My favorite way to make half-square triangles, or HSTs, is to cut them bigger than I need and trim them down. It takes a little extra time, but the points come out sharp and the blocks land at the size they’re supposed to.
That wasn’t always the case for me.
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 quarter-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.
Plenty of new methods and rulers have come along since then. The new rulers are clever and make life easier. But my favorite, the one that finally got my points sharp and my blocks accurate, is the two-at-a-time version where you cut your squares oversized and trim them down after sewing.
It’s a bit tedious. I know. Once your half-square triangles are square and your points are sharp, you’ll forget you ever cared about the extra step.
What You’ll Need
- A square ruler with a 45-degree line
- A rotary cutter
- A fabric-safe marking pen or pencil
- Your cutting mat
A Quick Note on Sizes
There are two terms worth knowing when making and trimming HSTs.
- Finished size is the size your half-square triangle will be once it’s sewn into the quilt. The seam allowances are tucked behind the seam.
- Unfinished size is the size you cut and trim to before piecing. It’s the finished size plus a half inch, which accounts for a quarter-inch seam allowance on each side.
So, a half-square triangle that finishes at 2 inches in the quilt is 2 ½ inches unfinished.
Step 1: Cut Your Squares Bigger
Take the finished size you want and add 1 inch. This is the size square you will cut to start. For example, a 2-inch finished half-square triangle starts as a 3-inch square. That gives you the space you need to make your HSTs slightly larger so you can trim them to size.
Step 2: Draw Your Diagonal Line
Draw a diagonal line from corner to corner on the wrong side of the lighter fabric. Stack the two squares right sides together. Draw a line from corner to corner on the wrong side of the lighter fabric.
Step 3: Sew on Either Side of the Line
Sew a quarter-inch seam on each side of the line you drew. Cut the squares apart along the line, press the seams open or to the dark side (your choice), and you’ll have two oversized HSTs ready to trim.
Trimming in Two Passes
To keep the diagonal seam perfectly centered, trim in two passes. Don’t try to do this all at once. Your ruler will shift, and your points will suffer.
Pass One
Place the HST on your cutting mat with the diagonal seam running from corner to corner. Line up the 45-degree line on your ruler with the diagonal seam. Don’t worry about the exact final size yet. Just trim off any fabric sticking out beyond the ruler on the top and right side.
I cut as close to the edge as I can on this first pass. If the ruler shifts, I still have plenty of room for a second attempt.
Pass Two
Rotate the HST 180 degrees so the trimmed edges are now on the bottom and left. Line up the 45-degree line on your ruler with the diagonal seam again. Then line up the unfinished size marks on your ruler, which is the finished size plus a half inch, with the trimmed edges of the half-square triangle.
Trim the top and right side one more time.
There’s not much room for error on this second pass, so double-check everything is lined up before you cut.
And That’s It
Cut bigger, sew, press, and trim in two passes. You’ll end up with a half-square triangle that’s square, the right size, and ready to nest into sharp, matched points.
If you want to put this technique to work on a quilt full of half-square triangles, check out my School Spirit quilt pattern. It’s made up of half-square triangles and quarter-square triangles, so you can show off your sharp points, and it’s a great way to practice this method on something you’ll love when it’s finished.
