4" gap is 4-1/2" on center. On the open side of an IRC stair the gap can be 4-3/8" (R321.1.3 Exception 2). Most shops lay out a little under the limit so a picket a hair out of plumb doesn't fail.
What is the 4 inch sphere rule?
A guard can't have an opening a 4" ball would pass through (R321.1.3). That covers the space between pickets, between a picket and a post, and under the bottom rail, because the rule runs from the walking surface up. The IRC has two exceptions: the triangle formed by a riser, a tread and the bottom rail can pass a 6" sphere (R321.1.3 Exception 1), and the open side of a stair can pass 4-3/8" (R321.1.3 Exception 2). The IBC uses the same 4" (1015.4), lets the part of a guard from 36" to 42" up pass 4-3/8" (1015.4 Exception 1), and allows 6" at the stair triangle (1015.4 Exception 2).
How tall does a guard rail have to be?
36" under the 2024 IRC (R321.1.2), measured from the deck or floor, and 42" under the IBC (1015.3). On the open side of an IRC stair it can be 34", measured plumb from the nosing line (R321.1.2 Exception 1). A guard is required where the walking surface is more than 30" above the floor or grade within 36" of the edge (R321.1.1, 1015.2). Some states raise the IRC height, so check locally.
How many balusters do I need for a 10 foot railing?
On the calculator's starting railing (3 posts of 2" × 2" × 1/8" square tube, 1/2" square bar pickets, IRC level edge) a 10'-0" run has 2 bays of 4'-9" clear. Each bay takes 12 pickets for a 3-15/16" gap, so 24 pickets in all. Wider pickets or more posts change the count, so put your own sizes in.
How much does a steel railing weigh?
The 10 foot railing above comes to 115.4 lb of steel: 57.9 lb of pickets, 25.5 lb of rail and 32.1 lb of posts, about 11.5 lb per foot of run. The weights use the Metal Weight Calculator's math, with mild steel at 0.2836 lb per cubic inch.
Is a 4 inch gap between balusters OK?
On paper, yes. The IRC says a 4" sphere must not pass (R321.1.3), and a rigid 4" ball doesn't fit through a 4" opening. In the field, pickets wander a little when you weld them, so a layout right at 4" can fail on one bay. Lay out to a target like 3-3/4" with the tighter gap option to leave room.
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