Why is a manhole cover round? A Microsoft interview question.

OK, so it probably isn’t an “official” interview question (if there are any), but I have asked it during interviews in the past. For me, it is simply a way to help loosen up the interviewer and have some fun, but if the only answer I get is “I don’t know, why is it?”, then that gives me the impression that the person either isn’t trying to really interview, or that they just aren’t very creative.

So, why is a manhole cover round? Heck if I know, but here are some good answers that I’ve heard over the years:

  1. Because the hole is round (duh!)
  2. Because animals dig round holes, so it feels natural to humans too
  3. Because a circle offsets the straight lines of a city
  4. Because it is easier to roll the cover some distance than carry it
  5. Because it won’t fall into the hole – but, the same is true for an equilateral triangle
  6. Because it is easier to pour hot metal into a circular mold than one with sharp corners

I suppose there are more, and there is probably some web site with the “correct” answer. But I like these because I can remember the individuals who have told them to me over the years. What’s your answer?

Comments (23)

  1. Leon O'Brien says:

    Because its the most efficient use of space in consideration of the types of elements entering the hole. One could imagine reducing the circumference to a point where there is no room to move but up or down, whereas with a square there would be some air space….thats my answer anyway 😉

  2. Brian Groth says:

    Cool answer Leon! Now if we combine the ideas of an ever shrinking concept of "now" (what is the smallest unit of time anyway?) with an ever reducing circumference, can we move subatomic particles at light speed through our manhole?

  3. 1) So that people of all sizes and shapes (up to a point) can go through the hole when they need to do work on the tunnels.

    2) The tunnels are pipes so its easier to go with the whole theme or circularity (pipes being circular and all). :-)

    3) I think if you stress test an equilateral triangle and a circle you will find that the circle dissipates the stress more evently at all points. For instance, stress at a corner of a equilateral triangle would have a different effect than when the same stress is applied to the center of that equilateral triangle.

    4) The equilateral triangle like the circle wont fall into the hole but if you wanted to put the cover on, then for the equilateral triangle you would have to position it into 3 positions only. With a circle that isn’t the case so laying it out is much easier.

    5) Like jar tops, the man hole covers can be locked into place. (The same idea as the safety caps of prescription bottles). I’m not sure they actually do this but it would be a neat idea.

    6) Manhole covers are round in only some parts of the world. They are squares or even rectangles in other areas. For pics check these links out.



    Hope this helps. :-)

  4. Scott says:

    They’re round so they can’t fall in the hole.

  5. Don Newman says:

    Actually an equilateral triangle could fall through. Draw a line from one tip to its opposite side midpoint (creating two right angle triangles). Using A^2 + B^2 = C^2 and assigning a length of 1 unit to the side of the triangle, B = to the side you divided in half, and A = the line your drew, you get A^2 + 0.5^2 = 1^2 and solve A = 0.866 units. That means if you hold the triangle flat against one of the sides at a right angle to the ground and with the point sideways instead of up or down it will easily fit (depending on the thickness of the manhole cover).

  6. Duncan says:

    Manhole covers are round because manholes are round. The real question is – why are manholes round? … because the round cross-section tube is a stronger shape than any other shape tube.

  7. James says:

    So they can’t fall down the hole obviously. Another thought is so that they are easy to move around once it is out of the hole. You can roll it :)

  8. Bill Gates says:

    So that it is easier to close the hole without having to position the cover in any particular way. And to think I recruited you!

  9. Nirmal V Raghavan says:

    I think because, one need not waste time aligning the cover to the hole……!

  10. anonymouse says:

    One of my favourite bits of humour:

    Round Manhole Covers, or: If Richard Feynman applied for a job at Microsoft

    "Why are manhole covers round" is one of the eternal questions in job interviews, and so it seems to be at Microsoft. The desired and politically correct answer to the question is: "Manhole covers are round because round is the only shape that can never fall into the manhole and hurt someone (with the hole of the same shape, but slightly smaller size than the cover)". And the answer is wrong.

    Let’s ask Mr Feynman:

    "Interviewer: Now comes the part of the interview where we ask a question to test your creative thinking ability. Don’t think too hard about it, just apply everyday common sense, and describe your reasoning process. Here’s the question: Why are manhole covers round?

    Feynman: They’re not. Some manhole covers are square. It’s true that there are SOME round ones, but I’ve seen square ones, and rectangular ones.

    Interviewer: But just considering the round ones, why are they round?

    Feynman: If we are just considering the round ones, then they are round by definition. That statement is a tautology.

    Interviewer: I mean, why are there round ones at all? Is there some particular value to having round ones?

    Feynman: Yes. Round covers are used when the hole they are covering up is also round. It’s simplest to cover a round hole with a round cover.

    Interviewer: Can you think of a property of round covers that gives them an advantage over square ones?

    Feynman: We have to look at what is under the cover to answer that question. The hole below the cover is round because a cylinder is the strongest shape against the compression of the earth around it. Also, the term "manhole" implies a passage big enough for a man, and a human being climbing down a ladder is roughly circular in cross-section. So a cylindrical pipe is the natural shape for manholes. The covers are simply the shape needed to cover up a cylinder.

