Why Your HSTs Aren’t Big Enough to Trim
Ever cut your half square triangles exactly the size the pattern told you to, sew them the way you were supposed to, and still don’t have enough to trim? If that’s happened to you more than once, it’s not bad luck. There’s an actual reason for it — and once you see it, it’s an easy fix.
Why this happens
There’s real math behind those starting square sizes patterns give you — it’s not arbitrary. That math is what tells you how much extra room you need beyond your finished HST size to account for your seams.
But there’s a second factor that matters just as much, and it’s the one that trips people up: your actual seam allowance. That math only works if your seam really is a true quarter inch. Sew a little wide, even by a hair, and you can follow the pattern exactly and still come up short at trim time.
What’s in the video
In the video I walk through where those starting square sizes actually come from, then show you a real side-by-side comparison — three HST units, same starting size, sewn with slightly different seam widths — so you can see with your own eyes how much that difference matters.
If you’ve been consistently short on trimming room, this will tell you why, and what to check first.
What to do about it
Two options, and you can use either one or both:
Sew closer to a true scant quarter inch. Worth testing your seam allowance if you haven’t checked it in a while.
Or just cut your starting squares a little bigger. Look through your pattern, find which pieces are becoming your HST units, and add 1/8″ to those cut sizes. Just double check you’ll still have enough fabric to do that, and test it once before you commit to a whole quilt’s worth.
More help with HSTs
Want the actual trim technique next? Here’s how I trim HSTs — 3 methods. Or if you want to start from the basics, this is the HST fundamentals post.
Happy sewing! 😉
Want more of the why behind quilting technique — not just the how?
