Half square triangle calculator
Pick your finished HST size and method, and get the exact squares to cut, the trim size, and how many squares you need.
Enter your numbers
How it is calculated
Two at a time: draw a diagonal on the back of the lighter square, sew 1/4 in on both sides of the line, cut on the line. The exact starting square is the finished size plus 7/8 in; most quilters cut at finished plus 1 in and trim, which is what the recommended setting uses.
Eight at a time (Magic 8): sew 1/4 in on both sides of both diagonals, then cut in quarters and on both diagonals. Starting square is (finished + 7/8) x 2 exactly, or (finished + 1) x 2 for trimming.
Four at a time: sew 1/4 in around the whole square, cut both diagonals. It is fast but the resulting edges are bias, and it always needs trimming; the tool sizes the square so units trim cleanly to finished plus 1/2 in.
Worked example
For 3 in finished HSTs: 2 at a time uses 4 in squares (3 7/8 exact), 8 at a time uses 8 in squares (7 3/4 exact), and 4 at a time uses 5 1/2 in squares. All trim to 3 1/2 in unfinished. To make 24 of them 8 at a time, cut 3 background and 3 print squares.
Common questions
- What size squares make 2 1/2 inch finished HSTs?
- Two at a time: 3 1/2 in squares (3 3/8 exact). Eight at a time: 7 in squares. Four at a time: 4 3/4 in squares. Every unit trims to 3 in unfinished.
- Should I cut HST squares oversize?
- Most quilters do. The extra fabric lets you trim every unit to a perfect square, which makes the whole quilt fit together better. The cost is a little fabric and the trimming time.
- Which HST method is best?
- Two at a time keeps edges on the straight grain and suits small batches. Eight at a time is the fastest for many identical units and also keeps straight-grain edges. Four at a time is quick but leaves bias edges, so press and handle those units gently.
- What does finished versus unfinished mean?
- Finished is the size the unit shows in the completed quilt. Unfinished is finished plus 1/2 in, the size it measures before it is sewn in. You trim to the unfinished size.
Related tools
SewCalc results are careful estimates based on standard practice. Fabrics, brands and personal technique vary, so always confirm measurements against your own pattern or project before cutting.