What Is the 4 Ball Rule for Railing?

Edited by: View Mate All Glass Railing

The 4 ball rule for railing, also known as the 4-inch sphere rule, is a widely adopted safety standard in modern buildingcodes. It states that a 4-inch (102 mm) diameter sphere must not be able to pass through any opening in a railing system. This regulation is designed primarily to prevent children from slipping through gaps and falling from elevated areas.

The rule is commonly enforced in residential and commercial applications, including balconies, decks, stair railings, and guardrails, under international codes such as the International Building Code (IBC) and International Residential Code (IRC). For stair railings, some exceptions allow slightly larger openings in specific locations, but the general principle remains focused on child safety.

Glass railings, aluminum railings, cable railings, and vertical picket systems are all required to meet this spacing limitation. As open-concept designs trend upward in 2025, manufacturers increasingly engineer railing systems to balance transparency with strict code compliance

Understanding the 4 ball rule is essential when selecting or designing railings to ensure safety, regulatory approval, and long-term usability.