Half Square Triangle Calculator

This half square triangle calculator helps determine the size of fabric squares needed to create half square triangles, which are fundamental components in many quilt designs.

HSTs are created by sewing two squares of fabric together diagonally and then cutting them apart to form two triangular units.

You want to make a 4-inch finished HST, you would typically start with two 5-inch squares. The hst calculator takes into account the seam allowance and the fact that you’re working with the diagonal of the square. By inputting your desired finished size, the calculator provides the initial square size needed, saving time and reducing fabric waste.

Half Square Triangle 4 Time 2 Time Size Chart

Half Square Triangle Formula

The formula for calculating the starting square size for a Half Square Triangle is:

Finished HST size + 7/8 inch = Starting square size

A finished HST of 3 inches: 3 inches + 7/8 inch = 3 7/8 inches

This formula accounts for the 1/4 inch seam allowance and the diagonal cut. The extra 7/8 inch ensures that your HST will be the correct size after trimming.

What is a triangle half of a square?

A triangle half of a square is literally what it sounds like – a triangle that forms half of a square when two of them are placed together. In quilting terms, this refers to a Half Square Triangle (HST).

You take a 5-inch square and cut it diagonally from corner to corner, you’ll end up with two triangles. Each of these triangles is half of the original square. When these triangles are sewn together along their long edges (the hypotenuse), they form a new square that’s slightly smaller than the original due to seam allowances.

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