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Cutting Triangles from Rectangles

by Caryl
(San Antonio, TX)

I, too, am a new quilter and know that this should be easy to find...I just can't.

When cutting triangles from a square, you add 7/8" to insure that you have your correct 1/4" allowance. Is there a similar formula when you intend to cut triangles from a longer rectangle (3" x 6")?


There is no standard number to add to both the length and the width of a rectangle when you are cutting for divided rectangles. That is because the legs of the triangle are two different lengths which results in different angle measurements.

Where There's a Tool, There's a Way

Tri-Recs Ruler by EZ Quilting
There is, however, a two piece ruler called the "Tri Recs Tools" made by the EZ Quilting. One ruler is called the "Tri Tool", the other the "Recs" tool. With it, you can use your rotary cutter to speed up the cutting process.

To use this ruler system, fabric strips are cut the finished width plus 1/2" for the shape you'll create. The size strips you need are conveniently marked on the rulers, themselves.

Use both the "Tri" and the "Recs" tools/rulers together, cutting one "Tri" and 2 "Recs" shapes, and you'll create a triangle within a square. Click here to learn and see how Tri Recs Rulers are used to do this.

Use just the "Recs" tool, cutting two pieces and you'll create a divided rectangle that is twice as tall as it is wide. The largest finished rectangle you can create with these rulers is 3" x 6". I've used the ruler for a "Faceted Jewels" quilt pattern by Glad Creations. The ruler was a nice addition to my quilting tools box. It did the job it was designed for.

The Recs ruler has a corner cut out of it known as the "Magic Angle" that helps you to accurately line up your shapes for sewing. A very nice feature, indeed. No guessing how the pieces line up.

Tried and True Templates

Cut your rectangle template in half diagonally
If a new ruler isn't in your future or if your rectangle isn't twice as tall as it is wide, then it's time to make a template.

With your rotary cutter and ruler, cut a rectangle the finished size required by the pattern.

Then cut it in half on the diagonal.

Notice, I've used paper because it's handy. I prefer to cut my templates instead of drawing them. If the pencil is the least bit rounded, the templates could be off by as much as 1/8" each. That would create a big problem if there were many of these divided rectangles in each block.

Cut your rectangle template in half diagonally
Tape or glue one of the resulting triangles in place on top of another sheet of paper.

Now with your ruler and rotary cutter, cut out the new template adding a 1/4" seam allowance to each side.

When I cut my shapes from the fabric, I'll place the 1/4" line of my rotary ruler on the turquoise edge of the inner triangle. Having the seam allowance represented by another color of paper makes it easier for me to see and cut accurately.

No matter which method you choose, good luck with your rectangles.


Julie Baird

Link to Free Quilt Block Patterns Library

Comments for Cutting Triangles from Rectangles

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Cutting rectangle on diagonal
by: AnonymousLy nda

Is there a way to measure for cutting a RECTANGLE on the diagonal? Perhaps a formula? Templates are definitely out.

From the Editor:

If you are making Half Rectangles, using a template, paper piecing or a specialty ruler is the way to go. (Try the Tri Recs and use just the Recs part). I'll be writing a couple of tutorials for this unit later this year.

Unfortunately, there are several tutorials on the Internet that provide numbers, but there's a lot of fudging in them. Some are just plain wrong.

I wish I could be of more 'easy' help, but the math is a bit funky and not particularly rotary cutter friendly.

~ Julie

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