So Just Why Are Manhole Covers Round?

Interviewing for technical positions can be tricky, but it can also be fun. Expect odd questions, and plan on being able to think on your feet.

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So Just Why *Are* Manhole Covers Round?

Well I’ll tell you, but if all you want is the answer then you completely missed the point of the question.

 

“Why are manhole covers round?” is a now famous interview question considered anywhere from insightful and thought provoking to downright stupid by people on the receiving end.

Why do interviewers ask it?

Well, to be clear, it’s so well known these days that that they shouldn’t really be asking it anymore.

Originally the question was asked simply to see how interviewees responded.

The idea wasn’t to see what the candidate knew, but rather to get a sense for an interview candidate’s ability to think on his or her feet outside of their area of expertise.

The way a candidate approaches problems, thinks of and presents potential solutions, defends them or discards them are all exceptionally relevant to the interviewer.

It’s a simple question. There are simple answers. But the great not-so-secret about this question, and indeed these types of questions in general, that everyone seems to miss is that it’s not about the answer! The interviewer cares much more about how you arrive a whatever answer you get than
what that answer is.

Are you thinking logically? Is there method or madness to the solutions you consider, accept, and discard? Are you even willing to entertain such an absurd question?

If you’re now getting yourself all wrapped up about the answer, if you think there’s a right or wrong answer, then I say again: you’re missing the point! It’s not about the answer – it’s about how the candidate responds to the question.

ManholeOh. You still want the answer.

Fine.

Remember, there is no single or “correct” answer.

The most common answer is simply that a round manhole cover can’t fall into the hole it covers.

But I did tell you that there’s more than one answer. Heavy, round manhole covers are easier for one person to move by standing it on end and rolling. Also, you don’t have to “align” a manhole cover to it’s hole. Any orientation will do.

My late father, a mechanical engineer, claimed an answer I hadn’t heard before: that round manhole covers are easier to manufacture. That’s a nifty answer and it would have done great in the interview except he also claimed that was the only reason and all the others were wrong. Whoops.

And for the record, round is not the ONLY shape that can’t fall into its hole. There’s a class of such shapes referred to as “Reuleaux polygons“. The common characteristic is that they share a constant width or diameter.

But if you walk into an interview having simply memorized answers there’s a good chance you’ll fail miserably. What interviewers care about is that you know why, and how and much, much more.

Just knowing an answer isn’t enough.

This is a minor update to an article originally posted : October 2, 2003

There are 66 comments:

  1. Hugh Reply

    The Dean of the School of Industrial Engineering at Purdue asked our Sr. class the same question. I did NOT gain points by answering “because the holes are round.” He was not amused.

  2. Damaraju Pawan Kumar Reply

    Its not that man holes should be round . there are many shapes which work with same efficiency.

  3. marysh Reply

    Hi,
    I was asked this question in an intreview today, and asked to give 3 answers. I could think of only one, and so was asked to think of more answers, andd so I looked up this website!

    thanks
    Mary

  4. Leo Reply

    I think twice about working for someone who’s still actually using this question in an interview.

  5. Katherine Reply

    I think I had a very unusual answer. My ex used to work for the Parks Dept. and one day was pulling a lawn mower behing him. He was not looking and actually fell into a manhole. Therefore it is to keep people from falling in. Next, my 10yr. old son had a friend who said he had thrown his Aladdin’s lamp into the sewer. When my son tried to take the man hole cover off he smashed his fingers. Next, to keep children from getting in. And third the best I could come up with is to let people get in there with the rats and stuff to work on pipes and things. 1,2,3 had a reason for each. We’ll see if I get the job. :-)

  6. JP Reply

    Leo

    I think its a crime you are “hawking” this crap for $20 on the net.

  7. Greg Reply

    Note: This has nothing to do with a person being a Square.

    Circles and Squares were created equal.

    Have a happy yellow “smiley face” sticker kind of a day!

