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So Just Why Are Manhole Covers Round?

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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.

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The interview question

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

And, to be clear, it’s so well known these days that it simply shouldn’t be getting asked any more.

If the answer misses the point, why was it used in the first place?

The journey is the goal

It’s all about how you respond to the question.

The idea wasn’t to see what you know, but rather to get a sense for your ability to think on your feet outside of your area of expertise.

The way you approach problems, think of potential solutions, and present, defend, or discard them are all exceptionally relevant to the person asking the question.

It’s a simple question. There are simple answers. But the thing about this question – and indeed these types of questions – is that it’s not about the answer! Everyone seems to miss that.

The interviewer cares much more about how you arrive at whatever answer you give 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 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 you respond to the question.

Answers

ManholeBut you still want an 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. The lips that keep it from falling in when properly in place make a hole smaller than the cover itself. That keeps it from falling in regardless of how it’s placed in, on, or over the hole.

Another thought is that heavy, round manhole covers are easier for one person to move: you stand it on end and roll.

You don’t have to “align” a manhole cover to its hole. Since it’s round, any orientation will do.

My late father, a mechanical engineer, provided an answer I hadn’t heard before: round manhole covers are easier to manufacture. That’s a nifty answer, and it would have been great in an 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 an entire class of such shapes, referred to as “Reuleaux polygons“. One common characteristic is that they share a constant width or diameter.

Meanwhile, back at the interview

If you walk into an interview having simply memorized answers, there’s a good chance you’ll fail miserably.

Expect lots of questions about why the answer you recite is correct, and how you arrived at that answer, step by step. That’s what interviewers care about: the whys and hows behind your answer.

Just knowing an answer isn’t enough.

And, for the record, if anyone really is using this question in a real interview after all this time, I’d question their interviewing skills. There are appropriate ways to bring it in, but still … there are other, better, and lesser-known alternatives.

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109 comments on “So Just Why Are Manhole Covers Round?”

  1. 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.

    Reply
  2. 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

    Reply
  3. 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. πŸ™‚

    Reply
  4. 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!

    Reply
  5. 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? πŸ™‚

    Reply
  6. 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*

    Reply
  7. 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?

    Reply
  8. 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?

    Reply
  9. 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.

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

    Reply
  11. 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)

    Reply
  12. 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.

    Reply
  13. 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?

    Reply
  14. 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.

    Reply
  15. 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?)
    πŸ˜‰

    Reply
  16. 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!!!

    Reply
  17. 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.

    Reply
  18. 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.

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

    Reply
  20. 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.

    Reply
  21. 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.

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

    Reply
  23. 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.]

    Reply
  24. 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

    Reply
  25. 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?

    Reply
  26. 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…….

    Reply
  27. 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.

    Reply
  28. 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

    Reply
  29. 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.

    Reply
  30. 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

    Reply
  31. 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.

    Reply
    • From my observations manhole covers are not rolled back into place. First they are usually removed by inserting a hooked rod (https://www.grainger.com/category/manhole-cover-lifters/confined-space-equipment/safety/ecatalog/N-hlk) into a small hole or notch in the cover. This allows the worker to lift one edge of the cover above the surface of the street and drag it out of the way exposing the hole. When done he repeats the operation dragging the cover back into place with the rod before removing it. No rolling involved.

      Everyone that remarked on rolling the cover back into place failed to mention how the cover was removed in the first place. There are no places to grasp the cover only the small hole I mentioned above. That implies a tool of some kind to get a grip on the cover. If the cover is raised on its edge to move it this would be a safety issue for sure because lowering it to a flat position when removing or replacing it would be a potential finger crusher hence the tool to keep tender fingers far away from the cover when it’s being moved. Toes, hopefully, are being protected by steel toed boots.

      Reply
        • No it won’t. This specialised drill is called a morticer (or mortiser). You can look it up on Wikipedia. In my opinion round manhole covers are preferred because of all the practical reasons given above: ease of manufacture; ease of removal and replacement (no need for alignment); equal distribution of forces; and sewers are round. But there is one overwhelming case for rectangular manholes – when the hole itself is rectangular! And why should a hole be rectangular? When it is intended to give access not to the sewage system but conduits carrying gas pipes and/or electricity cables. These may require meters and/or monitoring devices, and these cannot easily be attached to a concave surface.

          Reply
  32. 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.

    Reply
  33. 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.

    Reply
  34. 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
    Reply
  35. 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

    Reply
  36. 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
    Reply
  37. 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

    Reply
  38. “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…

    Reply
  39. 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)

    Reply
  40. 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 β€œon 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.

