Equal infill spacing / Arithmetic study
Illustrative defaults. The target is your design input, not a code limit supplied by this tool. Confirm applicable requirements and construction tolerance separately.
14 openings, including both ends. Actual calculated gap: 3.750 in.
Level opening only. Stair slope, bottom gaps, irregular infill, post spacing, load and fastening are excluded. Above 100 balusters, the drawing switches to a five-baluster spacing detail; the calculated total and worksheet still cover the full opening.
For N balusters of width B within clear opening L, each of N + 1 equal gaps is (L − N × B) ÷ (N + 1). Choose the smallest whole N that does not exceed your target. Use actual measured widths, and allow the responsible installer to account for tolerances.
At a 72 in opening with 1.5 in balusters and a hypothetical 3.75 in target, the result is 13 balusters and 3.75 in gaps. This is an arithmetic example, not a universal installation recommendation.
Coordinate the complete assemblyUse the opening study to describe the geometry you are considering. A measured opening does not establish that a guard, handrail or connection satisfies the requirements for its location. Keep those decisions separate so the sketch can be reviewed without appearing to approve an assembly.
Record whether the element is a guard, a graspable handrail, a decorative cap or a combination. Note stairs, changes in level, corners and attachments to the building. These conditions can introduce different questions even where the material looks the same.
State where the opening was measured and whether dimensions are clear openings or center-to-center distances. Record the installed components and any uncertainty. Do not transfer a measurement from a photograph to the final layout without checking it on site.
An owner wants to replace the cap on an existing guard. The cap dimensions help describe the new piece, but they do not establish the condition of the posts or their attachments. The handoff should identify the cap change and retain a separate unresolved question about the supporting guard.