Half-Square Triangle Calculator: Cut Size and Squares Needed

By Tam Vu · Updated October 13, 2026

For the common two-at-a-time method, start with a pair of fabric squares. Sew on both sides of a diagonal, cut between the seams, and square the two half-square triangle (HST) units to size. This calculator uses a ⅞ in cut-size allowance over the finished unit size, rounded up from the geometric estimate described below.

Why add ⅞ in?

The diagonal seam and the ¼ in seam allowance mean the two sewn triangles need trimming after they are opened. The geometric estimate for the starting square is the finished size plus about 0.854 in: ½ in for the two seam allowances, plus ¼√2 (about 0.354 in) for the diagonal geometry. Rounding that allowance up to ⅞ in gives a repeatable cut increment.

The extra is for trimming accuracy. It does not change the finished size you enter, and the actual sewn units should be squared to the size required by your pattern.

How many squares to cut

Each pair of squares makes two HST units. Divide your desired unit count by two and round up: an even count uses one square for every two units; an odd count leaves one extra unit. For example, 24 units need 12 squares of each fabric. For 25 units, cut 13 squares of each fabric to make 26 units, then keep the extra for another block or a test.

This is a starting-size calculator for the two-at-a-time method. Other methods, specialty rulers and pattern instructions may specify a different cut size. Follow your pattern when its directions differ.

Source

• Cut-size allowance and unit counts are geometry and arithmetic: ¼ in seam allowance, diagonal relation √2, and rounding the 0.854 in allowance up to ⅞ in.