Why are Manholes Covers Round?

manhole cover

This one is only loosely defined as a “brainteaser.” In fact, it’s often used as an interview question to test a candidate’s approach to problem-solving.

It’s thought that Microsoft was the first employer to use this interview brainteaser.

The question is simple:

What makes manholes round?

Ponder it and come up with as many answers as possible to compare against ours.

Firstly, the pragmatic (or you could say sceptical) among you might’ve thought, “are they always round?”

Well, it’s not easy to answer that for sure, but for the most part, manhole covers are round. In the USA, they’re pretty much always round. In the UK, drain covers and so on are often square or rectangular, but access holes designed for humans are round.

So why are they round? There are four main reasons.

  1. Safety: Round manhole covers are less likely to fall into the manhole. For example, with a square cover, the diagonal distance across the length of the square is 1.414 times the length of the sides. That means it can be dropped into the hole at certain angles, which is potentially dangerous. Circles circumvent this issue – you can’t fit it into the hole by merely rotating the shape.
  2. Convenience: Round manhole covers can be easily rolled into place. This is an advantage, as manhole covers can be very heavy.
  3. Universal design: Round manhole covers can be used in any orientation and don’t require precise alignment when put in place – you can place the shape over the hole at any angle, unlike a square or rectangle, where you’ll have to line up the sides and corners.
  4. Cost-effectiveness: Round shapes have less surface area than similarly-sized angled shapes. A round manhole cover uses less material than a square cover of identical length and width.

So there we have it, four reasons why circles make good manhole covers. Can you think of any others?


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