Inputs
in
Measured between the inner faces of newel posts.
in
Clearance from left post to first baluster.
in
Clearance from right post to last baluster.
in
in
Common residential limit is 4 in (101.6 mm). Verify locally.
When set, results compute using this count and check compliance.
Results
Baluster count
—
Interior clear gap
—
Edge clear gaps
—
Center‑to‑center spacing
—
Max clear opening
—
Compliance check
—
Formula used
Let L = opening length − left offset − right offset
Let B = baluster width
Let g_max = allowed maximum clear opening
Let mode weighting β = 1.0 (Equal), 0.5 (Half-edge), 0.0 (Zero-edge)
Auto-count (Equal mode):
Choose smallest integer N satisfying
(L − N·B)/(N + 1) ≤ g_max
i.e., N ≥ (L − g_max)/(B + g_max)
Then the equal clear gaps are g = (L − N·B)/(N + 1)
Generalized gaps with edge weighting β:
Weighted gaps count W = (N − 1) + 2β
Interior gap g_i = (L − N·B)/W (for N ≥ 1 and W > 0)
Edge gap g_e = β · g_i
The governing clear opening is max(g_i, g_e)
Center‑to‑center spacing:
s_cc = B + g_i (interior)
This tool selects the minimal baluster count that meets the specified opening limit in Equal mode when count is not locked. Always confirm against your local building code.
How to use
- Enter the clear span between inner faces of the posts.
- Optionally set end offsets to fine‑tune edge reveals.
- Enter baluster width and your maximum allowed clear opening.
- Pick a layout: Equal, Half‑edge, or Zero‑edge.
- Click Compute. Review gaps, count, and compliance.
- Optionally lock a baluster count to test alternatives.
- Export results via CSV or PDF for submittals or records.
Example data table
| Opening | Offsets L/R | Baluster | Max Opening | Mode | Count | Interior Gap | Edge Gap | Center‑to‑Center | Compliance |
|---|---|---|---|---|---|---|---|---|---|
| 72 in | 0 / 0 in | 1.25 in | 4.00 in | Equal | 13 | 3.98 in | 3.98 in | 5.23 in | PASS |
| 72 in | 0.5 / 0.5 in | 1.25 in | 4.00 in | Half‑edge | 13 | 4.12 in | 2.06 in | 5.37 in | FAIL |
| 1800 mm | 0 / 0 mm | 32 mm | 100 mm | Equal | 17 | 98.9 mm | 98.9 mm | 130.9 mm | PASS |
Example rows are illustrative. Use Compute for your exact inputs.
FAQs
Many jurisdictions limit the clear opening to 4 in (101.6 mm), intended to prevent a 4‑inch sphere from passing. Always verify your local code requirements before building.
Yes. Choose Equal for uniform edges, Half‑edge for smaller end gaps, or Zero‑edge for tight ends. The governing opening is the largest gap reported.
Enter a count in “Lock baluster count”. The calculator will compute gaps for that count and show pass/fail against the selected opening limit.
Toggle “Use mm” to switch between imperial inches and metric millimeters. The opening limit field automatically updates the default to 101.6 mm in metric.
Interior center‑to‑center spacing equals baluster width plus the interior clear gap. This is often useful when laying out marks along the rail.
This model assumes a straight rail measured horizontally between posts. For curves or stairs, measure along the rail and confirm with local stair/guard requirements.
Increase the baluster count or width, adjust edge offsets, or select a layout with more interior gaps. The tool highlights when the governing opening exceeds the selected limit.
Disclaimer: Spacing rules vary by jurisdiction and occupancy. Use this as a planning aid only and verify with applicable codes and an authority having jurisdiction.