Half Square Triangle Calculator
Type the size, get the square to cut for two, four, eight, and sixteen at a time, how many squares that is, and how much fabric. Then see exactly how to sew each one.
Type the size your pattern gives you and pick Finished or Unfinished to match (if the pattern says “trim to 3½″”, that’s unfinished). Add how many HSTs you need and every card below fills in the square to cut, how many to cut from each fabric, and the yardage. Leave Cut a little big on unless your ¼″ seam is dead accurate. Scroll down, pick the method you like, and follow its pictures. Nothing to submit; it updates as you type.
How the math works
Every half square triangle starts life as a square of two fabrics sewn together on the diagonal. The whole calculation is “how big does that square need to be so that, after the ¼″ seam and the cut, what’s left measures the unfinished size?” Three formulas cover it, all assuming a ¼″ seam, and the big-batch methods are just those formulas doubled:
- Two at a time (and cut triangles): finished size + ⅞″. Or + 1″ if you’re trimming, which you should be.
- Four at a time: unfinished size × 1.414 + ½″. The 1.414 is √2, because the square’s outside edge becomes the HST’s diagonal.
- Eight at a time: twice the two-at-a-time square, because each quarter of it is literally a two-at-a-time unit.
- Sixteen at a time: twice the four-at-a-time square. Once the outside and both center lines are sewn, each quadrant has a seam on all four edges and gets cut on both diagonals, which is exactly four at a time, four times over.
Every cut size is rounded up to the next ⅛″, the finest line on a quilting ruler. Rounding down is how units come up short.
Questions people ask
Why 7/8″? Where does that number come from?
The seam runs corner to corner, on the diagonal, and a ¼″ seam measured across a diagonal eats about 0.354″ of the square’s width (¼″ × √2). Add the ½″ the straight sides need (¼″ each side) and you get 0.854″, which every ruler rounds to ⅞″. It works if your seam is dead on. Most people’s isn’t, which is why the calculator defaults to cutting 1″ bigger and trimming.
Finished vs. unfinished: which one do I type in?
Whichever your pattern gives you. Finished is what the HST measures once it’s sewn into the quilt with its neighbors. Unfinished is what you trim it to before that, which is always ½″ bigger. If the pattern says “trim to 3½″”, flip the toggle to Unfinished and type 3.5.
Which method should a beginner use?
Two at a time. One drawn line, two seams, one cut, and every edge you handle is on the straight grain. Once your ¼″ is reliable, eight at a time makes the same units in a quarter of the time. Four and sixteen at a time are quick but leave all the outside edges on the bias, so save them for when you’re comfortable with starch and a light touch.
What is a bias edge and why does everyone keep warning me about it?
Fabric stretches very little along its threads and a lot on the diagonal between them. A cut on that diagonal is a bias edge. It will stretch as you sew and press, and an HST with a stretched edge doesn’t fit its block. Cutting triangles one at a time puts a bias edge on the seam you’re sewing; four and sixteen at a time put bias on every outside edge. Two and eight at a time keep the bias on the inside, where the seam already holds it.
What seam allowance does this assume?
¼″, the quilting standard, and what every printed pattern assumes. If you sew a scant ¼″ you’ll come out a hair big, which trimming takes care of.
How is the yardage worked out?
Squares are cut from strips across the width of the fabric, assuming 40″ of usable width off a 42″–44″ bolt after selvages. It counts how many squares fit across one strip, how many strips you need, and rounds up to the next ⅛ yard. It’s per fabric, for the count you typed, and doesn’t add any extra for mistakes. Buy a little more than it says.