Half Square Triangles (HSTs) are a quiltmaker's best friend! They are a building block in lots of classic and modern quilts. Know how to calculate, cut, and sew them, and you'll be on your way to designing and sewing your own quilts!
This method makes half square triangles two at a time. I like it because it doesn't require sewing along a bias edge, which can stretch and warp as you sew, making a wonky block. So even if I only need 1 half square triangle, I'll make two and save one for later. :)
How to calculate and cut HST's
What size do you want your finished HST's to be? Start with that number, and then add 7/8'' or 1''. Add 7/8'' to your finished HST number and if you sew them precisely, they will end up the required size exactly. More often than not, I'll add 1'' to the finished size so I can trim them up (just in case my stitching is less-than-precise).
Choose two fabrics. Cut two squares the required size (finished unit + 7/8'' or 1''), one from each fabric.
How to Sew HST's
Place the two squares right sides together. Then draw a line (any pencil or pen will do) diagonally from one corner to another. This is your cutting line, not your sewing line.
Sew 1/4'' from each side of the drawn line. Easy, huh? If you don't have a 1/4'' foot for your sewing machine, you may want to draw two stitching lines 1/4'' from either side of your cutting line to give yourself a guide.
Note: I am actually stitching a scant 1/4''. That is just my preference. It helps make up for the tiny bit of fabric loss when I press my seams to the side. I'm going to trim the blocks anyway, so they will be just the right size in the end.
Cut along the cutting line and press both blocks open.
Trim the blocks to the correct size, if needed, and cut off the triangle dog ears. Done!