    Reply
  41. 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.

    Reply
    • Agreed. While a section of the company may desire a new hire, most HR departments seem to be guided by a solitary mission – Just Say No.

      Reply
  42. 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.

    Reply
  43. Thanks Leo for this question. It was never asked in my interviews for IT jobs. But I gave it a thought and my answer is – manhole is to service the pipes which are round and in ancient days may be the pipes it self were coming out of ground on surface to enable servicing. The practice continued even though now, the pipes themselves rarely come out directly out of ground.

    Thanks a lot for a thought provoking topic

    Reply
  44. In Australia the Chinese dug round or oblong shafts looking for Gold, so evil
    beings etc. etc. had no corners to hide in, makes sense to me.
    Funny thing is they did not have Covers on them, not round or oblong, probably not invented then.
    This is Ridgey Didge, Fair Dinkum.

    Reply
  45. Reminds me when I worked for the bookstore and I was interviewing candidates. I always asked, “Why do you want to work here.” Nine times out of ten the answer was “because I like books.” Those candidates never interested me very much. I was more interested in those who came up with a different answer. They were the ones who were probably telling the truth.

    Reply
    • I had an interviewer ask me, “What makes you think you want to go to work in a foundry?” I told him that I had a one year old and a pregnant wife, and they expected me to keep them fed. He told me to come to work Monday, but I had another job by then.

      Reply
  46. Your father was probably right, Leo. When the first manholes were made, probably in ancient Rome or earlier, they probably just went with the easiest method of production without any of the other considerations. Since all those other factors were covered, there was no reason to change the shape.

    Reply
  47. Great question, lots of good answers. Also, it’s heavy and would be easier to roll it away from the hole rather than carrying it away.

    Reply
  48. I think the bigger question is: Why are manholes round? I think the answer is: Because corners invite stress cracks, hence the rounded corners on airplane windows, and in most machine parts i.e. fillets and rounds.

    Reply
  49. Mamholes are round for the same reason tunnels are round. It’s more efficient to build a round passage than, say, a square one. It’s wasted effort to square off corners, which are offer no advantage. Corners are wasted space.

    A shorter reason is this. Manhole covers are round because manholes are round. Manholes are round because people are roundish and it’s more efficient to build manholes without corners. See above.

    Reply
  50. Edited version. I am a native English speaker who has studied the language, something that might not be obvious when mispellings and extra words find their way into my writings.

    Manholes are round for the same reason tunnels are round. It’s more efficient to build a round passage than, say, a square one. It’s wasted effort to square off corners, which offer no advantage. Corners are wasted space.

    A shorter reason is this. Manhole covers are round because manholes are round. Manholes are round because people are roundish and it’s more efficient to build manholes without corners. See above.
    REPLY

    Reply
  51. I think the appropriate response to this question should be “Pardon me, but how do manhole covers pertain to your open position as swing shift WalMart(r) Greeter?” (Or to really dumb-down the gig, insert “TSA Agent” here.)

    Reply
  52. Has anyone given the manufacturing process much consideration to the equipment, forms and man hours to make changeovers? Different equipment and forms, time for change over from different forms and sizes, creating round to square form connectors etc. I was in manufacturing for 20yrs and there are many facets to calculate costs and performance.

    Reply
  53. Round manhole covers are positioned down inside the round hole onto a ledge that encircles the hole. This allows the cover to be level or near level to the surface of the roadway. That ledge is smaller in diameter than the diameter of the round cover. That keeps the round cover from falling into the hole which could injure or kill someone below. (Edit: I am not aware how square or triangle covers are prevented from falling through the hole.)

    Reply
    • It seems that there are various answers addressing the covers of non-circular shape, but they all seem to address a need. Small covers that allow access to something like a valve on a water service line are often of these other shapes. The need is to easily identify a particular facility’s infrastructure, as just one example. To my knowledge, these other shapes use alternate means to keep them from falling in the hole. Small covers often have rather large flanges that prevent one from finding the smallest dimension of the cover that might fit inside the hole. In fact, sometimes a small cover is allowed to fall inside the hole…IF the cover is easily removed and reinserted into the opening correctly. When I worked with high tension lines in underground vaults and conduits, the openings with demanding “jobs” tended to be square, opening on fixed hinges. If you would see repair crews lowering a new transformer into a vault, or watch how a crew would use special tools and pulleys to “pull” very heavy cables, you might discern the logic.

