Half Square Triangle Calculator
Calculate precise cutting measurements for quilting half square triangles
Calculate HST Measurements
Choose whether to calculate cutting size from finished size or vice versa
The size of the completed half square triangle unit
HST Formula
Raw Cut Size = Finished Size + 7/8"
Finished Size = Raw Cut Size - 7/8"
Diagonal = Finished Size × √2 (≈ 1.414)
Why 7/8"? This accounts for ¼" seam allowances on all sides plus the bias stretch that occurs when cutting triangles.
How to Make Half Square Triangles
Cut and Mark
Cut two squares using the calculated raw size. Draw a diagonal line from corner to corner on the lighter fabric.
Draw Sewing Lines
Mark lines ¼" on each side of the diagonal line. These are your sewing lines.
Sew and Cut
Place fabrics right sides together. Sew along the ¼" lines, then cut along the diagonal line.
Press and Trim
Press seams toward darker fabric. Trim dog ears and square up to finished size if needed.
Common HST Sizes
Quilting Tips
Use a ¼" seam allowance for accurate results
Press seams toward the darker fabric
Use a rotary cutter for precise cutting
Square up finished HSTs to exact size
Trim dog ears for cleaner pressing
Use 100% cotton fabric for best results
Understanding Half Square Triangles
What Are Half Square Triangles?
Half square triangles (HSTs) are fundamental quilting units consisting of two right triangles sewn together to form a square. They're one of the most versatile building blocks in quilting, used in countless traditional and modern quilt patterns.
Why the 7/8" Rule?
- •Accounts for ¼" seam allowances on all four sides
- •Compensates for bias stretch when cutting triangles
- •Allows for minor cutting and sewing variations
- •Provides enough fabric for squaring up
Essential Tools
Cutting Tools
- • Rotary cutter for clean, straight cuts
- • Square quilting ruler (6½" or larger)
- • Self-healing cutting mat
- • Fabric scissors for detail work
Measuring & Marking
- • ½" quilting ruler for seam lines
- • Water-soluble fabric marker
- • Mechanical pencil for fine lines
- • Template plastic (optional)
Pro Tip: Always test your ¼" seam allowance before starting a project. Consistent seam allowances are crucial for accurate HST sizing.