Half-Square Triangle Calculator

Two-at-Once only needs a small fixed add-on (7/8 inch) because a single diagonal cut removes a thin strip of fabric. Four-at-Once and Eight-at-Once involve two diagonal cuts (an X) through the same square, which removes a wedge of fabric whose area grows with the size of the square rather than staying fixed -- so the starting square has to scale up with the finished size, not just add a constant. Using the wrong shape of formula for these methods can undersize your fabric by a full inch or more on a typical block.

It depends on the method: Two-at-Once and Eight-at-Once both bake in a fixed standard quarter-inch seam allowance, so changing Seam Allowance only moves the Unfinished HST Size output for those two, not the starting square. Four-at-Once's starting square formula is built from the unfinished HST size, which does mechanically scale with Seam Allowance, so it's the one method here where a wider or narrower seam allowance moves the starting square -- but treat that as a reasoned approximation rather than confirmed math, since published Four-at-Once references parametrize the underlying constant to a fixed standard seam, not a variable one.

It's how many finished half-square triangles you get out of one pair of starting squares (one light fabric square and one dark fabric square, both cut to the Starting Square Size) using the selected method. Two-at-Once yields 2 HSTs per pair, Four-at-Once yields 4, and Eight-at-Once yields 8, because each method packs more diagonal cuts into the same pair of squares.

Two-at-Once needs the smallest starting square and wastes the least fabric per HST, and it's the easiest to sew accurately since there's only one seam line to keep straight. Four-at-Once and Eight-at-Once trade some extra fabric and a slightly larger starting square for speed -- you produce more HSTs per cutting-and-sewing pass, which matters more on a quilt that needs dozens or hundreds of identical units.

Treat it as a minimum rather than a final cutting number. It does not build in extra room for fabric shrinkage, stretch along the bias edges some methods create, or the trimming margin many quilters add so the sewn HST can be squared up perfectly before final trim. Many quilters round up to the nearest common ruler increment (an eighth or quarter inch) for easier cutting.