They are only round when they cover a round hole. I've seen a number of covers that are other shapes, often square.

There are engineering reasons to prefer a round hole, but there are sometimes other considerations which push a different shape.

> Why are manhole covers round?

Manhole covers exist in several shapes, as illustrated on the Wikipedia page.

https://en.m.wikipedia.org/wiki/Manhole_cover

That is the argument but actually any lipped shape can give this guarantee. Even historically manhole and utility covers had to be somewhat flush to the ground and so they've always had to be lipped for this.

You can see lots of utility covers that are hinged, square, or other options, but all are lipped for safety. While a circular profile makes the lipping easier, it doesn't seem to influence many utility covers.

You can easily put a square or a rectangular utility cover into the hole, because the length of the side is less than the diagonal. This isn't possible with a circular cover because the diameter is uniform.

The parent’s point is that a lip prevents this for every shape, by making the hole smaller than the side-length.

Only with a sufficiently large lip. The diagonal of the hole has to be less than the shortest side of the lid. As I picture it in my mind this is an excessively large lip.

If you have a very rectangular shape then the lip would have to be large.

Stop for a moment next time you walk in an urban environment. Look for circular utility covers you see in your urban environment as opposed to squaed ones. Look at the features of the circular ones vs the non-circular ones.

One of the reasons this myth irritates me so much is that everyone is so certain that they know exactly the answer but their actual daily experience doesn't line up with the results at all.

I've heard several people offer explanations from a geometric safety option (which is attractive for free-standing covers) to the simplicity of manufacturing (e.g., that it's very easy to make circular molds and get even density compared to square molds) to simply what the contractor suggested. I've also heard people suggest that metal cylindrical templates were something very common to manufacture for a variety of industrial uses.

I'm not certain, that's why I asked the question in the first place.

I'm not sure what features I would look for because the lip on a rectangular cover would be under the lid. How do I know that the rectangular covers have sufficiently large lips to prevent the lid from falling in? I'm willing to accept that in some cases the lids are rectangular and there is a risk of them falling in.

If only this were true for certain utility holes in my neighborhood with rectangular lids. I I've sprained my ankle a few times. I mean, they won't fall in accidentally because of the lip. But you can lift one out and put it in the hole, due to negligence or a taste for vandalism.

> You can easily put a square or a rectangular utility cover into the hole

Then why do rectangular (including square) and triangular manhole covers exist? Rectangular ones are quite common, triangular less so.

Grandparent is correct, lips are what, in practice, prevents manhole covers from falling in, not being circular (which many are not.)

Well maybe in some cases the risk of the lid falling in is acceptable. Or maybe there are other mitigations in place.

I don't know why rectangular utility holes exist, I assume cost.

FYI, I've personally dropped a rectangular manhole cover into a properly lipped hole. They do fit all too well.

Sewer grates are frequently square. For what it's worth.

(Not always, but quite frequently.)

They have the same gravitational-gradient dynamic as manholes.