Baluster spacing on angled stair rails

Choose a measuring convention and keep it

Horizontal dimensions can be convenient when transferring a plan or checking plumb balusters. Along-rail dimensions can be convenient when marking the actual sloped rails on a bench. Both can describe the same bay, but only after converting width, opening and spacing consistently.

A measured rail angle is useful on an existing installation. Matched rise and run are useful when planning a new one. Avoid mixing a whole stair height with a footprint that counts a different number of intervals; that creates an angle unrelated to the repeating stair pitch.

Define where the opening is measured

On a sloped guard, the distance along the rail and the horizontal distance between vertical balusters are not the same. The applicable opening limit may be checked differently depending on the guard system and local rules.

Calculate the rail slope from rise and run

Choose Stair rail in the calculator, then Rise & run. Enter one finished riser height and one tread run, excluding nosing. Alternatively, measure a matching vertical rise and horizontal run between the same two points on the rail. Use Known angle when you have measured the rail directly.

A 7.5-inch rise and 10-inch run give an angle of about 36.87 degrees. Do not divide the full floor-to-floor height by the stair footprint to find this angle: the landing configuration can give different numbers of rises and runs.

Rail angle = arctan(matching rise ÷ matching horizontal run)

Keep vertical balusters vertical

When balusters remain plumb under an angled rail, their intersection points along that rail spread differently than a horizontal layout. Establish a consistent datum line, transfer centres vertically, and verify the clear opening at the controlling location.

Mock up one section

• Confirm post faces and rail angle.
• Use actual baluster width and connector geometry.
• Lay out the first and last baluster before filling the field.
• Check openings with the required gauge or measurement method.
• Verify that fasteners and rail connections suit the complete guard system.

Use one measurement convention

Choose whether the clear opening was measured along the rail or horizontally. Switching that control converts the measurement while retaining the same bay. Enter the actual width of each vertical baluster and the desired clear gap horizontally. The tool divides both width and gap limit by the cosine of the angle before laying out centres along the rail.

At 30 degrees, a 1.5-inch vertical baluster occupies about 1.732 inches along the rail, and a 4-inch horizontal gap corresponds to about 4.619 inches along the rail. Projecting only the member width would use the wrong gap convention.

Along-rail opening = horizontal opening ÷ cos(angle)
Along-rail width = horizontal width ÷ cos(angle)
Along-rail gap limit = horizontal gap limit ÷ cos(angle)

Worked stair bay and fitting room

At a 7.5-inch rise and 10-inch run, a 72-inch horizontal opening measures 90 inches along the rail. With 1.5-inch vertical balusters and a 4-inch horizontal target, 13 balusters create fourteen 3.75-inch horizontal gaps. The first centre is 5.625 inches from the lower post along the rail; the centre-to-centre spacing is 6.5625 inches along that rail.

Extra balusters tightens the interior spaces while preserving the chosen end condition: equal end gaps or end balusters against the posts. Use cumulative centre marks from one post rather than repeatedly adding rounded spacing. The diagram’s member heights and connections remain schematic; it does not specify baluster cut lengths.