Half Square Triangle Calculator — HST Cutting Sizes

For the 2-at-once method, cut squares 2.5cm larger than the finished HST size — for a 7.5cm finished HST, cut 10cm squares. The exact imperial figure is finished + 7⁄8" (2.2cm), which covers a ¼" (6mm) seam each side of the diagonal plus the cut. The 2.5cm default rounds that up so you can trim each unit down to an accurate finished size.

Layer a light and a dark square right sides together, stitch a scant ¼" (about 0.6cm) all the way around the outside edge, then cut on both diagonals. This yields 4 HSTs from one pair of squares. Cut squares to the UNFINISHED HST size (finished + ½" / 1.27cm seam allowance) divided by 0.64 — for a 7.5cm finished HST the unfinished size is about 8.8cm, so cut about 13.7cm squares, then add a small trim margin (0.5cm default). Unlike the 2-at-once method this is a multiplicative rule, so the extra fabric grows with the HST size rather than being a fixed addition.

Cut two squares sized (finished × 2) + 4.5cm — roughly double the size of a 2-at-once square — then draw both diagonals, stitch a scant ¼" each side of both lines, and cut on the drawn lines plus horizontally and vertically through the centre. One pair of these larger squares makes 8 HSTs, which is why it is the most fabric-efficient method for big batches.

Yes, always trim HSTs to exact size after pressing. This ensures accuracy when joining blocks. Use a square ruler with a diagonal line for precise trimming.

Press seams towards the darker fabric to prevent show-through. Press flat first to set the stitches, then open the unit and press from the right side. Some quilters prefer pressing seams open for flatness.