# Why are manhole covers always round?

Written by Isha Chakraborty

Bio

These circular disks, which get ignored on a daily basis, help connect the city’s sewer systems and electricity. Needless to say, these circular 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. 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:

However, this is not the case when it comes to circular shafts and covers. You can rotate a circular 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:

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.

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