Why is a man hole cover round?

Question

Adam · Accepted Answer

There are a few answers that would qualify as "correct", but I suspect this question won't be making the rounds much longer as this is becoming the stereotypical case example question which is meant to "see how you think".

A circular manhole cover can never fall into the hole, as a square cover could by turning it on a diagonal, for example.
Also, a worker can easily roll the cover without every having to lift it.

Dave · Answer

The real reason manhole covers are round is because the manhole is round.  It certainly wouldn't do to put a square cover on a round hole.  (However, that's likely not the answer they are looking for.)

Sigiloso · Answer

Weight displacement. So the cover does not fall into the hole no matter where it is or what is on it.

Tim · Answer

In addition to Adam's answers, they are also round because they will fit on the manhole regardless of which way you put them on.

CN · Answer

This was a Microsoft interview question at one point. It is round so that the cover can't fall into the hole the way a square or rectangular cover could if inserted diagnally. It's also easier to move (by rolling it).

Tim · Answer

It's also the maximum coverage for the minimal amount of material - making it the cheapest shape in terms of material cost.

Jeff in Boston · Answer

Because if it were star-shaped it would have pointy bits that would be ouchy and tear your clothing.

KC · Answer

In addition to Adam's observations (which contain the very best answers to this question), a round manhole cover has no sharp corners, so if for any reason it did not sit right in the manhole, and a car or truck ran over it, it would be less likely to puncture the vehicle's tire.

Maryann · Answer

Because the manhole pipe is round.

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