What size to cut for 2 1/2 inch half-square triangles?

For 2 1/2 in finished half-square triangles, cut 3 3/8 in squares to make 2 at a time, or 6 3/4 in squares to make 8 at a time. Trim units to 3 in.

Two-and-a-half-inch HSTs pair neatly with 2 1/2 in strips and precut squares.

3 3/8 in squares: 2 HSTs at a time.

6 3/4 in squares: 8 HSTs at a time.

Trim each unit to 3 in before piecing.

How it's worked out

2 at a time: finished size + 7/8 in = 2 1/2 + 7/8 = 3 3/8 in. Draw a diagonal, sew 1/4 in either side, cut on the line.
8 at a time: 2 x (finished + 7/8 in) = 6 3/4 in. Draw both diagonals, sew 1/4 in either side of each, then cut horizontally, vertically and on both diagonals.
Unfinished size = finished + 1/2 in = 3 in. Many quilters cut squares 1/8 in larger and trim for accuracy.

Planning the whole quilt? The Quilt Planner has a backing and binding worksheet, fabric requirements and a cutting chart for every project.