Half-Square Triangle Quilt Block Pattern

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How to Sew a Half-Square Triangle Quilt Block

The Half-Square Triangle (HST) is one of the most versatile and foundational blocks in quilting. By piecing together two triangles to form a square, you can create countless geometric designs — from pinwheels to stars and beyond!

Materials Needed

• Fabric in at least two different colors or prints
• Ruler
• Rotary cutter or fabric scissors
• Sewing machine
• Iron and ironing board

Step-by-Step Instructions

1. Cut two squares of fabric to the same size. For a 4" finished block, cut 5" squares.
2. Place the two squares right sides together, edges aligned.
3. Draw a diagonal line from one corner to the opposite corner on the wrong side of one square.
4. Sew a ¼" seam on both sides of the marked line.
5. Cut along the marked line to create two HST units.
6. Press the seams open on each unit.
7. Trim the dog ears (small triangles that stick out beyond the seam) from each unit.
8. Place the two HST units right sides together, seams matching.
9. Sew a ¼" seam along one side.
10. Open the block and press the seam open.
11. Trim the block to your desired finished size.
12. Repeat to create as many HST blocks as needed for your quilt.

No-Bias HST Technique

This technique eliminates the need to directly manipulate the stretchy bias of the triangle. It uses two easy-to-cut squares and produces two HSTs at once.

Step 1: Draw the Diagonal Line

On the back of the lighter fabric, draw a pencil line diagonally from corner to corner.

Step 2: Sew Both Sides of the Line

Stack a pair of light and dark squares right sides together. Sew a ¼" seam allowance on each side of the line.

You will end up with something like this:

Step 3: Cut Along the Diagonal Line

Step 4: Press the Seams

Press the seam to set it, then press toward the darkest fabric.

Step 5: Trim to Size

Line up the 45° diagonal angle on your ruler with the seam, then carefully trim with a rotary cutter.

How to Resize Quilt Blocks

The first step in modifying any quilt block is to decide on your finished block size. You can base this on doubling a pattern, cutting it in half, or working with your available fabric.

Note: When working from a pattern's cutting instructions, remove the seam allowance before scaling. For example, if your pattern calls for 3½″ squares, subtract the seam allowance (½″), double the finished size (3″ → 6″), then add the seam allowance back (½″) — giving you a 6½″ cut piece.

Resizing Square Blocks

Add ½″ to your finished block measurement. For a 4″ finished square, cut a 4½″ square of fabric.

Resizing Rectangular Blocks

Add ½″ to both the length and width. To double a 3″ × 4″ block, cut a 6½″ × 8½″ rectangle.

Resizing Half-Square Triangle Blocks

Add 7/8″ to the desired finished block size. For a 4″ finished HST block, cut 4⅞″ squares.

Resizing Quarter Square Triangle Blocks

Add 1¼″ to the desired finished block size. For a 4″ finished block, cut 5¼″ squares.