How to space balusters on a stair rail

Preserve the intended opening as the rail slopes

The eye usually reads vertical balusters as a repeated rhythm, but the tape may be running along an angled rail. A plumb member occupies a longer distance along that slope than its horizontal width. Treating those measurements as interchangeable can give the wrong count and gaps.

Choose an opening target with room for actual stock variation and fitting tolerances, within the requirements for the guard. Then mark cumulative centres from one post. The objective is a consistent finished opening, not a convenient repeated number on the tape.

Measure along the rail

For a stair guard, measure the clear opening between post faces along the sloped rail. Measure every cumulative centre mark from the same inside post face along that line; do not repeatedly add a rounded spacing.

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)

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.

Keep gap and rail measurements distinct

Maximum gap is entered horizontally between vertical members. The opening between post faces and all centre marks are measured along the sloped rail. The planner shows both the resulting horizontal gap and its along-rail distance. Check the actual opening with the gauge or test required for the assembly.