Half-Square Triangle Cut Size
Cut square sizes for half-square triangles two, four or eight at a time and for quarter-square units, from the finished size and your seam allowance, in or cm.
The starting square for the method you use, rounded up to your step, the unfinished size to trim to, how many squares of each fabric a count of units needs, and all four methods side by side.
Example: A 3 in finished HST with ¼ in seams: 3.854 in squares two at a time (3⅞ rounded), 5.450 in four at a time, 7.707 in eight at a time; a quarter-square unit needs 4.207 in.
The ⅞ rule,
derived.
Where the published “add ⅞ inch” and its cousins come from, how the four methods differ, why the page works from the seam allowance, and what oversize and rounding do.
Two at a time
Two squares right sides together, a diagonal drawn, a seam sewn a seam allowance s either side of it, cut on the line: two half-square triangles. The seam runs along the diagonal, so the seam allowance at the diagonal edge costs s√2 measured along a side, and the two straight edges cost s each. The square is F + s(2 + √2): at ¼ in that is F + 0.854 in, which the rule of thumb rounds to ⅞. Eight at a time is the same construction on a square twice the size, cut on both diagonals and both centre lines: 2 × [F + s(2 + √2)], or 2F + 1¾ at ¼ in.
Four at a time and quarter-squares
Sewing all four edges of two squares and cutting both diagonals gives four units whose outer edges are on the bias; the unfinished unit’s diagonal is S − 2s, so S = (F + 2s)√2 + 2s. A quarter-square (hourglass) unit is made of four triangles whose long edge is the outside: offsetting the seams inward by s on a right isosceles triangle shortens the hypotenuse by 2s(1 + √2), so the square is F + 2s(1 + √2) — F + 1.207 in, the published 1¼.
Rounding and oversize
Cut sizes are rounded up to the step you choose (⅛ in, ½ cm, or none for a rotary ruler with finer marks); rounding up only ever adds fabric, which is trimmed. Oversize is an amount you add to the finished size on purpose, to cut every unit back to an exact unfinished size after pressing. The unfinished size to trim to is always F + 2s.
What the number does not settle
The seam allowance is yours — ¼ in and 0.75 cm are common, but a scant ¼ in is a different figure. Thread take-up and pressing loss are why quilters oversize; the page does not estimate them. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.