# Manhole covers, again **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [June 9, 2000, 3:25am UTC](https://boards.straightdope.com/t/manhole-covers-again/21835 "2000-06-09T03:25:35Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![TRH](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TRH](https://boards.straightdope.com/u/TRH)\ **Post date:** [June 9, 2000, 3:25am UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/1 "2000-06-09T03:25:35Z")
Isn’t it possible that at least one reason manhole covers are round is that a circle is the largest area with the minimum perimeter possible? This translates into a cover that doesn’t weigh as much or use as much steel as any other shape of reasonably-useful size would. ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [June 9, 2000, 5:15am UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/2 "2000-06-09T05:15:45Z")
Welcome to the SDMB, and thank you for posting your comment. Please include a link to Cecil’s column if it’s on the straight dope web site. To include a link, it can be as simple as including the web page location in your post (make sure there is a space **before** and **after** the text of the URL). Cecil’s column can be found on-line at this link: [Why are manhole covers round? (06-Jan-1984)](http://www.straightdope.com/classics/a1_247a.html) The column can also be found on page 247 of Cecil Adams’ book “[_The Straight Dope_](http://www.straightdope.com/otheroutlets/books.html)”. If you want to save on steel, wouldn’t an ellipse or an oval with the same diameter as a circle save even more volume? (edited to correct link - zut alors!) (this post has been edited by Arnold Winkelried) ---
**Author:** ![Chas.E](https://avatars.discourse-cdn.com/v4/letter/c/e99b99/32.png) [@Chas.E](https://boards.straightdope.com/u/Chas.E)\ **Post date:** [June 9, 2000, 7:11am UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/3 "2000-06-09T07:11:58Z")
Cecil blew it, and gave the wrong answer. It appears that he didn’t even look at the question closely. He answered some other question entirely. This whole topic was covered in depth many years ago, by Martin Gardner in his column in Scientific American magazine. I grew up reading Gardner’s “Mathematical Games” column, everything I know about math I learned from him (much to the displeasure of my math teachers) and I also learned much from Gardner’s skeptical attitude. Cecil has much to learn from Martin Gardner. But anyway, Gardner offered the manhole puzzle in his column, and then debunked the incorrect but useful answers. Q: Why are manhole covers round? Wrong answers: 1. Because round manhole covers can’t drop into the hole (this is the best of the wrong answers). Cecil uses this answer, he’s very close, but wrong. 2. Because other shapes (i.e. squares) rattle more when traffic rolls over them and are more likely to be bounced off of the manhole. 3. Economical use of materials, most strength for the least metal. Right answer: A: Manhole covers are round because the manholes are round. Now if Cecil had been asked, “Why are manholes round” then he might have been close to the right answer. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [June 9, 2000, 12:56pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/4 "2000-06-09T12:56:57Z")
**Chas** says, > [@](#): > > Right answer: > > A: Manhole covers are round because the manholes are round. Oho! Interesting. And manholes would be round because a) sort of what **TRH** says: Economical use of materials; i.e., fewest bricks required to make the biggest hole, and/or b) A round manhole resists soil pressure better? Question, though: The way you stated the correct answer implies (at least to me) that manholes came first, and covers were made later. Alternatively, it implies that the design requirements of manholes outweigh any requirements on the covers. Which, if either of these, is true? And **Arnold** , I think the correct URL is: [http://www.straightdope.com/classics/a1\_247a.html](http://www.straightdope.com/classics/a1_247a.html) ---
**Author:** ![whitetho](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whitetho/32/1059_2.png) [@whitetho](https://boards.straightdope.com/u/whitetho)\ **Post date:** [June 9, 2000, 1:17pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/5 "2000-06-09T13:17:14Z")
Also, the Straight Dope Message Board discussion which lead to Cecil’s update is at [Why are manhole covers round?](http://boards.straightdope.com/sdmb/showthread.php?threadid=2255) ---
**Author:** ![Arnold\_Winkelried](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@Arnold\_Winkelried](https://boards.straightdope.com/u/Arnold_Winkelried)\ **Post date:** [June 9, 2000, 3:02pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/6 "2000-06-09T15:02:29Z")
Merci, zut, pour avoir corrigé mon erreur. What was I thinking? TRH, you might want to read the original discussion that whitetho thoughtfully mentioned, someone posted this statement: «One property specifically of the triangular version (of a Rouleaux “shape”) is that it’s the shape with the minimum area that won’t fall through the hole.» ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [June 9, 2000, 3:53pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/7 "2000-06-09T15:53:00Z")
Il n’y a pas de quoi, Arnold. Although that’s about as far as my high school French will take me. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [June 9, 2000, 6:52pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/8 "2000-06-09T18:52:11Z")
Chas.E, zut’s on the right path. What about manholes drives them to be round? It’s not the shape of the tunnel, as that can change at any time under the street. In fact, bricks stack better in rectangles than circles. 😉 What drives manholes to be round? Because they want a shape that has max area with min perimeter that will not fall through. And no alignment required. Thus the **cover** drives the design of the manhole shape, not vice versa. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [June 10, 2000, 5:47pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/9 "2000-06-10T17:47:51Z")
Round covers not only don’t need any alignment, they _roll_ to help in their own transportation. At least that’s my story, and I’m sticking to it until a sewer worker comes on here and tells us he would never do that anyway. ---
**Author:** ![sublemon](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@sublemon](https://boards.straightdope.com/u/sublemon)\ **Post date:** [June 13, 2000, 4:12pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/10 "2000-06-13T16:12:56Z")
Why are manhole covers round? I’ve been thinking about this question for several years. Well, not constantly… Anyway, couldn’t it be because some sort of drill is used to make the manhole, and it is just easier to make a round hole? Gabe ---
**Author:** ![TRH](https://avatars.discourse-cdn.com/v4/letter/t/4da419/32.png) [@TRH](https://boards.straightdope.com/u/TRH)\ **Post date:** [June 16, 2000, 4:33pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/11 "2000-06-16T16:33:45Z")
_Any_ shape of cover will fall through a hole of the same shape and size, even a Rouleaux triangle. What normally keeps a manhole cover from falling through is not actually its shape but the narrow lip around the edge of the hole, only about 1 inch wide, which actually makes the hole slightly smaller than the cover. When in place, the cover sits on this lip. That said, there are only a very few shapes that cannot be turned so as to fall through a matching lipped hole. A square cover, for instance, will easily fall through by aligning it across the square hole’s diagonal. An oval will slip through if turned vertically, long axis up and down, and aligned along the hole’s long dimension. The only two shapes I can figure out that will not fall through a matching hole with a reasonably small lip are the circle and the Rouleaux triangle. (Obviously, make the lip wide enough and you could block any lid from going through…but then, the hole wouldn’t be useable. We’re talking real-life practicality here.) Tangentially, an article in today’s Washington Post, [http://www.washingtonpost.com/wp-dyn/articles/A2194-2000Jun15.html](http://www.washingtonpost.com/wp-dyn/articles/A2194-2000Jun15.html), about manhole covers getting flipped by underground explosions, mentions a _square_ manhole cover in Wisconsin that got thrown clear over a building. I’ll leave the regional jokes to other people… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 17, 2000, 2:36am UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/12 "2000-06-17T02:36:26Z")
Well, you can make a Rouleaux shape for any odd number, and they’ll all have that property. Of course, once you get past three sides, they start looking an awful lot like circles. ---
**Author:** ![emoryj](https://avatars.discourse-cdn.com/v4/letter/e/eb9ed0/32.png) [@emoryj](https://boards.straightdope.com/u/emoryj)\ **Post date:** [June 27, 2000, 7:17pm UTC](https://boards.straightdope.com/t/manhole-covers-again/21835/13 "2000-06-27T19:17:41Z")
ok, so there are some good reasons for square manholes. i bet that the real reason is that the circle dissipates tension and stress (from above and the sides) better than any other shapre, triangles and squares included. on other shapes, stress would be focused on the points proportionally to the # of sides. more sides, more places to dissipate stress. since one definition of a circle is a shape with infinite sides and angles it follows that it is the shape best suited to standing up under stress. maybe.