Half Square Triangle Calculator

What Is a Half Square Triangle?

A half square triangle (HST) is a fundamental quilting unit made by sewing two right-angle triangles together to form a square. Half square triangles are essential building blocks for countless quilt patterns, from simple designs to complex geometric arrangements.

Getting the measurements right is critical for successful HSTs. Our half square triangle calculator helps you determine the correct raw fabric size based on your desired finished square size, accounting for the standard 7/8 inch seam allowance.

The Formula

The calculation is simple: add 7/8 inch to your desired finished square size to get the raw fabric square size. For example, for a 4-inch finished square, cut your fabric squares to 4 7/8 inches. The diagonal of the raw square is then used to cut two triangles.

How to Make Half Square Triangles

1. Cut two fabric squares to the calculated raw size.
2. Place the fabrics together with right sides facing each other.
3. Mark a diagonal line from one corner to the opposite corner.
4. Sew 1/4 inch on each side of the marked diagonal line.
5. Cut along the marked diagonal line to separate into two units.
6. Press open with the seam allowance toward the darker fabric.
7. Trim the dog ears at each end of the diagonal.

For larger projects, check out our Fabric Calculator to estimate total yardage needed.