Quilt HST Calculator: Half-Square Triangle Cut Size and Squares

Work out HST cut size before you cut. Enter finished HST size and HSTs needed, and see cut square size, squares per fabric, total squares and HSTs made, with the formula in steps.

Calculate your HST cut size

Finished HST size in

HSTs needed

HSTs needed is a whole number, greater than zero. Count HSTs by fabric pair, not by quilt, if different fabric pairs use different finished sizes.

This calculator uses the two at a time method only. Each pair of squares makes 2 HSTs, so the square count is given per fabric pair, not per finished HST.

This calculator uses the two-at-a-time method only. Two squares, one of each fabric, sewn on the diagonal and cut apart, make 2 HSTs. For other HST methods, use the QST Calculator for quarter-square triangles, or the Fabric Calculator for fabric yardage.

Your result

Enter your measurements, then calculate.

How the calculator works

Step 1: Cut square size

Cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches, for the two-at-a-time method. The extra 7/8 inch covers seam allowances on the diagonal seam and trimming to finished size. For finished HST size 4 inches, cut square size equals 4 plus 0.875, equals 4.88 inches, rounded at two decimal places, which is 4 and 7/8 inches.

Step 2: Squares per fabric

Squares per fabric equals HSTs needed divided by 2, rounded up to a whole number, because each pair of squares makes 2 HSTs, and a partial pair cannot be sewn. For HSTs needed 20, squares per fabric equals 20 divided by 2, equals 10 squares per fabric.

Step 3: Total squares and HSTs made

Total squares equals squares per fabric times 2, because each pair uses one square of each fabric. For squares per fabric 10, total squares equals 10 times 2, equals 20 squares. HSTs made equals squares per fabric times 2, because each pair makes 2 HSTs. For squares per fabric 10, HSTs made equals 10 times 2, equals 20 HSTs, exactly the number needed, with no extra HST.

Assumptions, shown next to the result

This calculator uses the two-at-a-time method only, with seam allowance 0.25 inch on each side, as is standard for piecing. Cut square size is used as entered in the unit mode you choose, with no extra added for mistakes or shrinkage, beyond the 7/8 inch already in the formula. If HSTs needed is an odd number, squares per fabric is rounded up, and HSTs made will be one more than HSTs needed, which is one extra HST, not a mistake.

HST cut size chart

This chart uses the same formula as the calculator: cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches, for the two-at-a-time method.

Finished HST size	Cut square size
2 inches	2.88 inches (2 and 7/8 inches)
3 inches	3.88 inches (3 and 7/8 inches)
4 inches	4.88 inches (4 and 7/8 inches)
6 inches	6.88 inches (6 and 7/8 inches)
8 inches	8.88 inches (8 and 7/8 inches)

Chart values are a guide for the finished sizes shown. Your own finished size may give a different result. Use the calculator above for your own measurements.

Worked example, checked against the calculator

Finished HST size 4 inches, HSTs needed 20.

Cut square size: 4 plus 0.875 equals 4.88 inches, which is 4 and 7/8 inches.

Squares per fabric: 20 divided by 2 equals 10 squares per fabric.

Total squares: 10 times 2 equals 20 squares.

HSTs made: 10 times 2 equals 20 HSTs, exactly the number needed, with no extra HST.

Frequently asked questions

How do I calculate half-square triangle cut size?

For the two-at-a-time method, cut square size equals finished HST size plus 7/8 inch, which is 0.875 inches. For example, a 4 inch finished HST needs 4.88 inch cut squares, which is 4 and 7/8 inches.

What is a half-square triangle?

A half-square triangle, or HST, is a square made from two right triangles, sewn together on the diagonal, with two different fabrics. Its finished size is the size in the finished quilt, after seams are sewn.

How many squares do I need for HSTs?

For the two-at-a-time method, each pair of squares, one of each fabric, makes 2 HSTs. Squares per fabric equals HSTs needed divided by 2, rounded up to a whole number. For example, 20 HSTs need 10 squares per fabric, 20 squares total, and make 20 HSTs exactly.

🪶