The reason for manhole covers is usually round

The manhole cover in a round shape allows easy installation, transportation as well as preventing risk of leakage.

The round lid is easier to install into the manhole.

Why are manhole covers often circular? The manhole cover is quite heavy, so the round shape allows it to be transported easily from one place to another by rolling.

In addition, the round lid is also easier to install into the manhole. While the square or triangular lid needs to correct the angle and the edge.

Besides, the edge of the manhole usually has a small groove, which makes the crater smaller than its lid, preventing it from falling into the hole during installation. Meanwhile, the square or rectangular lid can still fall into the manhole. Do you know why?

That's because the edge of the square or rectangle is always smaller than their diagonal. The circular manhole cover is never trapped because of how you rotate it, the pit cover is always wider than the crater.