Guard Baluster / Picket Count (4-in Sphere Rule)
Balusters on a guard or fence spaced so a 4 in sphere cannot pass.
Example
You enter
- Clear span between posts (in) 96
- Picket width (in) 1.5
- Maximum clear gap (in) 4
You get
- Pickets 17
- Gaps 18
- Actual clear gap 3.92 in (meets the IRC 4 in sphere rule)
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopepickets = ceil((clear span - max gap) / (picket width + max gap)); actual gap = (clear span - pickets x width) / (pickets + 1). A 96 in span with 1.5 in pickets holds 17 pickets at a 3.92 in gap; 3.5 in wide pickets take 13 at 3.61 in. The clear span and picket width both drive the count. The guard height and load are checked separately.
Formula or tablepickets = ceil((rail_clear_in - max_gap_in) / (picket_width_in + max_gap_in)); gaps = pickets + 1; actual_gap_in = (rail_clear_in - pickets x picket_width_in) / gaps.
Source and editionGuard baluster spacing by name (the IRC 4 in sphere rule); first-principles equal-spacing arithmetic.
Free accessThe equal-spacing geometry is public first-principles; the 4 in sphere limit is the code rule, stated by name.
What governsEstimate. AHJ and licensed professional govern.
API field namesrail_clear_in, picket_width_in, max_gap_in, pickets, gaps, actual_gap_in
- Sphere rule 4 in maximum guard opening (IRC R321.1.3 [R312.1.3 in IRC 2021]); 6 in at the riser-tread-bottom-rail triangle, 4-3/8 in for a guard on the open side of a stairIRC
- Equal spacing pickets and gaps evenly distributed across the clear spanfirst principles