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Half-Square Triangle Cutting Calculator

Size the starting squares for half-square triangles that finish at your measurement with 2, 4 or 8 at a time, and see how many squares and units to cut.

the sewn-in size of one short side, before any seam allowance
switching converts the sizes already typed
per side; quilting is 0.25 in (6.35 mm)
whole units; the tool rounds up to whole pairs
4 at a time puts the bias on the outside edges
Enter the finished size to see the starting square.

The cut size is the exact minimum for the seam allowance you enter: each starting square is the finished triangle plus the allowance on all three sides, which is the finished size plus 2.4142 allowance widths, scaled by 1 for 2-at-a-time, 1.4142 for 4-at-a-time and 2 for 8-at-a-time. It is rounded up to the next 1/8 in (or whole millimetre) so you can cut with a normal ruler. That leaves no trimming room, so cut 1/8 to 1/4 in larger if you like to square units up after pressing; trimming size is the finished size plus twice the allowance. Figures assume a straight-grain starting square and a seam allowance held exactly; a scant seam or a stretchy bias edge changes the result. Everything runs in your browser with no storage, uploads or network access.