The half-square triangle formulas
A half-square triangle (HST) is a square made of two right triangles. The extra you add to the finished size covers the seam allowances on the diagonal and around the edges:
- 2 at a time: cut squares = finished size +
7/8". Pair a light and a dark square, draw a diagonal line, sew ¼" on both sides, cut on the line. Two HSTs per pair. - 8 at a time: cut squares = (finished size +
7/8") × 2. Sew ¼" on both sides of both diagonals, then cut horizontally, vertically and on both diagonals. Eight HSTs per pair. - Trimming: many quilters add 1" instead of 7/8" (or 2" for 8-at-a-time) and trim each unit to finished + ½". It takes a little longer but gives perfectly square, accurate units.
Example
For 3" finished HSTs with the 2-at-a-time method, cut 3 7/8" squares. Each light/dark pair makes two units that measure 3½" unfinished and finish at 3" in the quilt.
How the yardage is worked out
Squares are cut from strips across the width of fabric (WOF). The calculator fits as many squares as possible into your usable width, works out how many strips you need, and rounds the total up to the next ⅛ yard, the smallest cut most shops sell. Consider buying a little extra for mistakes or straightening the fabric.
Frequently asked questions
Why do you add 7/8" to half-square triangles?
The diagonal seam takes up more fabric than a straight one. Adding ½" for the outer seam allowances plus about ⅜" for the diagonal gives 7/8". It is the standard allowance for the two-at-a-time method.
Should I make HSTs oversized and trim them?
If accuracy matters, yes. Bias edges stretch and pressing can distort the unit. Adding 1" and trimming to finished + ½" gives crisp, square units.
What size do I cut for 8-at-a-time HSTs?
Double the 2-at-a-time size: (finished + 7/8") × 2. For 3" finished units, cut 7¾" squares. Many quilters use (finished + 1") × 2 and trim.