Why are manhole covers round?

We probably walk over dozens of manholes every day, often not even taking a moment to think about why manhole covers are round. But, there’s actually a really important reason why a manhole is circular!

A peculiar reason is the geometric property of the circle. First of all, when you rotate a square which is of 2 parallel lines first pushes apart and then comes back whereas, in a circle a.k.a a curve of constant width, the line stays exactly the same distance apart. The reason involves making sure we don’t fall down the manhole, i.e. objects that are circular cant fall through other things that are shaped identical to themselves. It’s simply the geometry that saves millions of lives. But occasionally you will find manholes of a rouleaux triangle shape because they are similar to circles.

As we know circle shapes are effective for resisting the compression of the earth around it, the reason why underground pipes, transportation tunnels are circular which makes it facile to dig because round castings are easier to manufacture. Weight also plays a salient role, since manhole covers are heavy weighing up to 250 pounds or 114 kilos, the round shape made it easier for the workers to move and slide by rolling them. The cover is wider than the opening which made it the safety reason. It also fits the human body in a round hole and won’t have to align the cover to fit it on top of the hole.

The query on why the manhole cover is round was popular back in the ’90s when this was asked in a job interview for either Google or Microsoft.