Measure the opening before choosing a count
Use the clear distance between the inside faces of the end posts or newels whenever possible. If your measurement includes both posts, enter the separate finished width of each post. Do not assume both posts are identical without measuring them.
Use finished dimensions: Measure the actual baluster face that occupies the opening. Nominal lumber and manufacturer names may not equal the installed width.
How the two mounting layouts work
Inside-mounted: Every baluster remains inside the clear opening. The calculator divides the unused distance into equal gaps at both ends and between every baluster.
Face-mounted: The first and last centerlines align with the inside post faces. Half of each outside baluster overlaps a post face. Verify adequate mounting surface, fastener edge distance, and product instructions.
Level and stair rail measurements
Level-rail marks follow the horizontal opening. Stair-rail marks follow the sloped rail. In stair mode, the calculator uses the entered angle to account for the distance a plumb baluster occupies along that rail. The visual is a layout aid, not a fabrication drawing.
Model residential codes commonly regulate guard openings and contain different exceptions for stair guards and triangular tread openings. The calculator deliberately treats the maximum gap as a user-selected planning limit and does not identify a layout as jurisdiction-approved. Review the 2021 IRC guard provisions only as a model-code reference, then confirm the code and amendments actually adopted locally.
Worked spacing example
For a 72-inch clear opening, 1½-inch balusters, equal end gaps, and a selected 4-inch maximum, 13 balusters occupy 19½ inches. The remaining 52½ inches divide across 14 gaps:
(72 − 13 × 1.5) ÷ 14 = 3.75 inches
The center-to-center spacing is 3.75 + 1.5 = 5.25 inches.
Frequently asked questions
How many balusters do I need?
Enter the clear railing opening, actual baluster width, and your selected maximum gap. The calculator adds enough balusters to keep the equalized gap at or below that selected limit.
Where do I measure a railing opening?
The simplest measurement is between the inside faces of the end posts or newels. You may instead enter the full outside-to-outside section and provide each post width.
What is the difference between inside-mounted and face-mounted layouts?
Inside-mounted mode keeps every baluster inside the opening and uses equal end gaps. Face-mounted mode aligns the first and last baluster centerlines with the inside post faces, overlapping half of each outside baluster onto a post face.
Can I use this for stair balusters?
Yes. Enter the section length along the sloped rail and the stair angle. The resulting marks follow the rail line, but every actual opening—including triangular tread openings—must be field-checked.
Does a calculated gap guarantee code compliance?
No. The maximum gap is selected by the user. Applicable rules vary by project, adopted code, jurisdiction, product, and local amendments. Confirm requirements with the building department.
Continue planning
Use the stair calculator for straight-flight rise and run geometry, or browse all DIY calculators.
Safety disclaimer
This calculator does not design or approve a guard, handrail, post connection, structural attachment, or fall-protection system. Verify every opening, height, load requirement, fastener, material listing, permit, and local amendment before construction. Review the measurement disclaimer.