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Why Are Manhole Covers Round?

Updated on April 30, 2013
Why are manhole covers round?
Why are manhole covers round?

Why Are Manhole Covers Round?

Why are manhole covers round?

This might not be something you have ever thought about, or asked yourself. We don't often walk with our heads bowed down so it is not surprising that you may not have even noticed them.

Sure we all know that streets and sidewalks have manholes which typically lead to the underground sewers and dark dingy places but just why were they made round, and not square or any other shape?

Is this question just as random as asking why a pizza is round? With an answer simply, "Because it is" or is there are logical reason?

Why Are Manhole Covers Round? Why A Circle Shape?

There are many reasons why the circle shape is the most common shape used for a manhole. Let's delve a little further.

  • A circular manhole cover cannot fall through its opening. Under the cover is a little 'lip' or edge which means that the actual hole is slightly smaller.
  • It is easier to dig a round hole.
  • Pipes and drains are circular.
  • It can be easily moved by being rolled.
  • If it were to be displaced by a car or something else, it could more easily be slotted back into position, compared with a square shape which would need its sides to be matched.
  • There are no sharp edges to damage anything from driving over it if it was dislodged.

Circles are so designed that they have a constant width. Geometry clarifies this by stating that its width is defined as the distance between two parallel lines, however in a circle's case, the width is its diameter.

None of these may be the exact reason why they were initially designed this way and none of these may be the true answer but they are logical conclusions.

Why are manhole covers round?
Why are manhole covers round?

Why Are Manhole Covers Not Square, Oval or a Triangle?

Let's consider a square.

A square cover be tipped on its axis and potentially it can drop through a hole on its diagonal.

How about an oval shape?

Same as a square, however with greater ease could this drop through if dislodged.

Triangles?

Science shows us that a round manhole cover is not the only shape that will not fall into a hole. In fact a triangle, more specifically an equilateral triangle (three sides of identical length) will not fall in either.

So why then was a triangle not used as a standard manhole cover? Put simply it is quite hard to move, or roll for that matter, and a triangle, like a square, if dislodged it may have one of its points raised from the road or sidewalk which could cause damage to a car, or someone could trip over it.

There is no denying the fact that there are some manhole covers which are square, some are hinged and some are triangle however for the majority, circular is the way to go.

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Manhole Cover Art

Despite the definitive reason, manholes are magnets for some wonderful street art, or at least inspire many to design sculptures depicting the circular nature of them.

The few selected here for show are not even a drop in the ocean of the number designs found around the world.

It seems that not only are their circular shapes a drawcard for colors and themes, but surprisingly there are even manhole cover groups; supporter groups or fan clubs. Yes, manholes have a fan club and it spans the world over.

If you are wanting more information, you might like to join in. When you see a manhole cover that grabs your attention, take a photo of it and share it with your club.

From the research I have undertaken it seems that there are some very knowledgeable people in the world who can not only identify a manhole design, but its origination, its meaning, its manufacture and its location. Truly amazing.

You can even find pendants in the design of manholes. What will they think of next!

Vintage Postcards "Will Drop in Soon, Look for me" Copyright 1905. JRH. Chicago. Postmarked 1906.
Vintage Postcards "Will Drop in Soon, Look for me" Copyright 1905. JRH. Chicago. Postmarked 1906.

Did you know?

A manhole cover usually weighs in excess of 50kg. This is a design element to ensure that when traffic passes over them, they are not dislodged. More to the point, they are typically made from cast iron and/or concrete. Despite their weight, they have been known to go missing, or be stollen.

An original Interview Question

History shows that the question of 'Why are manhole covers round?' came to fame in a interview process undertaken by Microsoft. The aim was not to actually understand the reason, but to access the applicant approach and response to such a question; arguably a psychological analysis.

The aim was not to actually get an answer, but to view the logical thought process used in coming to a conclusion. In today's world I would argue that such a question would not be acceptable.

Regardless of the definitive reason, it appears that for the most part, manhole covers are round. This is the shape of choice.

Why? It is a simple enough question, with a simple enough answer.

Is that good enough for now?

This is just one of those questions which if asked 100 times could receive 100 varied responses. A bit of trivia. Next time you have a family gathering, pose the question.

All Hubs are Original Material by 'Work At Home Mums' ©

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    • thumbappoo profile image

      thumbappoo 5 years ago from Kerala, India

      Thanks very much for asking this thought provoking question again. Easiness of manufacturing must be the most important criteria. Not only for us, even for god also. That is precisely why most of the natural objects we see around us are circular. Right from the humble dew drop to the astonishing solar system of heavenly bodies, most of the natural objects are circular (spheres and ovals included). What about trees and plants? They are not circular. Now, take the cross sections of the different parts of a tree. Aren’t they nearly circular? Not only that, if you allow any tree to grow undisturbed, the overall shape of the foliage will become spherical in shape!

      This is applicable to animals also.

      Aesthetically also, circle is the most preferred shape. Think of the many beautiful things around us. At least one of their dimensions is circular, isn’t it? Circles, spheres, ovals, or smooth (read round) corners…

      Now take this test. Close your eyes and think of as many types of holes as you can, both natural and artificial. How many of them are not circular?

      Interesting hub.

      TC & Regards

    • LA Elsen profile image

      LA Elsen 5 years ago from Chicago, IL

      Funny and interesting. All I know is that when I was very young I used to think they looked like giant Oreo Cookies. I really haven't given much thought to them until now. (Although now I want a cookie.) I did not know that about Microsoft either.

    • Patty Kenyon profile image

      Patty Kenyon 5 years ago from Ledyard, Connecticut

      Very Interesting!! This is something that I never thought about...but definitely interesting!!