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Shop Math: How to Find the Center of a Circle



The Voice: Hey, Kevin. What are you doing?

Kevin Caron: I'm here in the studio doing some homework for one of my YouTube videos. I had a question the other day from one of my YouTube subscribers: "How do you find the middle of a circle if there's no center mark?"


Well, if you guys are anything like me, and you've been away from school for awhile, you forget some of the math stuff you learned early on, so I keep this great book in my work area.

The Voice: What's it called?

Kevin Caron: It's called "Math to Build On". It contains all the formulas you need for figuring out the center of a circle; figuring out the length of objects; figuring out how to bend, and so forth. All the formulas are right inside one little book: Math to Build On by Johnny and Margaret Hamilton. Get this over at Pipefitters.com. Go check them out.

So, you want to find the center of a circle? Measure it; it's twelve inches. What's the center? It's got to be six inches somewhere, right?

The Voice: Yep.

Kevin Caron: Take your pencil and just mark it at six inches. Is that going to tell you right where the center of the circle is? Not yet.

Get another mark at six inches and yet another mark at six inches. Where all the marks meet up, there's your center.

The Voice: Well, that seems easy.

Kevin Caron: If you have a tape measure. Do you want to cheat a different way? If you happen to have a compass with a pencil or a piece of chalk for working with steel, you can get on the outside of your circle and just draw a line. Come over to the outside of the circle about a third of the way over; a quarter of the way over and draw another line; one more, just to be sure. Do three of them. Draw a line, and where they intersect is your center. It's that easy.

If you take the diameter, divide it by two, you get the radius. And that's that dimension right there, from the outside of the circle to the center.

Measure in three ways; where they all meet, there's your center.

OK, class over. Back to work.

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