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Stairs & stringers guide · 6 min read

How to space balusters on a stair rail

Correct vertical-baluster width for the stair angle, then create equal centre marks along each sloped post bay.

01

Measure along the rail

For a stair guard, measure the clear opening between post faces along the sloped rail. Transfer the first centre and repeating on-centre marks along that same line.

02

A vertical member projects wider on a slope

A vertical baluster intersects a sloped rail at an angle, so it occupies more distance along the rail than its horizontal actual width. The calculator corrects that effective width before dividing the remaining opening.

Effective width along rail = actual width ÷ cos(stair angle)
03

Check the completed openings

  • Use actual baluster width, not nominal size.
  • Keep balusters plumb unless the system is specifically designed otherwise.
  • Verify the top and bottom of every opening; geometry and fabrication tolerances can create a larger point.
  • Confirm opening limits, climbability, strength and fastening requirements locally.

Equal spacing is only one part of guard safety. A calculated gap does not validate the posts, rails, balusters or their connections.

Put it to work

Use the numbers on your job.

Enter the real field dimensions in the matching UC Field Tool, then verify the result against the actual assembly and applicable requirements.

Open baluster calculator
Next guideMeasure total rise

Plan carefully. Build to the rules that apply.

UC Field Guides and Field Tools are planning aids, not engineered designs, permits, product instructions, or code approvals. Verify field conditions, materials, loads, connections, drainage, guards, dimensions, and every other requirement with the governing code, manufacturers, local authority, and qualified professionals for your project.