Half Square Triangles (often shortened to HSTs) are one of the most useful building blocks in patchwork and quilting. They appear in everything from traditional quilts to modern designs but they do need accurate trimming to make sure everything fits together neatly.
In this post, I’ll walk you through how to trim down a Half Square Triangle step by step, using a square quilting ruler with a 45° diagonal line.
What you’ll need
- Your sewn Half Square Triangle (pressed and slightly oversized)
- A square quilting ruler with a 45° diagonal line
- Rotary cutter
- Cutting mat
Tip: HSTs are usually made slightly larger than needed and then trimmed back — this is what gives you accuracy later on.
Step 1: Press your HST
Before trimming, make sure your Half Square Triangle is pressed flat.
- Press the seam open or to one side — whichever you prefer — but be consistent.
- The block should lie flat with no tucks or bubbles.
This is important, as trimming a block that hasn’t been properly pressed can throw off your measurements.
Step 2: Line up the diagonal seam
Place your HST on the cutting mat and position your ruler on top.
- Line the 45° line on your ruler directly over the seam line of the HST.
- Take your time here — this alignment is the key to getting a perfectly square block.
Your ruler should sit comfortably over the block, even if the fabric edges extend beyond it.
Step 3: Set the finished size
Still keeping the diagonal line matched to the seam:
- Slide the ruler so that the horizontal and vertical measurements match your required unfinished block size.
- For example, if you need a 2½” unfinished HST, make sure the ruler markings read 2½” on both sides.
At this stage, you’ll usually see excess fabric sticking out on all sides — that’s exactly what we want.
Step 4: Trim the first two sides
Holding the ruler firmly in place:
- Trim along the right-hand side of the ruler.
- Then trim along the top edge of the ruler.
You’ve now created two clean, square edges at the correct size.
Step 5: Rotate and trim the remaining sides
Rotate the block 180 degrees.
- Line the ruler back up, using the freshly cut edges to sit exactly on the measurement you need.
- Double-check that the diagonal seam still runs through the corners correctly.
Trim the final two sides.
Step 6: Check your finished HST
You should now have a perfectly trimmed Half Square Triangle:
- Square
- The correct size
- With the diagonal seam running neatly corner to corner
This accuracy makes such a difference when joining blocks together — especially in quilts with lots of points meeting.
A few extra tips
- Trim every HST, even if it looks close enough — consistency is what matters.
- Keep your ruler and cutter sharp for clean edges.
- If you’re trimming lots of HSTs, take regular breaks — accuracy drops when we rush.
Half Square Triangles may be simple, but trimming them properly is one of those small skills that really elevates your patchwork. Once you get into the rhythm, it becomes second nature — and your blocks will thank you for it.
Happy sewing ✂️🧵