How to Trim Diamond in a Square units

Let's talk about the "Diamond in a Square" or "Square in a Square" unit. As simple as the appear, these guys can be a little bit tricky. For the Mosaic Star quilt pattern (releasing Nov. 17!) I have you make the units just slightly oversized so that you can trim them down to be exactly what you need. That being said, these units are a little finicky in the sense that your seam allowance must be an accurate 1/4" because you do not have any wiggle room for the center square. For whatever size Diamond in a Square unit you're making, your center square should FINISH 1/2" smaller than what it was to begin with. If your seam allowance is too large or too small, you might not have enough fabric to trim while still keeping a 1/4" overlap on all 4 sides of the unit. Let's take a closer look.


Whenever you go to trim this unit, take the unfinished measurement and divide it in half. In this instance, I need my unit to measure 4.5" square (unfinished), so my landmarks will be 2.25". You can see these landmarks indicated by the red dots in the diagram below. 

Place your ruler on top of the unit and align each of the four corners with the 2.25" marks on your ruler. So the top and bottom corner will be aligned with the vertical 2.25" line on your ruler. And the left and right corners will be aligned with the horizontal 2.25" line on your ruler.

Then you'll want to double check that the the ruler gives you 1/4" from the corner of the diamond to the edge of the ruler along the right and top edges (opposite if you're left handed) as indicated by the red arrows. Then trim those two sides.

Now that you have two edges that are perfectly square, you'll need to square up the other two edges. Rotate the unit around and do the same. Be sure you keep your 2.25" landmarks right over each of the four corners and that you have 1/4" between the corners of the diamond and the edge of the block. Then trim the remaining two sides.

Looking at the unfinished unit, you'll see that there are 1/4" overlaps between the corners of the diamond and the edge of your unit. Those 1/4" overlaps will disappear whenever you go to sew this unit in your quilt, leaving you with beautiful, perfect points!

I hope you found this tutorial helpful! If you need further trimming advice, check out these trimming tutorials:

And if you want to try all of these techniques in one place, be on the lookout for the Mosaic Star pattern release on November 17th! 


Happy sewing!