Half Square Triangle Calculator
Calculate raw fabric square size for half-square triangles (HST) in quilting. Enter finished size for precise cutting dimensions with seam allowance included.
About This Calculator
This Half Square Triangle (HST) Calculator helps quilters determine the exact raw fabric square size needed to create half-square triangles of a desired finished size. Whether you are a beginner learning quilting basics or an experienced quilter planning a complex quilt pattern, this tool ensures accurate fabric cutting every time.
A half-square triangle is a fundamental quilting unit — a right-angle triangle sewn together with another to form a square block. The key to perfect HSTs is correct sizing: the raw fabric square must be cut larger than the finished size to account for seam allowances. The standard formula adds 7/8 inch (0.875 inches) to the desired finished size: Raw Square = Finished Size + 7/8 inch. For quilters who prefer the oversize and trim method, we also provide the trim-to size using the +1/2 inch allowance, which lets you sew the HST slightly oversized and then trim it down to perfection.
Our calculator also provides the diagonal length of the raw square, which helps with fabric layout planning when cutting multiple HSTs from a single strip of fabric. The size comparison chart shows how your chosen size fits within the range of common HST sizes from 2 to 6 inches, making it easy to plan your quilt design.
Regional Notes
Global: Half-square triangles are measured in inches across all regions (US, UK, India, and worldwide) as the quilting industry standard uses imperial measurements. The 7/8 inch seam allowance is universal. Fabric widths are typically 44 to 45 inches (112 to 115 cm) globally for quilting cotton.
US: Quilting is most popular in the United States, where fabric is sold by the yard (36 inches). Our results help you plan yardage requirements efficiently.
UK: UK quilters may find fabric sold in meters (100 cm). Convert results using the factor 1 yard = 0.914 meters, or use our conversion calculators for precise metric measurements.
India: Quilting fabric in India is often sold in meters; the standard fabric width is approximately 44 inches. The same HST formulas apply since measurements are in inches.
Frequently Asked Questions
What is a half square triangle in quilting?
A half square triangle (HST) is a quilting unit made by cutting a square diagonally to create two right-angle triangles. These triangles are then sewn together along the long edge to form square blocks in quilt patterns.
How do I calculate the raw fabric size for a half square triangle?
Add 7/8 inch (0.875) to your desired finished size. For example, for a 4-inch finished HST, cut your raw square at 4 7/8 inches (4.875). Our calculator does this automatically for any finished size you enter.
What is the 7/8 inch seam allowance for?
The 7/8 inch (0.875) represents the standard seam allowance for HSTs. When you sew two triangles together, 1/4 inch on each side (total 1/2 inch) is consumed by the seam. The remaining 3/8 inch accounts for the fabric lost when cutting the square diagonally.
What is the oversize and trim method for HSTs?
Many quilters prefer cutting the raw square slightly larger (adding 1/2 inch instead of 7/8 inch), sewing the HST, and then trimming it down to the exact finished size. This ensures perfectly sized HSTs. Our calculator shows both the standard and trim-to sizes.
Can I use this calculator for metric measurements?
While the calculator works in inches (the quilting industry standard), you can convert your desired finished size to inches first. One inch equals 2.54 cm. The 7/8 inch seam allowance equals approximately 2.2 cm.
What are common half square triangle sizes?
Common finished HST sizes range from 2 inches to 6 inches, with 2.5, 3, 3.5, 4, 4.5, and 5 inches being the most popular choices. Our size chart shows the corresponding raw square dimensions for these standard sizes.
How many HSTs can I make from one strip of fabric?
The number depends on your raw square size and fabric width (typically 44 to 45 inches for quilting cotton). Divide the fabric width by your raw square size (rounded down), then multiply by the number of strips you cut. Our diagonal length result helps with layout planning.
What tools do I need to make half square triangles?
Essential tools include a rotary cutter, a square quilting ruler with a 45-degree line, a 1/2 inch quilting ruler for seam marking, and an iron for pressing. Many quilters also use pre-marked patterns or specialty HST rulers for faster production.