

So Just Why Are Manhole Covers Round?

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.


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.


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.


Manhole But you still want an answer.


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?”


  2. Hi,



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

    Circles and Squares were created equal.


    So how? 🙂














    (Should it be personhole cover?)








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



    Other “Ideas” around the Manhole.

    [Content copied from Wikipedia removed.]


  25. Leo


    Any idea?

  26. the simple answer is round hole square peg…..
    there are many answers to this question…….

  27. man holes are round for may reasons,
    safety- they cannot fall through their own hole.





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




    • From my observations manhole covers are not rolled back into place. First they are usually removed by inserting a hooked rod ( 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.

        • 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.



  34. Other possible answers:


    and, finally,




    “No Hire”



    Good times…

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




    • 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.


  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

  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.

  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.

    • 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.

  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.

  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.

  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.

  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.

  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.


  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.)

  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.

  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.)

    • 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.

    • 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.

  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.

    • 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.

  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.

  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.”

  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.

    • 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.

  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.

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

  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!


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