  8. qUARKA Reply

    Because they are manufactured in that way. Next question is “Why factories do that?” I do not know. I don’t have a degree in this industry.
    So how? :)

  9. Elphaba Reply

    You can fit the same size fat guy into a circular hole and a square hole, so why not just save material and make a circle. Ehehe…*picturing a fat guy jumping into manhole*

  10. Phil Reply

    I just asked someone this question at an interview (I know, I should be shot). The reply was “what’s a manhole cover?”. Serves me right, huh?

  11. Monica Rodriguez Reply

    I am posing this question as a mystery for my geometry students to solve. I am not allowing the use of the Ineternet, but I want to give them clues to solving the mystery. Can you tell me how heavy most manhole covers are?

  12. Jeremy Smith Reply

    One reason sewer covers are round is that no matter what position it is in, it won’t slip through the hole. Circles and squares are not made equally because if you turn a square sewer cover perpendiculer to the ground and diagonally to the hole, and let go, you will hear a clang or possibly a crash as the thing hits the bottom of the sewer.

  13. John Hills Reply

    The question implies that a manhole cover is round universally and eternally. This could never be proved. The question should not have been asked.

  14. Jennie Reply

    In Hong Kong, I’ve witnessed countless square manhole covers, as well as those that consist of 2 triangular covers that make up the square shape. I am totally convinced by the logical (+ health & safety) fact that round covers could not fall into the hole, among others. Though I am not too sure giving this answer would guarantee a total success at the interview – interviewers look for your reasoning right? So simply saying I’ve found opinions that say so isn’t gonna impress.

    Try this instead: How many trees are there in England? (I was asked that yesterday)

  15. Stacy Reply

    Why does it never occur to anyone that manhole covers are round because the manholes themselves are round?

  16. ruchi Reply

    Round manhole covers can be tightened to the manhole by providing grooves or threading to it.

  17. Vance Reply

    I think if I were asked this question in an interview my response (I’m fairly confrontational) would be to ask the interviewer, what are you REALLY looking for? Abstract thought? Logic? Problem solving?

    Ive worked in and around manholes for years and in all parts of the world. I have seen square manhole covers. I have seen square manhole covers fall into holes because they are very heavy, not a little bit awkward, and have a tendency to slide, and the square shape allows for it. I have never seen a square manhole cover fall on anyone and safety is NOT a factor because noone would be working IN a manhole while someone was adjusting a manhole cover above them. Safety IS a factor when the poor schmuck had to lift the thing OUT of the manhole.

    I suspect that the reasons that most if not all new manhole covers are now manufactured round are that 1-the cover will never fall into the hole 2-It can be moved fairly easily, 3-The round shape easily facilitates work being performed in the manhole with the most efficient use of space.

    Maybe the best interview answer is that if ‘I’ were developing a manhole that i was going to be required to use on an everyday basis, and considering my work requirements and all other considerations, round manholes would be the logical choice for me. I cant say why others choose to.

  18. Holly Johnson Reply

    The first answer that came into my mind was, “Because most modern men aren’t square.” Does that qualify me as logical? By the way, what the f… is a manhole cover, actually?

  19. e hansen Reply

    The shaft leading up to the manhole is round and as we all know a round shaped ‘tube’ is not only stronger than a square shaped ‘tube’ but it also uses less material to manufacture. Think about why submarines are round shaped, they have pressure all around it trying to crush it. The same principle applies to a manhole. Therefore the covers are round. Plus they have the added advantage that they will not fall into the hole like a manhole of a different shape would.

  20. Moe Reply

    I grew up in Nashua, NH and remember manhole covers there were triangular. I was told it was because they wouldn’t rattle.
    (Should it be personhole cover?)
    ;-)

  21. Carrie Reply

    My first thought was that if the man in the manhole had to get equipment down there (wires, tools, etc.) the items would not get stuck as easily on a rounded surface. A square surface has corners that items would get caught on. Now that I’m thinking about it…boy am I missing something here!!!

  22. Cecelia Nosa Reply

    When I was asked why Manhole covers are round the first thing that came into my head was because men are not square, silly but true.

