How to Sew a Half-Rectangle Triangle in Any Size

Half-rectangle triangles may seem just like half-square triangles, but they’re not quite the same. Half-rectangle triangles require a little more planning, but once you get the hang of it, you can use these shapes to bring a whole new design element to your quilting.

What is a half-rectangle triangle?

A half-triangle rectangle is just that: an elongated triangle that forms half a rectangle. This shape can be used to make diamonds, zig zags and other interesting quilt designs.

half-triangle rectangle

How to construct half-rectangle triangle units

Most quilters’ first instinct is to make a half-triangle rectangle the same way they make half-square triangles. 

But I’ll let you in on a secret: This doesn’t work! You can see below that you end up with an odd kite shape.

A little more planning goes into sewing a half-triangle rectangle.

Guidelines for sewing half-rectangle triangles

There isn’t a hard and fast formula to make half-rectangle triangles, but there are guidelines you can follow.

Start with the same size rectangle you want to finish with, and then add ¼” to the width and ½” to the length.

So, if you want the resulting half-rectangle triangle to be 3½” x 7½”, start with two 3¾” x 8″ rectangles. These half-triangle rectangles would finish at 3½” x 7½”.

Easy half-rectangle triangles tutorial

Start with two rectangle fabric pieces.

I recommend using rectangles that have a 1:2 ratio for the length and width. A 1:2 ratio will result in a wider half-triangle rectangle rather than a long skinny shape. As the rectangle gets skinnier, it gets more difficult to work with.

This tutorial doesn’t require you to trim your half-triangle rectangles. If your units are coming out too small, increase the size a bit and then trim back down to the proper size. 

Step 1: 

Stack the two contrasting rectangles with all fabrics right side up.  

Step 2:

Cut each rectangle on the diagonal. Keep in mind that each diagonal will make a half-rectangle triangle that goes in one direction. In this case, I’m cutting each pair of rectangles in the opposite directions.

Step 3:

Sew the cut triangles back together, using two different fabrics and leaving a ¼” overlap on each side. Use a ¼”seam allowance.

half-triangle rectangle

Tip: Use starch to prevent the bias edges from stretching.


Step 4:

Press the half-rectangle triangles open. Each pair of half-rectangle triangles is going in the opposite direction.

Four of these half-rectangle triangles make an elongated diamond. You can easily repeat these steps to make multiple diamonds or other shapes. 

Make any size half-triangle rectangle

This same method works to make just about any size half-rectangle triangle. Below is a skinnier rectangle. These starting rectangles are 2¾” x 8″. The resulting half-triangle rectangles are 2½” x 8″.

Now we have two diamond shapes the same height but different widths. Experiment with different size starting rectangles to get various half-rectangle triangles.

A common problem for diamond units

My first instinct was to sew the half-rectangle triangles together below to make diamond blocks. The side seam allowances kept coming up short, as you can see below.

half-triangle rectangle

A simple solution to this seam allowance problem is to use a bigger seam allowance. To create a diamond shape, first sew the top two half-triangle rectangles together.  Then sew the bottom two half-triangle rectangles together. 

Now comes the step where the seam allowance will change.  Use a 3/8″ or ½” seam allowance when sewing the top and bottom units together.

Now that you know how to sew half-rectangle triangles, you can start dreaming up new quilt designs using this shape!