Sphere Rule

Definition

The sphere rule is a guard opening limit: a 4-inch (102 mm) sphere must not pass between balusters or pickets at any height. The US IRC allows 4⅜ in on the open sides of stairs, and other countries set their own limits, typically 100–120 mm; confirm the adopted code. Equal gaps mean n balusters create n+1 spaces: gap = (opening − n × width) / (n + 1). Posts are not counted — enter the clear opening between them. The same math spaces a fence if you raise the allowed gap.

Calculators that use this

• Fine woodworking & cabinetry

  Baluster / Spindle Spacing Calculator

  Evenly space balusters or pickets between posts so the gap stays at or under the 4-inch (102 mm) sphere rule.

• Carpentry & framing

  Stair Rise and Run Calculator

  Size stair risers, treads and stringer length from total rise and a target step height. Works in inches or millimetres.