Why are manhole covers round?
Short answer
Round covers are hard to drop through their own openings because their width is the same in every direction. They also have no corners, can be rolled when moved, and spread loads around their rims.
How it works
A circle has the same distance from its center to its edge in every direction. That means a round cover whose diameter is larger than the opening cannot fall through it when turned. A square cover, by contrast, can present its diagonal and sometimes pass through an opening of the same general width.
Manholes provide access to underground sewers, water pipes, and other utilities. Because their covers can be heavy, rolling one is often easier than carrying it. A circular shape also avoids sharp corners and makes it practical to support the cover around its rim.
Key point
Not every access cover is round. Square and rectangular covers are also used when the underground equipment or street layout calls for them. Round covers are simply a useful combination of safety and practical handling.