Half Square Triangle Calculator (HST cutting size: 2-at-a-time, 8-at-a-time, trim)

Understanding the half square triangle calculator — concept and principle

This calculator gives the size of squares to cut for half square triangle (HST) quilt units, for three common methods, and how much fabric you need. Quilters use it to cut accurately for any finished size and to avoid running short of fabric.

Add ⅞ inch to the finished size: with the default inputs the result is 7.8 in of each fabric (≈ 20 cm).

How it is calculated

Cut square size for a finished size F (in), with ¼ in seams:

Two at a time: F + ⅞ in (2 HSTs per pair of squares).
Eight at a time: 2F + 1¾ in (8 HSTs per pair).
Oversize and trim: F + 1 in, then trim each unit to F + ½ in (2 per pair).

Squares per fabric = units ÷ HSTs per pair, rounded up. Squares per strip = usable width ÷ cut size, rounded down. Strips = squares ÷ squares per strip, rounded up. Fabric length = strips × cut size.

The principle

A finished HST is cut ½ in larger than its finished size, because each edge loses ¼ in to the seam. The diagonal seam also sits ¼ in from the cut bias edge, and measured along the square's side that offset is ¼ × √2 ≈ 0.354 in. So the exact minimum square is F + 0.5 + 0.354 = F + 0.854 in, which quilters round up to ⅞ in. The eight-at-a-time method sews around both diagonals of a larger square and cuts it into eight. Cutting 1 in oversize and trimming gives the most accurate units.

Worked example

Default inputs: finished 3 in, two at a time, 24 units, usable width 42 in.

Cut square = 3 + 0.875 = 3.875 in (9.8 cm).
Squares per fabric = 24 ÷ 2 = 12.
Unit before sewing in = 3 + 0.5 = 3.5 in; exact minimum = 3.5 + 0.354 = 3.854 in.
Squares per strip = floor(42 ÷ 3.875) = 10; strips = ceil(12 ÷ 10) = 2.
Fabric = 2 × 3.875 = 7.75 in, shown as 7.8 in (20 cm) of each fabric.

Each point is the calculator re-run with only “Finished HST size (in)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Finished HST size (in)	Cut square size (in)
0.75	1.625
1.5	2.375
2.25	3.125
3	3.875
3.75	4.625
4.5	5.375
6	6.875

How to read the result

Buy a little more than the fabric length to straighten the cut edge and allow for shrinkage if you pre-wash. If accuracy matters, for example for small HSTs, choose the oversize-and-trim method; it uses more fabric but gives crisp points. For many identical units, eight-at-a-time is fastest.

Common mistakes

Using + ½ in instead of + ⅞ in for the two-at-a-time method.
Sewing an inconsistent seam allowance.
Pressing with steam and stretching the bias edges.
Forgetting that each fabric needs the same number of squares.

Keep in mind: Fabric widths vary after removing selvages and shrinkage. Measure your own usable width before cutting.

Frequently asked questions

What size square for a 3 in finished HST?

3⅞ in for the two-at-a-time method.

Why ⅞ in and not ½ in?

The diagonal seam adds about 0.354 in on top of the ½ in seam allowance, which rounds up to ⅞ in.

Which method is most accurate?

Cutting oversize by 1 in and trimming to size.

Sources: Geometry of a ¼ in seam allowance on a 45° diagonal (offset ¼ × √2), as used in standard quilting cutting charts

Checked 9 Nov 2026. Geometry of the ¼-in seam allowance on a 45° diagonal (offset ¼ in × √2); the ⅞-in, 1¾-in and 1-in additions are the usual quilting roundings of that minimum; usable fabric width is an example value.

Your numbers