About the Half Square Triangle Calculator
A half-square triangle (HST) is one of the most common quilt block units: a square made from two contrasting right triangles joined along the diagonal. This calculator uses the standard two-at-a-time method to tell you exactly what size to cut your starting squares, how many you need, and how many strips to cut from your fabric.
The standard formula
With the two-at-a-time method, you place two squares of fabric right sides together, draw a diagonal line from corner to corner, sew a straight seam a standard 1/4 inch from the line on both sides, then cut apart on the drawn line and press each piece open. This single pair of squares yields two finished half-square triangle units.
- Cut square size = Finished HST size + 7/8 inch (0.875"). This is the widely used quilting convention for a standard 1/4 inch seam allowance — it covers both seams plus a small trimming margin so the pressed unit squares up cleanly.
- Unfinished block size = Finished HST size + 1/2 inch. Once pieced, the HST square itself (before it is sewn into the quilt top) carries a 1/4 inch seam allowance on each of its four sides.
- Starting squares needed = number of HST units ÷ 2, rounded up. Every pair of squares produces exactly two HSTs, so odd quantities still require a full extra pair.
Worked example
To make 20 HST units finishing at 4 inches: cut squares at 4 + 0.875 = 4.875 inches (4-7/8"). You need 20 ÷ 2 = 10 starting squares. On fabric that is 42 inches wide (usable, after trimming selvedges), you can fit floor(42 ÷ 4.875) = 8 squares per strip, so you'd cut ceil(10 ÷ 8) = 2 strips that are each 4.875 inches wide.
How to get the best results
- Measure and cut with a rotary cutter and quilting ruler for accuracy — small cutting errors compound across many HSTs.
- Sew a true, consistent 1/4 inch seam allowance; the 7/8 inch add-on assumes this standard width.
- Always trim (square up) finished HST units to their exact unfinished size before assembling a block, especially with the two-at-a-time method.
Practical context
This calculator handles the standard two-at-a-time cutting method, the most common approach for HSTs. Other methods — such as the four-at-a-time or eight-at-a-time techniques used for making many identical HSTs at once — use different starting square sizes and are not covered here. When in doubt, cut a test unit and measure it before committing to a full strip of fabric.