      Reply
    • Triangular covers wouldn’t fall in, square ones would only be prevented from falling in with a huge lip in relation to the hole. Small covers don’t present a problem because the covers are light and the hole is usually shallow enough to retrieve the cover easily.

      Reply
  54. Retired Civil Engineer here… I got the answer from a respected (and quite old) Civil Engineer over 50 years ago! The simple answer, “Because it is the only shape that…with a rim of small size…will not fall into the hole”, is the correct answer. But Civil Engineers did not “invent” the concept. Similar circular covers go back at least 2000 years, and archeologists believe the concept, going back even further, was developed by potters. Apparently, many cultures have teapots and vessels that display this quality. Being one who has to know the “real” answer to such questions, I searched, and it seems my Civil Engineering Elder is correct.

    Reply
    • What you say is true, but I’m not sure that was the original reason for that design. I tend to believe that since the default shape for pottery is round and the default shape for a hand dug hole is approximately round, it just happened that way and since the design worked, it was used for everything else.

      Reply
  55. True, lipped circles and triangles can’t fit through the hole in the ground.
    But you can easily make a square lipped manhole cover by simply making the length of the edges equal to the length of the diagonal. You need only a compass and a straightedge.

    Reply
  56. There is a, possibly apocryphal (but I hope not), exam question from years ago that asks “using only a standard barometer, how can you estimate the height of a tall building?”. Supposedly one bright student who did not want to provide the obvious answer, provided several alternatives including

    You drop the barometer from the roof and time the descent, then calculate the height using the known acceleration due to gravity.

    You go to the main flooor stairwell and climb each floor, measuring the height in “barometer units”. You calculate the height by measuring the length of the barometer and multiplying.

    You use the barometer as the weight on a pendulum and, using a long string, lower it from the roof until it is close to ground level. Start it swinging and time the periodicity of the swing…

    And my absolute favourite…

    Take the barometer to the building custodian and say “I have here a very fine barometer which I will give you if you tell me the height of the building.”

    Reply
  57. Am I missing something? If the hole is larger than the cover, the cover can still fall into the hole, no matter what the shape. Right?
    Hole —> O
    Cover —> o
    Result–The cover will fall down into the hole.

    Reply
    • The bigger question is, unless this is a manhole company,why is the question even asked? What ever happened to talking to the person,find out what they’ve done,feel what your guts tell you.

      Reply
  58. You are giving the current crop of people to much credit.

    3/4 of the college kids can not tell you who the Vice President of the United States is.

    I doubt there are many who could come close to a reasonable answer to this question.

    Reply
  59. A “Reuleaux polygon.” Interesting! — I’ve never heard of them before! I must/i> read that Wikipedia article…!

    Reply
  60. Leo:
    Over fifty years ago I started my first week of “shop training” with then one of the twenty one Bell Telephone Franchised Companies throughout the United States! Today, young folks would not have an idea who “Ma Bell”, a rotary phone or a switch hook would have in common in today’s communication world. Answer: dial tone.

    But back to my story, one part of our Rookie training besides how to climb telephone poles, fend off barking dogs who would rather take a bite out of your leg than eat the dog biscuit you personally purchased at the 7-11 out of your wages that morning was to go around and ask the “old timers” on the line crews why Manhole covers were round?

    The exercise was more ritual, more rite of passage, more pure science out in the cable storage yard than just a straight answer. All of the “newbies” would be gathered one morning with instructions to follow the commands of one of the old timers and pay close attention to what was being taught!

    Slowly, metal parts that were assembled by a construction crew to form up the likeness of a “manhole: section that would be positioned below grade or street level would come to shape. With little imagination, access into the dark, dank, wet and winter chilled vaults containing communication cables snaking out of one conduit hole and back into another, begin to take place, as if travelling a silent and swift journey.

    To understand the design and function of a true fitting manhole cover “up close and personal” only requires to be elected by your peers of the “Junior Pole Climbers Club” to maneuver a 300 LB cast iron cover using only a single, iron hooked tool that latches into the center opening grab hole of the cover. Swinging, coaxing, grunting or gently pleading this flat cover into moving the slightest of inches towards the mating ring design can be ever wished for by the inexperienced rookie.

    Moving a manhole cover is only an exercise of a person’s mind and body strength attempting to overcome the forged chemical bonding of ferrous materials that can easily mandate who will become the winner of an impromptu manhole cover lifting contest! The round design of the cover is claimed to have numerous purposes: stop vibrations, accommodate round pipes or to prevent covers from falling inward. Regardless of the final answer, either down to economy of casting materials or esthetic designs, a square manhole cover just can not be rolled to the vault for placement on top!

    Reply

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