About the Half-Square Triangle Calculator
This half-square triangle (HST) calculator tells you what size squares to cut so your HST units finish at the size your quilt pattern asks for. Enter the finished size and the number of units you need, pick the 2-at-a-time, 4-at-a-time or 8-at-a-time method, and it returns the cut size rounded up to the nearest 1/8", the number of square pairs to cut and the fabric each colour needs.
It is meant for quilters adapting a pattern to a new block size, scaling a quilt up or down, or choosing the most efficient way to make a big batch of HSTs. A comparison table shows all three methods side by side so you can see which one uses the least fabric and the fewest cuts for your project.
Sizes assume a standard 1/4" seam allowance and are derived from the geometry of each method rather than a lookup chart. Add a trimming allowance if you like to sew oversized units and square them up afterwards — many quilters add 1/8" to 1/2" for perfect points.
How to use the half-square triangle calculator
- 1Enter the finished HST size from your pattern (not the cut size).
- 2Pick 2-, 4- or 8-at-a-time and enter how many HST units you need.
- 3Optionally add a trimming allowance if you like to square up oversized units.
- 4Cut the listed number of squares from each fabric at the cut size shown.
- 5Sew, cut apart, press and trim each unit to the unfinished size.
Formula and method
Every HST finishes ½" smaller than its unfinished size because of the two ¼" seam allowances. In the 2-at-a-time method you sew ¼" either side of a diagonal, so the diagonal seam costs ¼" × √2 ≈ 0.354" of square width — which is why pattern charts say "finished size + ⅞"". The 8-at-a-time method is four 2-at-a-time squares sewn as one, so the square is doubled.
The 4-at-a-time method sews around the outside of two squares and cuts both diagonals, so the original edge becomes the HST diagonal: the square must equal the unfinished size × √2 plus ½". The calculator computes the exact value, adds any trimming allowance, and rounds up to the next ⅛" so you never cut too small. Yardage assumes squares are sub-cut from width-of-fabric strips the same width as the square.
- S
- Size of each square to cut (inches)
- F
- Finished HST size (inches)
- √2
- ≈ 1.41421, ratio of a square’s diagonal to its side
Worked examples
24 three-inch HSTs, 2 at a time
For a 3" finished HST the exact square is 3 + 0.5 + 0.354 = 3.854", rounded up to 3 7/8". Each pair makes 2 HSTs, so 24 units need 12 squares of each fabric. Ten 3 7/8" squares fit across 42" of fabric, so you need two strips (7.75", about 0.22 yd) of each colour.
32 four-inch HSTs, 4 at a time
The unfinished size is 4.5", so the square is 4.5 × 1.41421 + 0.5 = 6.864", rounded up to 6 7/8". Each pair yields 4 HSTs, so 8 pairs make 32. Six squares fit per 42" strip, so two 6 7/8" strips (13.75", about 0.38 yd) of each fabric are needed.
40 two-inch HSTs, 8 at a time
The 8-at-a-time square is 2 × (2.5 + 0.354) = 5.707", rounded up to 5 3/4". Five pairs give 40 HSTs, and seven squares fit across the fabric, so a single 5 3/4" strip of each colour (about 0.16 yd) is enough.
Frequently asked questions
What size square do I cut for a half-square triangle?+
For the classic 2-at-a-time method, cut squares 7/8" larger than the finished size: a 3" finished HST needs 3 7/8" squares. Many quilters cut 1" larger instead and trim the sewn units down for accuracy.
What is the formula for 4-at-a-time HSTs?+
Multiply the unfinished HST size (finished + 1/2") by 1.414 and add 1/2", or approximately finished × 1.414 + 1 1/4". Round up to the next 1/8". Note that all four edges of these HSTs are on the bias, so handle them gently.
How do I make 8 HSTs at once?+
Cut two squares at twice (finished size + 7/8"), draw both diagonals on the lighter one, sew 1/4" on each side of both lines, then cut horizontally, vertically and along both diagonals. A 3" finished unit needs 7 3/4" squares.
Which HST method is most accurate?+
The 2-at-a-time method keeps the outer edges on the straight grain, so units stay square and are easy to trim. The 8-at-a-time method also gives straight-grain outer edges but has more seams meeting at the points. The 4-at-a-time method is fast but leaves all four outer edges on the bias, which can stretch, so oversize slightly and trim.
What size should I trim HSTs to?+
Trim each unit to the finished size plus 1/2" — for example 3 1/2" for a 3" finished HST — lining up the 45° line of your ruler with the diagonal seam.