🧵 Half Square Triangle Calculator
Get the exact starting square size to cut for your half square triangle (HST) quilt blocks, plus how many squares you’ll need.
How half square triangles are cut
A half square triangle (HST) quilt block is commonly made by pairing two same-size squares of fabric, drawing a diagonal line, sewing a quarter-inch seam on each side of the line, then cutting along the line to yield two finished HST units.
The formula
Starting square size = Desired finished HST size + 7/8 inch. This extra 7/8 inch accounts for the seam allowances on both sides of the diagonal cut. The unfinished block size (before it’s sewn into the quilt top) is the finished size plus 1/2 inch.
Since each pair of starting squares yields 2 HST units, the calculator rounds up to the number of square pairs needed to reach your target quantity.