    Interviewer: Do you believe there is a safety issue? I mean, couldn’t square covers fall into the hole and hurt someone?

    Feynman: Not likely. Square covers are sometimes used on prefabricated vaults where the access passage is also square. The cover is larger than the passage, and sits on a ledge that supports it along the entire perimeter. The covers are usually made of solid metal and are very heavy. Let’s assume a two-foot square opening and a ledge width of 1-1/2 inches. In order to get it to fall in, you would have to lift one side of the cover, then rotate it 30 degrees so that the cover would clear the ledge, and then tilt the cover up nearly 45 degrees from horizontal before the center of gravity would shift enough for it to fall in. Yes, it’s possible, but very unlikely. The people authorized to open manhole covers could easily be trained to do it safely. Applying common engineering sense, the shape of a manhole cover is entirely determined by the shape of the opening it is intended to cover.

    Interviewer (troubled): Excuse me a moment; I have to discuss something with my management team. (Leaves room.)

    (Interviewer returns after 10 minutes)

    Interviewer: We are going to recommend you for immediate hiring into the marketing department."

  11. Atilla Ozgur says:

    This is the answer, I would give.

    When you heat metals, they expand. Assume that you choose any shape other than circle, when they expand you can not know which parts becomes longer than other parts. But with circles they expand (almost) uniformly. So even with expanding they are easier to remove and put back in. I mean when they expand they do not get stuck.

  12. Actually, Scott is right. Manhole covers are round because a round cover cannot fall in a round hole. The maximum diameter is the same at every cross-section of the cover and at every cross-section of the hole. And the maximum diameter of the cover is always greater than that of the hole. Therefore, the 50 lb chunk of iron can’t fall 10 feet through the hole it came from and crush the skulls of the workers below. As for the shape of the tunnel below mandating the shape of the manhole cover, there are plenty of square-walled manholes with round covers.

  13. Porticus says:

    Here is my answer:

    Microsoft secretly bought the company that makes all the manhole covers and, even though round is not the most efficient design, Microsoft controls the industry and will keep it round just because they think its the most efficient and most profitable.

  14. Rev says:

    And this is why I’ll never get a job at Microsoft. I don’t imagine you’ve had anyone reply with. "Yeah… I don’t answer stupid questions. I’m here to code, not play trivial pursuit."

  15. GUYS.

    Just read this. An answer to the age old question is given here.


    It also links to http://www.maa.org/mathland/mathland_10_21.html which are "Reuleaux polygons" (constant width or diameter).


  16. PS. Did u know that the location of manholes defines the type of manhole cover used. For instance, on a road where cars go over the manhole, the hole is actually indented into the road. And on sidewalks, the holes lay flat with the pavement. Obviusly they are flat on the sidewalk so that people don’t trip over the cover and then sue the city. But why the indentation for road manholes covers. I believe this has something to do with the shocks of cars. When one tyre of a car is suddenly allowed to "fall" the shocks will reduce the amount of stress on the manhole. Thus I would say that road manholes are more likely to be round. The stress (weight of the car/vehicle) is a big factor.

    Next time you see a manhole on the road, notice whether it has an indent or not? If it does, then the cities road works has done a good job and you should write a thank you note to your local road works. I’m sure everyone appreciates being told they did a good job right :-)

  17. For all you scientific types, heres architectural drawings of Manholes. http://www.msdgc.org/downloads/standard_drawings/

    The first thing I notice is that there are different types of manholes. For instance, the Drop Manholes are round but the Interceptor Manholes are rectangle. The rectangles are so that you can "intercept" more of the manhole contents than a round one. :-)

    Also I notice that the type of pipe underneath determines the style of the manhole. For instance, the Low Pressure Force Main with Air Release is used in areas where you don’t expect your manholes to be blown up by the sudden surge of content. However, if that is anticipated, then Watertight Manhole or Locking Lid Manhole would be used. After all you don’t want then manhole spouting its contents all over the pavement/road. That could be a health hazard.

    GOD I LEARNT SO MUCH ABOUT MANHOLES. This is sooo COOL!!! I loved looking at those drawings. Looks like the technical drawings I used to do in high school. :-)

  18. flying dragons says:

    Interesting question…however it’s obvious to me that the reason they are round is because of the movie industry…

    A round manhole cover being blown off into the air and then tumbling around on the ground, going faster and faster as it comes to a rest, adds to the drama of the scene.

    The sewer creatures are always serpent like… filling the entire pipe with their body not allowing one to escape…

    The Hero finds that it’s more difficult to negotiate a curved walking surface then it is a flat one, as he wanders through the lair of the serpent like creature.

  19. Venkat says:

    Its quite a common sense. Imagine a square man hole. There is a possibility of the lid sliding in because the diagonal of the suqare. While in a round shape there is no possibility of the Lid going in.

  20. A couple of years ago I wrote up “ Why is a manhole cover round? ” and got a few good comments on it.