What size to cut for 4 inch half-square triangles?

For 4 in finished half-square triangles, cut 4 7/8 in squares to make 2 at a time, or 9 3/4 in squares to make 8 at a time. Trim units to 4 1/2 in.

Four-inch HSTs make quick, bold blocks. Four of them make an 8 in finished block.

4 7/8 in squares: 2 HSTs at a time.

9 3/4 in squares: 8 HSTs at a time.

Trim each unit to 4 1/2 in before piecing.

How it's worked out

2 at a time: finished size + 7/8 in = 4 + 7/8 = 4 7/8 in. Draw a diagonal, sew 1/4 in either side, cut on the line.
8 at a time: 2 x (finished + 7/8 in) = 9 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 = 4 1/2 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.