Half Square Triangle Quilt Calculator
Free Half square triangle quilt tool for clothing & sewing. Enter your details to get instant, tailored results and guidance.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Cut Size (2-at-a-time) = Finished HST Size + 7/8 inch
For the two-at-a-time method, add 7/8 inch to the finished HST size for the cutting square. For four-at-a-time, add 1-1/4 inch. For eight-at-a-time, multiply the finished size by 2 and add 1-1/4 inch. These allowances account for seam allowances and diagonal sewing geometry.
Worked Examples
Example 1: Baby Quilt with 3-inch HSTs
Problem:Calculate fabric requirements for a baby quilt (36x48 inches) using 3-inch finished HSTs with the two-at-a-time method.
Solution:HSTs across: 36 / 3 = 12\nHSTs down: 48 / 3 = 16\nTotal HSTs needed: 12 x 16 = 192\nCut size: 3 + 7/8 = 3.875 inches\nSquares per row (42-inch fabric): floor(42 / 3.875) = 10\nSquare pairs needed: 192 / 2 = 96\nRows needed per fabric: ceil(96 / 10) = 10\nFabric per color: 10 x 3.875 = 38.75 inches = 1.08 yards
Result:Need 96 square pairs, 1.08 yards each of two fabrics, cut at 3.875 inches
Example 2: Pinwheel Table Runner
Problem:Make a table runner with 24 pinwheel blocks using 2.5-inch finished HSTs. Each pinwheel uses 4 HSTs.
Solution:Total HSTs: 24 pinwheels x 4 HSTs = 96 HSTs\nCut size (two-at-a-time): 2.5 + 0.875 = 3.375 inches\nSquare pairs needed: 96 / 2 = 48\nSquares per row (42-inch fabric): floor(42 / 3.375) = 12\nRows needed: ceil(48 / 12) = 4\nFabric per color: 4 x 3.375 = 13.5 inches = 0.38 yards
Result:Need 48 square pairs, 0.38 yards each of two fabrics, cut at 3.375 inches
Frequently Asked Questions
What is a half-square triangle (HST) in quilting?
A half-square triangle (HST) is one of the most fundamental and versatile quilt blocks in patchwork quilting. It consists of a square divided diagonally into two equal right triangles, typically made from two different fabrics. When multiple HSTs are arranged together, they can create dozens of classic quilt patterns including pinwheels, flying geese, chevrons, herringbone, and many star variations. The HST is sometimes called a 'triangle square' or 'half-square triangle unit.' Its versatility comes from the fact that by rotating and combining HSTs in different orientations, quilters can create an almost infinite variety of designs. Learning to make accurate HSTs is considered an essential skill for any quilter.
What is the 7/8 inch rule for half-square triangles?
The 7/8 inch rule is the standard formula for the two-at-a-time HST method: cut your starting squares at the finished HST size plus 7/8 inch (0.875 inches). This extra 7/8 inch accounts for the seam allowances on all sides after sewing and cutting. For example, if you want a 3-inch finished HST, you cut 3.875-inch squares. The math works because you need two 1/4-inch seam allowances (totaling 1/2 inch) plus the diagonal seam loses approximately 3/8 inch due to the geometry of sewing on the diagonal. Together, 1/2 plus 3/8 equals 7/8 inch. Some quilters prefer to add a full inch instead for a more generous trim allowance, which gives slightly oversized units that can be trimmed to exact size for maximum accuracy.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy