Math Puzzles

why are manhole covers always round


Table of Contents

1. Introduction
2. Poth holes
3. Why does long division work?
4. Conclusion
5. Frequently Asked Questions (FAQs)
6. External References


You must have seen manhole covers around your city or town a lot of times while traveling. You may have also wondered 'why are manhole covers round'? You may be surprised to know that this question is frequently asked in job interviews. This question will be answered with a simple math logic in this article.


These circular disks, which get ignored on a daily basis, help connect the city’s sewer systems and electricity. Needless to say, these circular manhole covers, do protect something really important.

But why are they always circular? Is it some weird fascination that engineers have for the shape, or is there a practical reason behind it being that shape?

You’ll be surprised to know that the major reason for manhole covers to be round is so that they don’t fall through the cylindrical shaft present beneath. Puzzled? There is actually simple math behind this logic.

First, it’s important to know that any other shape will actually directly fall through the shaft of the same shape. For example, a square cover, if not aligned with the shaft of the manhole properly, will straightaway fall through the shaft. You can see it for yourself:

aligned                                               not aligned


However, this is not the case when it comes to circular shafts and covers. You can rotate a circular manhole cover in any-which-way you want to without changing its diameter. In simpler words, a circle will never pass through another circle of the same diameter. So no matter which way you align it in the 3-dimensional space, it won’t fall through the shaft as shown below:


Horizontal alignmentvertical alignment



Poth holes

This phenomenon is mathematically known as the ‘curve of constant width’ – it means that a curve will always hold its width constant, no matter how you rotate it. In this case, the width of the circle is always the same as the diameter. Whereas the width of the square is not constant and can change depending on how you align it in a 3D.

There are a few more reasons for manhole covers to be round. Due to the heaviness of the manhole covers, they can be moved around conveniently if they are round. Also, round shapes are easier to manufacture than other shapes.

Now that you know why manhole covers are round, share this information with your friends and keep them in the know as well, because the more you know, the more you grow.

India’s #1 Math learning program Cuemath is now in the US, UK, and Canada. Click here to know more about our International program:


Related Articles
Access Personalised Math learning through interactive worksheets, gamified concepts and grade-wise courses
Learn More About Cuemath