How this instrument works
A half-square triangle (HST) unit is, geometrically, a square cut along its diagonal into two congruent right isosceles triangles, then re-pieced with a contrasting triangle to form a new square split diagonally by color. Because the finished unit needs seam allowance around all four edges, plus a little extra to account for fabric consumed in the diagonal seam itself, the starting cut square always has to be meaningfully larger than the block's finished size.
This calculator uses the widely followed convention for the efficient 'two-at-a-time' construction method: cut square size equals finished size plus 7/8 inch. That extra 7/8 inch covers a standard quarter-inch seam allowance on both sides of the diagonal cut line (a half inch total), plus a bit more to account for fabric lost to the diagonal seam's angle rather than a straight edge — the two-at-a-time method sews two full squares together, marks and stitches a quarter inch on each side of a diagonal line, then cuts apart on that line to yield two HST units from one pair of squares.
The second figure this calculator returns — finished size plus a full inch — is a 'trim-friendly' alternative some quilters prefer: cutting deliberately oversized, then squaring up (trimming) each finished HST unit down to its exact target size with a rotary cutter and ruler afterward. That extra margin forgives small stitching drift at the cost of a little more fabric and an added trimming step, which some quilters find worth the trade for cleaner, more consistently sized blocks.
- Enter your target finished HST size, in inches — the size the block will measure once sewn into the quilt.
- Read the exact cut-square size for the standard two-at-a-time piecing method.
- Read the trim-friendly cut size if you'd rather square up the block after sewing.
- Cut two squares at your chosen size, layer them right sides together, and mark, sew, and cut on the diagonal per your pattern's instructions.
Worked example — a 4-inch finished HST block
Targeting a 4-inch finished HST: exact cut size = 4 + 0.875 = 4.875 inches per square, using the standard two-at-a-time seam allowance.
The trim-friendly alternative for the same 4-inch target: 4 + 1 = 5 inches per square — a slightly larger starting square that leaves an extra 1/16 inch of margin on each side after sewing, giving room to true up the block to an exact 4.5-inch unfinished size (finished plus a standard quarter-inch seam allowance on each remaining edge) with a rotary cutter.
Questions
Why isn't the cut square just the finished size plus a quarter inch?
A quarter inch would only cover a single straight seam, but an HST unit is built by cutting on a diagonal line with a seam sewn on each side of that line, meaning two seam allowances are consumed, not one — plus a small additional margin because a diagonal seam eats slightly more fabric than a straight one at the same width. That combination is what adds up to the 7/8 inch total allowance.
What's the difference between the "exact" and "trim-friendly" cut sizes?
The exact size (finished plus 7/8 in) gives precisely enough fabric for the standard seam allowance with no room for stitching error, while the trim-friendly size (finished plus a full inch) deliberately over-cuts, then relies on squaring up the finished block afterward with a rotary cutter to true it to exact size — a small fabric and time trade-off some quilters make for more forgiving, consistently sized results.
How many half-square triangle units does the two-at-a-time method make per pair of squares?
Two — the method layers two full squares of contrasting fabric right sides together, sews a quarter-inch seam on both sides of a marked diagonal line, then cuts apart along that diagonal to separate the pair into two completed HST units in one pass, which is more efficient than cutting and piecing a single HST at a time.
Does this formula work for any seam allowance, or only a quarter inch?
This calculator's 7/8 inch allowance is specifically built around the standard quarter-inch quilting seam — a widely used default across quilting patterns, though not universal. If your pattern specifies a different seam allowance, the extra margin needed would scale differently than this calculator's fixed 7/8 inch figure.
Why is squaring up (trimming) recommended for pieced blocks?
Even careful piecing accumulates small inconsistencies from seam width variation or fabric stretch, and squaring up — trimming each block to an exact, consistent size after sewing — corrects for that drift so all your blocks match precisely when assembled into rows, avoiding the rippled or uneven look that slightly mismatched block sizes can cause in a finished quilt top.