Why Are So Many Manhole Covers Round? The Answer Is Smarter Than You Think

 

You've driven over thousands of them without giving them a second thought. Turns out, that circle is doing more work than you think.

Wait... What?

There's probably a manhole cover somewhere near you right now.

You've driven over them. Walked across them. Parked on them. Maybe you've even wondered what's underneath one.

But here's a question you may have never stopped to ask:

Why are so many of them round?

It seems like an oddly specific design choice. We make doors rectangular. Windows are usually rectangular. Boxes are rectangular. Sidewalk slabs are rectangular.

So why does civilization suddenly become obsessed with circles when we need a giant metal lid in the middle of the street?

Turns out, there's a very good reason.

Actually, there are several.

So, What's the Deal?

The most famous answer comes down to one wonderfully simple piece of geometry:

A properly sized round manhole cover can't accidentally fall through its own circular opening.

Think about a square lid for a moment.

A square is wider from one corner to the opposite corner than it is from one side to the other. Tilt it the wrong way over a sufficiently sized square opening, and suddenly you've created the possibility of the cover dropping through.

That's a particularly unpleasant possibility when the object in question is a very heavy piece of metal sitting above an underground utility space.

A circle doesn't have that problem.

Its width through the center is the same regardless of which direction you measure it.

Turn it 10 degrees.

Same width.

Turn it 90 degrees.

Same width.

Spin it around while dramatically pretending you're on The Price Is Right.

Still the same width.

As long as the cover is slightly larger than the opening supporting it, there isn't a narrower orientation that lets it slip through.

Simple.

Effective.

And suddenly that boring metal circle starts looking pretty clever.

Wait, It Gets Weirder

Safety isn't the only advantage.

A round cover also doesn't have to be lined up in a particular direction before it can be put back into place. There are no corners that have to match corresponding corners.

Then there's the weight problem.

These things aren't Frisbees.

Traditional cast-iron manhole covers can be extremely heavy, and a round object has another very convenient feature:

You can roll it.

The Wisconsin 101 project from the University of Wisconsin notes that the round design allows heavy covers to be rolled when workers need to move them.

So one basic geometric shape gives you a lid that:

• is difficult to accidentally drop through its opening,

• doesn't need to be rotated into a particular alignment, and

• can be rolled instead of carried.

For a chunk of metal we've collectively decided to ignore, that's a pretty impressive résumé.

Seriously?

Seriously: A circle isn't actually the only shape that can do the famous "can't fall through its own opening" trick.

And this is where our innocent question about street hardware begins turning into a math rabbit hole.

There's a shape called a Reuleaux triangle.

It looks a little like a triangle whose sides have been puffed outward.

But despite looking nothing like a circle, it has constant width. In other words, the distance between opposite parallel supporting lines stays the same as you rotate it.

That means a properly designed Reuleaux-triangle cover can share the same useful can't-slip-through property. Florida State University's mathematics materials use precisely this shape in discussing the manhole-cover puzzle.

So technically, the answer isn't:

"Manhole covers have to be round."

It's more like:

"Circles happen to be an exceptionally practical solution."

Which is much less catchy during a job interview.

The Rabbit Hole

Speaking of interviews, the manhole-cover question became famous as a brainteaser-style interview question — the kind meant to see how someone approaches an unfamiliar problem rather than whether they memorized the answer beforehand. Researchers have even studied people's reactions to puzzle interview questions using the manhole-cover question specifically.

But there's another misconception worth clearing up.

Not every mysterious metal cover you see in a street is necessarily a sewer cover.

Manholes and similar access points can provide entry to all kinds of underground infrastructure and utilities.

And not every utility cover is round, either.

New York City's own 311 information distinguishes round manhole hardware from square or rectangular utility-access covers.

So if you're now preparing to walk around town triumphantly pointing at every rectangular metal plate and declaring that the engineers screwed up...

Maybe don't.

Okay, But Why Should I Care?

You probably shouldn't.

At least not in any life-changing sense.

Knowing why manhole covers are round isn't going to lower your electric bill, make your boss less annoying, or finally explain where all the missing socks went.

But that's exactly why it's interesting.

Somebody had to think about this.

At some point, engineers had to solve the very real problem of putting a removable, extremely heavy lid over a hole in the ground while making it safe, practical and manageable for the people who would work with it.

And the solution became so ordinary that millions of us drive across it every day without noticing it.

There are thousands of little decisions like that hiding in plain sight.

Which is precisely the kind of thing that can ruin a perfectly good five-minute internet break.

Wait, Seriously? Verdict

So yes, those manhole covers are round for a reason.

They're safer against accidentally falling through a circular opening, they don't have to be carefully aligned, and when a giant chunk of cast iron needs to move, being able to roll it is awfully convenient.

And just when you think you've solved the mystery, mathematics shows up with a weird rounded triangle capable of pulling off the same trick.

Now you're going to notice manhole covers everywhere.

You're welcome.

Your Turn

What's another completely ordinary thing you've seen a thousand times but suddenly realized you have absolutely no idea why it's designed that way?

Wait, Seriously? — The Internet's Most Interesting Five Minutes.