Why make them in batches
Half-square triangles turn up in almost every quilt pattern: stars, pinwheels, flying geese, chevrons. A throw can easily need sixty or more, and sewing them one pair at a time is where a weekend quietly disappears.
The magic-eight method makes eight at once from two squares of fabric. You sew two long seams, make four cuts, and end up with a stack of identical units ready to press.
Cutting your squares
Take the finished size you want, add 7/8", and double it. That is the size of the two squares you start with. For 3" finished triangles, cut two 7 3/4" squares, one light and one dark.
If you like a little room to trim, add another 1/2" to the square. Your units will come out slightly large, and trimming them back gives you crisp, accurate points.
Sew, then cut
Place the squares right sides together and draw both diagonals on the back of the lighter one. Sew a scant 1/4" on both sides of each line, so you have four seams in an X.
Cut the square in half across the middle, then in half the other way, then along each drawn line. Eight triangle pairs, already sewn.
Press, then trim
Press each unit open before you trim. Pressing can stretch the bias edge a little, and trimming afterward squares up whatever moved.
Line up the 45-degree mark on your square ruler with the seam, trim two sides, rotate, and trim the other two. A 6" square ruler is all you need for most sizes.
Let Patchwork do the counting
Lay out your quilt in the Quilt Builder and the cut list tells you how many half-square triangles each fabric needs. It plans them two at a time from squares cut on the diagonal; if you would rather sew them eight at a time, divide by eight to get the number of square pairs to cut.