Why are Manhole Covers Round?

Have you ever noticed that every manhole cover you see is round? Have you ever wondered how engineers came to this decision? Did you realize that the reasoning comes down to the geometry of shapes?

That’s right: manhole covers are round because circles are the only shapes that cannot fall through themselves.

Let’s examine some of the properties of shapes to see why this is true.

First, it’s important to recognize that the shapes will only fall through themselves when they are rotated to be vertical. If they were lying flat then they would be covering the hole successfully!

Next, let’s examine shapes with unequal sides beginning with a triangle. If the triangle had side lengths of 3, 3, and 2 feet then we could rotate it so that the side with length 2 feet was parallel to the ground. This would mean that the width of the manhole cover would now be 2 feet, with the width of the opening approaching 3 along the sides of the hole. Since 3 is greater than 2, the cover would be able to fall through the opening. Similarly for a 4-sided figure if one side is shorter or longer than the others then we will find the same result.

There is a pattern to notice from examining the previous shapes. The shortest width of each shape is compared to the longest length. But what if the covers were regular shapes with all sides and angles being the same?

Let’s look at a square with length of 1 foot on each . The shortest width of a square will be the length of a side. But how about the longest length? That can be found by measuring from the upper left-hand corner to the lower-right hand corner.

Using Pythagorean’s Theorem, we know that 1^2 + 1^2 = diagonal^2. So the diagonal is square root of 2, or approximately 1.414. This means that this square could still fall through itself.

So now our focus can shift to finding a shape whose longest length and shortest width are the same. But this would mean that we would need all widths and lengths to be the same because we cannot have the longest length be shorter than the shortest width. This leads us to a circle.

A circle will always have the width of its diameter no matter which way it is rotated, so this will be the shortest width. But the longest length will also be the diameter as well because any chord will be shorter than the diameter. So the circle is unique to all polygons and shapes in that it can never fall through itself.

Want to test geometry with physical objects? Since manhole covers are heavy you should explore this idea using Tupperware containers. See how many different lid shapes will fall into the container, and how many will not – then post your comments!


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This post was originally posted in the Everyday Explanations – Answers to Questions Posed on a Middle School Bus Ride by Sean Mittleman. We have his permission to re-post in the MSP2 blog.

39 thoughts on “Why are Manhole Covers Round?

  1. Your answer, that circles are the only shapes is inaccurate.

    Take a look at this explanation:
    http://ask-leo.com/so_just_why_are_manhole_covers_round.html says: “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.”

    Also, another class of shapes, might include a fractal such as the mandalbrot set which would not be able to fall through itself in most dimensions, although this might be much less practical to manufacture and deploy (although water-jets are getting cheaper at making intricate shapes).

    I look forward to hearing from you and hope you will update your answer.

    Win

  2. I do not agree, the man hole cover, needs a “lip”, “sit” on the perimeter of the frame for it to lay/sit, not only flat but flush with the surface, thereby, making the actual opening smaller than the cover. I believe the reason they are round is that they can be opened to any side. ( as we all know the circle has an infinite number of sides ).

  3. You cannot drop a circular cover through its frame. Anyone that has used a set of lifting keys to lift a cast iron cover will know what a precarious job the other shapes can be.

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  8. This is a really bad question and an even worse answer (so they don’t fall through). For starters, not all manhole covers are round (in Rome many are rectangular) and in some places they are hinged and cannot be completely removed (Poland). Aside from that inaccuracy, given a sufficiently large lip, ANY geometrical shape can be used without the cover falling through. A more rational explanation for the shape is that a circle is an efficient use of space and easy to prooduce both the cover and the hole. But a rational explanation is only speculation and not a factual answer.

    That’s the fundamental problem with the question. It’s great if you want people to speculate about reasons but it’s not a very good measure if you want people to research and provide factual answers. I think this is why the question is popular with Google and Microsoft–they specialize in providing answers but not providing facts. A factual answer (aside from “I don’t know”) would involve identifying the inventor of the circular manhole cover and trying to determine what factors drove the decision making process to arrive at a circular shape.

    Gotta go–the front door to my house just fell through the opening again. If only they made all doors circular…

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  11. Round man hole covers allow for equal distribution of weight and eliminate fatigue or stress (cracking) at corners. The plus after affect that has been broought up ist that prankster won’t drop them into the hole…well they could just as easily role them away, if that was actually the reason for the design in the first place. Here’s a similar design issue to ponder:
    The de Havilland DH 106 Comet was the first production commercial jetliner.[N 2] Developed and manufactured by de Havilland at its Hatfield, Hertfordshire, United Kingdom headquarters, the Comet 1 prototype first flew on 27 July 1949. It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wings, a pressurised fuselage, and large square windows. For the era, it offered a relatively quiet, comfortable passenger cabin and showed signs of being a commercial success at its 1952 debut.

    A year after entering commercial service the Comets began suffering problems, with three of them breaking up during mid-flight in well-publicised accidents. This was later found to be due to catastrophic metal fatigue in the airframes, not well understood at the time. The Comet was withdrawn from service and extensively tested to discover the cause; the first incident had been incorrectly blamed on adverse weather. Design flaws, including dangerous stresses at the corners of the square windows and installation methodology, were ultimately identified. As a result, the Comet was extensively redesigned with oval windows, structural reinforcement and other changes.

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  16. Yes, I am 100% agree with you. If by mistake we put the square shaped cover by diagonally it will fall inside the hole. But round shaped cover will never fall into the hole no matter how you put it on the manhole.

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