Why are manhole covers round?

The most common answer is that a round cover cannot fall into the hole it covers. A circle has the same width in every direction, so a cover resting on a lip slightly smaller than itself cannot be turned to slip through. The official answer from New York City's Department of Environmental Protection, as reported by Untapped New York, is that this is the principal reason. Several other practical reasons are also given, and Live Science notes that there is no single overarching answer. [1] [2]

A square or rectangular cover can be tilted diagonally and dropped through an opening of the same shape. An oval is longer than it is wide, so it can also be slipped through. A circle has constant diameter, so no orientation lets it pass through a slightly smaller circular hole. [3] [4]

Manhole covers are heavy, generally 200 to 300 pounds (90 to 136 kilograms), so a cover falling into a manhole would be a real hazard. Covers sit on a lip around the opening's edge. [1]

A round cover can be placed in any orientation, with no need to align it with the hole. Untapped New York presents this as a likely secondary reason, since a square, rectangular, hexagonal or oval cover would fit in only one or a few positions. [1] [5]

A removed round cover can also be moved by standing it on its edge and rolling it. Live Science adds that a round cover is cheaper to produce than a square one of comparable width because its surface area is smaller. [2] [5]

Josh Nelson of San Jose State University told Live Science that manholes are typically made with rotational tools, which produce circular shapes, so a circular cover fits best and anything else would be more complicated and less economical. The Ask Leo! author recounts that his father, a mechanical engineer, held manufacturing ease to be the only reason, a claim that other sources do not support. [2] [5]

No. Any shape of constant width cannot fall through a slightly smaller matching hole, and there are infinitely many such shapes. The simplest alternative is the Reuleaux triangle, built from three circular arcs, and it has the least area of the family. Such shapes are called Reuleaux polygons when built on polygons. Circles remain the practical choice, partly because of the other advantages above. [3] [5]

Manholes date to the mid-19th century, when cities built underground sewers and needed human-sized access points. Untapped New York says early covers were not necessarily round and could be square, rectangular or hexagonal. The question became widely known in the 1990s when Microsoft reportedly asked it in job interviews, apparently to invite creative answers rather than a single correct one. [2] [1]