  23. Jennifer Reply

    This is a good question to use in an interview with a future reference desk librarian. They get questions along these lines all day!

  24. terry coon Reply

    Why won’t a round cover fall in the hole? Seams to me it would unless the circumference of hole is less than that of cover.

  25. Rick Reply

    A round manhole cover would not fall through its circular opening????

  26. mark Reply

    Manhole covers are round simply because the friggen manholes are round. Why would you make a cover different from the opening?

  27. vivebrulee Reply

    Because round seals in moisture and hence the smell better, than straight edges. The edges would not seal as much during expansion and contraction. Ever wonder why tumblers and dive watches (mostly) are round too? That’s why.

  28. Bart Reply

    A round manhole cover will not fall through a round manhole because the actual circumference of the frame it sits in is slightly greater than the circumference of the whole itself, thus allowing it to lay flush with the street without falling into the whole. No matter how you position the cover, its smallest diameter (all diameters) is larger than any diameter of the hole.

  29. Ali Ree Reply

    I too, knew the answer would be simple…because holes are round….more specifically manholes are round…does that mean I’m simple???/

  30. Keenan Reply

    Well, I never really thought of that until I read it.

    But it totally makes sense, ok round the cover is larger than the opening! So it can’t fall in, even if you rotate it or turn it on it’s side.

    But… If you use a rectangle for example, still the same effect, but if you turn it on it’s side or rotate it, it could fall in, or not line up.

    Other “Ideas” around the Manhole.

    [Content copied from Wikipedia removed.]

  31. cameron Reply

    many of the above answers are true and correct when they are brought together. yes the hole is round and yes the seal is better and yes it cannot fall into the hole. there are other factors too, such as a circle being the strongest shape therefore alowing maximum strength and being able to cope with the huge volume of weight and pressure from above the street. the portability aspect of being able to manouvre the cover due to its rondness is also true. therefore manhole covers MUST be round!!!! and no i did not miss the point of the question

  32. Just J Reply

    Leo

    As your father claimed they were easier to manufacture round, and that suggestion was the *only* reason begs the question *why* are they easier to manufacture that way?

    Any idea?

  33. john neil Reply

    the simple answer is round hole square peg…..
    other answers ease of replacing in hole ..equal size means no manouvering to match hole .it simply drops in.
    there are many answers to this question…….

  34. mads Reply

    man holes are round for may reasons,
    safety- they cannot fall through their own hole.
    cost- they use less materials than other shapes because it’s circumfrence is smaller.

  35. Arvin Kumar Reply

    Round manholes covers will not fall in as a lot of people have commented above and so will other shapes but from the engineering point of view there is one advantage the round shape offers that other shape cannot match round shape does not have sharp corners that could pop out and injure animals in the days of the cart or damage cars today it also is less likely to have corners that break away thus requiring the manhole to be replaced as a triangle or sqaure shape would

  36. Richard Armstrong Reply

    They are round for ease of use, but the best reason is when a heavy object such as a car drives on top of them the weight of the car is evenly distributed along the outer edge of the manhole, thus equalizing the pressure on the whole unit. A square or rectangle will have uneven pressure on the edges and will fail over time.

  37. Moe E. Reply

    Hehe, I was asked this question in an interview and it totally threw me off. I had no idea where to begin so I started with the closest thing I could think of. Here’s what I said:

    The underlying sewers are all round. The pipes are all round. The round shape is chosen because a circle is the strongest shape for supporting the ground above it, stronger than a square or triangle.Therefore, since the underlying pipes as well as the pipe leading to the streets are round, the manhole cover would be round.

    It was a random answer I thought up on the spot.

    I’m shocked anyone is still asking this question, it’s gotten so much publicity.

    Leo
    23-Apr-2010

  38. John Withers Reply

    Just had a quick read through the comments and I’m amazed that my first thought doesn’t match anyone elses. Corners promote cracking and subsequent failure. Curves on the other hand do not. I was also surprised the most common answer was that a circular cover can’t fall in. That would be my last one. Think about the work necessary to move a square/rectangular manhole into a vertical position to enable it to fall through. It’s not something that could be done accidently by a vehicle. Finally I love the one about rolling the manhole being easier. That’s a great one. In summary my comment above pretty much proves the point of your article lol.

