Half Square Triangle Calculator
Cut size for HST units, and how many squares.
Calculate the cut square size for half square triangles from the finished size, and how many squares a given number of units needs.
Cut squares at
4.88"
8 squares for 8 units
The seven eighths is two quarter-inch seam allowances plus the extra the diagonal seam consumes. Each pair of squares yields two units, so an odd count still needs a whole pair and leaves one spare.
How the Half Square Triangle Calculator works
A half square triangle is cut from squares seven eighths of an inch larger than the unit will finish. That odd fraction is two quarter-inch seam allowances plus the extra the diagonal seam eats, and it is worth committing to memory because it is the single most used number in patchwork.
Also known as: HST calculator · quilt triangle calculator
Where the seven eighths comes from
A half square triangle is made by sewing two squares together on the diagonal and cutting them apart. The finished unit loses a quarter inch seam allowance on each of the two sewn edges, which is half an inch, and the diagonal seam consumes a further three eighths because it crosses the corner rather than running along an edge.
Half plus three eighths is seven eighths, and it is constant regardless of the unit size. A 3 inch finished HST is cut from 3⅞ inch squares; a 6 inch from 6⅞. Because it never changes, it is worth memorising rather than looking up.
Each pair of squares yields two units, so the square count is always even. Needing seven units means cutting eight squares and having one unit spare.
A worked example
Eight finished 4 inch units. Cut squares at 4⅞ inches. Eight units is four pairs, so eight squares in total — four of each fabric. Draw the diagonal on the lighter square, sew a quarter inch either side of the line, cut on the line and press.
Cutting oversize at 5 inches instead and trimming each unit back to 4½ inches unfinished costs about an eighth of an inch of fabric per square and removes all the accumulated error from marking, stitching and pressing. On a quilt where the HSTs must nest with other units, that trade is usually worth it.
Where the answer misleads
The seven eighths assumes a scant quarter inch seam — a thread or two under a true quarter — which is the quilting convention precisely because the fold of the pressed seam takes up a little fabric. Sewing a full quarter inch produces units consistently undersized by about a sixteenth, which compounds across a block.
Pressing matters as much as cutting. Pressing the seam open or to one side changes the finished dimension slightly, and pressing with steam can stretch the bias edge that runs along the diagonal. HSTs are bias-edged units, and handling them like straight-grain ones distorts them.
Finally, this method is one of several. Eight-at-a-time and magic-eight methods use different allowances entirely, so a cut size from one method will not work with another's instructions.
Frequently asked questions
What size squares do I cut for HSTs?
The finished size plus seven eighths of an inch. For a 4 inch finished unit, cut 4⅞ inch squares. Two squares sewn on the diagonal and cut apart yield two units.
Where does the seven eighths come from?
A quarter inch seam allowance on each of the two sewn edges is a half inch, and the diagonal seam consumes roughly another three eighths because the seam runs across the corner rather than along an edge.
Should I cut oversized and trim?
Many quilters cut a full inch larger and trim each unit square afterwards. It costs a little fabric and removes the accumulated error from cutting and stitching, which is worth it when the units have to nest precisely.
How many squares do I need?
Each pair of squares makes two units, so divide the units you need by two and round up for the number of pairs. Odd counts still need a whole pair, and you will have one spare.