Updated Nov. 11, 2025
When I first started quilting 25 years ago, the method for making half-square triangles was simple but limited. Cut your squares 7/8 inch larger than the finished block, stack them together, sew a 1/4-inch seam on both sides of a center line, cut them apart, and press. You should have the right-sized blocks following these steps.
Except I never did. Ever. It was so frustrating.
And while several new methods for creating half-square triangles have emerged and the rulers—oh, the rulers!—make the process clever, my favorite way to make half-square triangles is to make them larger than I need and then trim them down after sewing.
It’s tedious work, but I’ve really found that doing it this way has improved my accuracy so my points are actually sharp! And who doesn’t love that?!
It took me some time to perfect my trimming technique, so I wanted to share it with you to help save you time and frustration.
Make the Blocks Bigger
I add 1 inch to the finished size I want: so if you want your HSTs to finish at 2 inches, cut your squares 3 inches. This gives me plenty of room for mistakes or ruler wiggles while I'm cutting. Once I have my squares cut, I draw a line from one corner to the other. You'll sew a quarter-inch on each side of this line, but first... I know I say never to use your ruler marks on your cutting mats because they're notorious for being inaccurate. However, I find they really help when drawing my lines. I line up each corner along one line, set my ruler on top just to the side of that line, and draw with my fabric-safe pen or pencil.
Trimming in Two Parts
After sewing along the line you drew in the step above, cut the squares apart along that line and press open your half-square triangles. Once pressed, you're ready to start trimming. To keep your diagonal line perfectly straight, you will need to trim in two stages.
Part One
Align the 45-degree line on your ruler with the diagonal line on your square. I like to cut as close to the edges as possible on this first trim so I can try again if the ruler shifts; I'll still have plenty of room for a second attempt. Once everything is aligned, trim along the side and top, basically cutting off any fabric that extends beyond the ruler.
Part Two
Turn the block over. Now align the block with the 45-degree line on your ruler and the 2.5-inch marks for your finished square. Once everything is lined up, trim the fabric from the top and side again. There's not much room for error on the second trim, so make sure everything is correct before you cut.
And There You Have It
You should now have a perfectly trimmed half-square triangle. This method can be a bit tedious, but it's still one of my favorite ways to trim perfect half-square triangles!
