Why Are Manhole Covers Round?

February 14, 2024

Whether you’re a regular sidewalk observer or someone who gets asked this question on job interviews (and has to think quickly), there are many reasons why manhole covers are round. The short answer is because the manholes themselves are round, but the deeper truth is that several practical, engineering and economic decisions have been made to make something like a manhole cover circular.

The main reason is safety. Since each manhole cover sits on a lip that holds it in place, if the shape was square or rectangular, it could easily fall through. However, a circular cover is safe because its diameter doesn’t change no matter how it’s rotated or positioned.

Another reason is that, mathematically speaking, the circle is the strongest shape for a manhole cover to be in. Each point of its edge, which is only located on the lip that holds it in place, is an equal distance from the center. This makes it much stronger than a square or rectangular cover, where the corners may be weaker and more susceptible to pressure.

Also, by being circular, the cover is easy to roll around when it’s time to relocate it or clean the area below it. This ease of movement reduces the amount of time spent moving it and increases efficiency and cost savings, especially as there are fewer opportunities for theft or vandalism. Also, by being made of plastic instead of iron, it’s less prone to corrosion and requires fewer replacements.

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