How this calculator works
The two-at-a-time method starts with two squares of fabric placed right sides together. You draw a diagonal, sew a quarter inch on each side of it and cut along the line, which gives two half-square triangles. The parent square has to be larger than the finished square because the diagonal seam eats allowance along the hypotenuse. The traditional rule is to add 7/8 inch, or 2.22 cm, to the finished size. The finished diagonal is the finished square multiplied by the square root of two, about 1.414.
Formula
parent square = finished size + 7/8 inch (2.2225 cm). finished size = parent square - 7/8 inch. finished diagonal = finished size x 1.414. One parent square makes two half-square triangles.
Worked example
A finished 15 cm square needs a parent square of 15 + 2.2225 = 17.22 cm. The finished diagonal is 15 x 1.414 = 21.21 cm. One parent square yields two finished half-square triangles.
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Frequently asked questions
Why 7/8 inch rather than a full inch?
The diagonal seam allowance is shared between two triangles cut from the same square. Seven eighths is the tested allowance that leaves the finished square the intended size after pressing; a full inch works but needs trimming, and half an inch is usually too small.
Do I need to trim the squares after pressing?
Trimming to size gives cleaner points and more accurate blocks, especially on larger quilts. Many quilters add a little extra, press, then trim to the finished size. The chart gives the measured size before any extra trimming allowance.
Does this work for eight-at-a-time triangles?
No, the allowance differs because four seams meet in the centre. For eight-at-a-time, use a parent square of twice the finished size plus 1.75 inches. This page covers the two-at-a-time method, which is the most common.
What is the diagonal for?
The diagonal is the length across the finished square, which matters when you are matching the triangle to a border or when you cut setting triangles. It is 1.414 times the finished square, not the length of the seam.