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Half-square triangle calculator

Enter the finished size of your half-square triangle and how many you need. Two-at-a-time method.

Cut squares3 7/8"
  • 4 squares of each fabric make 8 units
  • Each unit measures 3 1/2" before it is sewn into the block
  • Prefer to trim? Cut at 4" and trim to 3 1/2"

Half-square triangle cutting chart

Finished sizeCut squares (exact)Cut squares (trim method)Unit before sewing
1"1 7/8"2"1 1/2"
1 1/2"2 3/8"2 1/2"2"
2"2 7/8"3"2 1/2"
2 1/2"3 3/8"3 1/2"3"
3"3 7/8"4"3 1/2"
3 1/2"4 3/8"4 1/2"4"
4"4 7/8"5"4 1/2"
4 1/2"5 3/8"5 1/2"5"
5"5 7/8"6"5 1/2"
6"6 7/8"7"6 1/2"

How it is calculated

Cut one square from each fabric. Place them right sides together, draw a diagonal line on the lighter one, and sew 1/4" on each side of the line. Cut on the line and press. You get two identical half-square triangles.

The squares are cut 7/8" larger than the finished unit: 1/4" for each of the two outer seams and 3/8" for the diagonal seam, which runs at an angle.

Many quilters add a full 1" instead and trim each unit to size after pressing. It uses a little more fabric and gives more accurate units.

Questions

What size squares for 3 inch finished half-square triangles?

Cut 3 7/8" squares for the exact method, or 4" squares if you want to trim. Each unit should measure 3 1/2" before it is sewn into the block.

How many half-square triangles does one pair of squares make?

Two. For eight units you need four squares of each fabric.

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