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Half-square triangle calculator: HST cut sizes

Find the square size to cut for half-square triangles made 2, 4 or 8 at a time, how many squares you need and how much fabric.

Finished HST size (in)

How many HSTs

Method

Usable fabric width (in)

Cut a little larger and trim

All three methods compared

Method	Exact cut size	Cut to trim	Squares per fabric	Yardage per fabric	HSTs made

The formulas

Two at a time: draw a diagonal on the lighter square, sew ¼″ on both sides, cut on the line. Cut squares = finished size + ⅞″.

Four at a time: sew ¼″ around all four edges of two squares placed right sides together, then cut both diagonals. Cut squares = (finished size + ⅞″) × 1.414.

Eight at a time: draw both diagonals, sew ¼″ on each side of both lines, then cut horizontally, vertically and on both diagonals. Cut squares = (finished size + ⅞″) × 2.

Exact sizes are rounded up to the next ⅛″. With trimming on, the calculator adds ⅛″ for two at a time and ½″ for the four- and eight-at-a-time methods, whose edges are on the bias. Trim every HST to the finished size + ½″.

How to use it

Enter the finished size of one half-square triangle, measured without seam allowances (the size it will be in the quilt).
Enter how many HSTs you need.
Compare the three methods: two at a time, four at a time and eight at a time. Pick the one you prefer.
Cut the squares listed for each of your two fabrics, sew, cut apart, press and trim each HST to the unfinished size.

Questions

What size square do I cut for a half-square triangle?

For the classic two-at-a-time method, cut squares 7/8 inch larger than the finished size. A 3 inch finished HST needs 3 7/8 inch squares. Many quilters cut 1 inch larger and trim for extra accuracy.

Why add 7/8 inch?

A 1/4 inch seam on each edge adds 1/2 inch, and the diagonal seam takes up extra fabric across the corner. Together that comes to 7/8 inch for two-at-a-time HSTs.

Which HST method is fastest?

Eight at a time is fastest when you need many identical HSTs, but every edge ends up on the bias, so press gently and trim. Two at a time is the most accurate and easiest for beginners.

What size should I trim my HSTs to?

Trim each unit to the finished size plus 1/2 inch. A 3 inch finished HST is trimmed to 3 1/2 inches, with the diagonal seam running exactly corner to corner.