Size two-at-a-time half-square triangles

Use the size after the unit is sewn into the quilt

Finished size is the visible size of the HST after it has been sewn into the surrounding block. Unfinished size includes the edges that those later seams will take up. A 4-inch finished HST with 1/4-inch seams needs to measure 4 1/2 inches before it is sewn into the block.

The starting squares are larger again. They must provide enough material for the diagonal construction and for squaring the unit. Keeping these three sizes separate helps prevent cutting a starting square at the finished size.

Understand the calculator's starting-square rule

Quilt & Piece uses: starter square = finished size + 2 × seam allowance + starting-square allowance. Its defaults are a 1/4-inch seam and a 3/8-inch starting-square allowance, which add 7/8 inch in total. That matches the traditional finished-size-plus-7/8 planning rule.

The 3/8-inch field is the allowance beyond the unfinished target; it is not an additional 3/8 inch on top of the traditional 7/8 inch. The cutting list rounds upward to the next 1/4 inch. If a custom seam allowance needs a larger starting square for the diagonal seam, the tool raises the cut size and tells you. Increase your allowance if you want more room to square up, and sew a test pair with your actual fabrics.

An example: 32 HSTs finishing at 4 inches

With the default seam and allowance, the calculated starter size is 4 + 1/2 + 3/8 = 4 7/8 inches. Rounded upward to the next 1/4 inch, the cutting list calls for 5-inch starter squares. The target to trim after sewing is still 4 1/2 inches, not 5 inches.

One pair of starter squares makes two HST units. For 32 units in the same two fabrics, cut 16 starter squares from each fabric: 16 pairs and 32 starter squares in total. A request for 33 units needs 17 pairs and produces 34 units, leaving one extra.

Make and check a test pair

Place the two starter squares right sides together and mark a diagonal on the wrong side of one square. Stitch on each side of that line using your planned seam allowance, then cut on the marked diagonal. Open and press the two units before measuring.

Square each unit to its unfinished target, with the ruler's diagonal aligned to the diagonal seam. If the test units cannot reach the target, adjust your stitching or use larger starter squares before cutting the rest of the batch.

Keep the cutting list specific

The tool counts one pairing of two fabrics. If your pattern has several color pairings, calculate and label each group separately rather than assuming the total can be divided equally among all fabrics. It does not calculate four-at-a-time or eight-at-a-time HSTs.

• Record the finished target and the unfinished trim size.
• Label starter squares by fabric and by pair count.
• Keep any extra unit from an odd quantity with the project.