  39. Glenn P. Reply

    Uh, Mike, I assume that’s a typo for “can’t”…?

    Heheheheh…

  40. set Reply

    we’ll i had this question asked some weeks ago, and my answer does not seem to match any. my answer was it’s round because the circle/cylinder is the one of the strongest shapes. if the manhole was square, expansion/contraction of the surrounding material (cement) will eventually make the square shape fail. not to mention that the circle shape works well with the vibrations created by the passing vehicles. I also added what bothers me is why are the manhole covers have a concave/dome profile shape.

  41. Muhammad Reply

    i was asked the same Q today at an interview. i answered ” thay are round so that pedestrians and bikes n vehicles tyres does not get hurt.haha.did not know what to say.

  42. B-retired Reply

    Other possible answers:
    “Why do you believe all manhole covers are round?”, challenging the validity of the presumption prior to spending resources to resolve the wrong problem.

    “If you accept the premise that some manholes are round, it would be only common sense to cover them with a matching shape”, applying the simplest of logic and again identifying the determinant factors, which may not be immutable.

    and, finally,
    “The fact that you are even asking me that question reveals more about the immature style in this company than you may have intended.” This shows a strong negotiating position.

    Actually that last one is more likely to get you shown to the door.

    Leo
    18-Mar-2011
  43. Saranya Reply

    I feel, in case of circle 1) weight covered on the lid is distributed evenly due to its shape, 2) if it slides from its position it will not drop inside till slided part is more than diameter of cover. But in case of rectangle if “slided part>=short side of rect.” it can slide

  44. David Lewis Reply

    Why are manhole covers round? Obvious! To provide a dumb but clever-sounding question for job interviewers who have no idea how really to assess a candidate.

    “No Hire”

    Leo
    23-Jul-2011
  45. Matt Reply

    Or, if diameter of round hole == width of square hole i.e. same ability to fit through it, the round one uses less material to make i.e. is cheaper

  46. J.M. Reply

    “Although the premise that manholes are all round has prevailing evidence on the streets, I noticed that at the threshhold of the parking lot, your company is using rectangular manhole covers, which shows a bold, innovative and cutting-edge approach to effectively adopting technology…..etc, etc” [stick around until the latest break for interviewer and watch them dash out to the parking lot/deck]

    Good times…

  47. A.V. Reply

    So, the answer for why manhole covers are round is the same as for why some persons are bisexual–“Any orientation will do”.

    Interesting. (provide recording of interview to HR compliance for sexual harassment review)

  48. bagelguru Reply

    Original manhole covers were made of wood core pieces of tree trunks: round, strong, cheap, abundant.

  49. Shuvo Reply

    What if I answer that Manhole Covers are round because “Manholes” are round…!!!! :p

  50. frederick myers Reply

    You will come across some tricky interview questions when it comes time to interview for a job. It may seem as though some job interview questions are designed to put you “on the spot.” To a certain extent they are.Peculiar and indiscriminate interview questions may not seem politically correct, but there is always an interviewer ready to throw one at you, and they expect an answer as well.

  51. Christo Reply

    The real question of course should be why manholes are round? The covers are obviously round to fit the manhole.

  52. Stevie Reply

    This is so insulting it is becoming nauseous. Have been through a few interviews in my time, know what I learned? If you can get through the HR interviewer, telling them anything they want to hear, you might just get a phone call. Now THAT’S insulting.

  53. Kate Reply

    I told my interviewer I knew the manhole answer already (fall in the hole) and he said there were three additional ones. Then I tried to guess: “well, I think one of the reasons would be with a circle there seems to be an equal distribution of weight, but with a rectangle for example or even a square there is more weight stress on a single point which over time may cause the cover to buckle.” He said he hadn’t heard that one and because it made sense he gave me the other three. That was a fun interview.

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