There is no immutable law demanding that manhole covers have to be round, so why are they? Indeed, there are some square or rectangular ones. However these are rare, generally only found in cases where there is a need for heavy equipment to go through them. The reality is that the vast majority of manhole covers are indeed round.

Why are manhole covers round?

There has to be some logic to it, surely. Alternatively, is it simply a case of manhole covers being made round today because they always have been round in the past? Actually, there are quite a few good reasons why. 

Firstly, there is the geometric fact that round manhole covers cannot fall through its own opening. Looking at this more technically, if you have a square cover the diagonal distance is 1.414 times the length of the sides. This means that if you were to drop a square cover onto its opening at a diagonal angle, it could easily fall through.

1. A circle is not the only shape that would solve this problem, by the way. A particular type of triangle called a Reuleaux Triangle, also fits the criteria, and would be just as suitable for stopping its cover from falling in.
2. Depending on the size of the hole being covered, and indeed, the material from which the cover is constructed, manhole covers may be heavy and difficult to remove. A round cover can be rolled away relatively easily.
3. When you do want to replace the cover there is no need to worry about alignment of a round cover - it can go on at any angle, unlike covers of other shapes.
4. Another factor in favour of circular manhole covers is that they are the most likely to drop back into position should they be bounced off by the forces of a heavy vehicle passing by.

Geometrical reason why are manhole covers round is that it gives the strongest shape. Each point of its edge, sitting on a lip on top of the hole, is the same distance from the center of the cover. In many ways, each of these points has the same strength as the corners of a square of rectangular cover.

Cylinder is the strongest shape for a hole, holding against the compression of the earth surrounding it. Thus, it is easier to dig and maintain round holes.

However, not all manhole covers lead directly down to round holes - often they lead to a larger cavern. Many underground structures are square or rectangular because they meet up with an intersection of pipes, and this makes it easier to have straight, flat sides, rather than circular ones.

Yet even in these cases, the actual manhole covers still tend to be round, because of all of the other reasons I’ve discussed here.

There’s also an economic reason. There is less surface area than for an equivalent sized square cover, and, therefore, less material needed to manufacture it.

So, as you can see, it is not just tradition that dictates why manholes are round. There are good practical reasons why manufacturers produce round sewer covers.