Enter the finished size of the HST (its size once it is sewn into the quilt) and how many you need. The calculator gives the square size to cut, how many squares of each fabric, the size to trim to, and how many spares you will have.
Worked example: 12 HSTs that finish at 3 inches
12 identical HSTs, 3 in finished (3 1/2 in unfinished), from two fabrics.
- Exact, two at a time: 3 + 7/8 = 3 7/8 in squares. Each pair makes two HSTs, so 12 / 2 = 6 squares of each fabric. No trimming; the units come out at 3 1/2 in.
- Oversized, two at a time: 3 + 1 = 4 in squares, 6 of each fabric, trimmed to 3 1/2 in.
- Eight at a time: 2 x (3 + 1) = 8 in squares make eight HSTs. 12 HSTs take 1 square of 8 in plus 2 squares of 4 in of each fabric, trimmed to 3 1/2 in, with 0 spare.
Result: 6 squares of 3 7/8 in of each fabric (exact), or 6 squares of 4 in to trim down.
HST formulas
- Unfinished size = finished size + 1/2 in (1/4 in seam allowance on each side).
- Exact two at a time: square = finished + 7/8 in. The extra 3/8 in over the seam allowance covers the diagonal seam, so a 2 in HST starts from 2 7/8 in squares. Accurate, but there is nothing left to trim.
- Oversized two at a time: square = finished + 1 in, then trim to finished + 1/2 in. QuiltLab's block construction recipes use it, because trimming fixes small sewing errors.
- Eight at a time: square = 2 x (finished + 1 in). Sew around both diagonals, cut through the middle both ways and on both lines, then trim. When the count is not a multiple of eight, the remainder is made two at a time.
- Squares of each fabric = HSTs needed / HSTs per pair, rounded up; any extra units are spares.
Assumptions
- Seam allowance is 1/4 in.
- All HSTs are the same size and made from the same two fabrics. Work out each fabric pair separately.
- Finished sizes from 1 in up, entered in sixteenths of an inch or simpler fractions (3, 3 1/2, 2 3/4).
- The oversized and eight-at-a-time sizes are the same formulas QuiltLab's block construction recipes use, checked by a geometry simulation in the test suite. A simulation is not a sewn sample: make one test unit before cutting everything.
How to make half-square triangles two at a time
Make two 3 1/2 x 3 1/2 in HSTs from two 4 in squares, then trim.
- Draw a diagonal line from corner to corner on the wrong side of the lighter square.
- Lay it right sides together on the other square, edges matched.
- Sew 1/4 in from the line on both sides, then cut on the line. You now have two units.
- Press each unit open, seam toward the darker fabric.
- Trim each unit to 3 1/2 x 3 1/2 in: lay the ruler's 45-degree line on the seam so the seam runs exactly corner to corner, trim two sides, turn the unit and trim the other two.
Frequently asked questions
What size square do I cut for a half-square triangle?
Add 7/8 in to the finished size for exact HSTs, or 1 in if you want to trim them down. For a 3 in finished HST, cut 3 7/8 in squares (exact) or 4 in squares (trim to 3 1/2 in).
Why add 7/8 inch instead of 1/2 inch?
A square only needs 1/2 in for its seam allowances, but a triangle also loses length along the diagonal seam. The extra 3/8 in covers that, so two squares cut at finished + 7/8 in give HSTs exactly finished + 1/2 in.
Is it better to make HSTs oversized and trim them?
Many quilters prefer it. Bias edges stretch and seams drift, and trimming lines the seam up corner to corner at the right size. It costs a little more fabric and a trimming step.
How many HSTs does the eight-at-a-time method make?
Eight from one pair of squares cut at 2 x (finished + 1 in). For counts that are not multiples of eight, the calculator makes the remainder two at a time so you do not end up with many spares.