There are a few reasons why manholes are round, the first being the shape does not allow for the cover to fall into the manhole. When you have a square manhole cover, you run the risk of it accidentally falling into the manhole as the diagonal length of the cover and the vertical and horizontal lengths are different. Whereas if you have a circular cover, the diameter is the same the whole way around, not allowing for the cover to fall into the manhole accidentally.

Another reason for manhole covers to be circular is to allow them to best handle the pressure of the earth. As with any opening into the earth, there is constant pressure from the earth to close the gap. This compression can easily lead to cracks in other shapes at their joining point. As a circle doesn’t have a joining point it is able to disperse the compression evenly, drastically reducing the risk of cracks. This is also why tunnel boring machines create circular holes.

The final reasons as to why manholes are round are because they are easier to manufacture and easier to move. As manhole covers can weigh between 10kg to 30kg, they are hard to lift and move no matter what shape they are. By making them round it allows the workers to lift and roll the cover along the ground, instead of lifting it off the ground.

Manhole cover shapes are generally dictated by the local or state government in partnership with the companies that are providing the underground utilities. This is why you will find circular, square, rectangle and triangle covers. The prominent thing to highlight here is that not all covers are manhole covers. Manhole covers are specific to the covers that are designed for workers to enter, whereas smaller ones are designed as direct access points to underground utilities.

Square manhole covers rarely cover a deep access point, rather they tend to cover large and shallow junctions. Reuleaux triangle covers are similar to circular covers in that they also have a constant diameter so cannot fall into a manhole. However, reuleaux triangle manhole covers are much more expensive to manufacture and are harder to move. In recent years there has been a push to streamline manhole cover shapes, with a focus on round covers.