Finding the Center of a Circle with a Quilting Ruler

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I didn’t realize when I started quilting just how much math is involved! I didn’t exactly doubt my math teaches in school when they said we’d need this stuff later in life, but I expected I’d be using it for something a little more, you know, scientific like laboratory experiments. Not warm and snuggly quilts.

No complaints here though! Thinking is good for the brain, so we gotta keep that gray wrinkly blob in tip top shape!

I’m currently working on a project that requires finding the center of a circle. Times like this I wish I had a compass laying around because that would make it easy, but for the time being I make my circles by finding some random round object around the house and trace it. Easy for making nice round circles, not so much for knowing where the center is.

Today I’m going to show you how to find the center using a common quilter’s tool – the quilting ruler.

Step 1: Draw a Chord

We’re starting off with some brain exercises today! Unless you’re one of the more mathematically inclined folks out there chances are you’re thinking “WTF is a chord?” At least I was.

A chord is simply a line that connects two points on circle.

You can draw a straight line right? Great! So draw a chord on your circle! (For ease of the next step it’s super helpful if the length of the chord is easily divisible by 2.)

Drawing chord on circle with quilting ruler

Step 2: Find the Midway Point

Measure the chord from circle edge to circle edge and mark the midpoint.

Finding midpoint of chord with quilting ruler

Step 3: Draw a Perpendicular Line from the Midpoint

We need to draw a 90° angle and luckily for us, quilting rulers have these really nice straight lines in them! Match up your chord with a line on the ruler at the midway point. (I know in the picture it looks a little off, but that’s because it’s taken at an angle. Make sure you’re looking straight down when you’re lining it up.)

Lining up chord midpoint with quilting ruler

Then draw your perpendicular line, making sure it extends farther than the center of the circle.

Drawing perpendicular line on chord with quilting ruler

Step 4: Repeat Steps 1-3

You’ll need a minimum of at least 3 but you can do more if you want to or for better accuracy.

Center of circle with chords and ruler

Now you know an easy way to find the center of a circle with just a ruler!

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