Half-Square Triangle Cut Sizes Explained

Half-square triangles (HSTs) are squares made from two right-angle triangles. They appear in countless blocks, and the most common question is simply: what size do I cut the squares?

Finished size versus cut size

Patterns give HSTs as a finished size: the size once the unit is sewn into the quilt, without seam allowances. The squares you cut are larger, because you lose seam allowance around the edges and along the diagonal seam.

The 2-at-a-time method

Place two squares right sides together, draw a diagonal line, sew 1/4 inch on both sides of the line, and cut on the line. You get two HST units.

Cut size = finished size + 7/8 inch.

Worked example: for a 3 inch finished HST, cut squares at 3 + 7/8 = 3 7/8 inches. Two squares give two units.

The 8-at-a-time method

Place two larger squares right sides together, draw both diagonals, sew 1/4 inch either side of both lines, then cut the square in half horizontally and vertically and along both diagonals. You get eight HST units.

Cut size = 2 x (finished size + 7/8 inch).

Worked example: for a 3 inch finished HST, cut squares at 2 x 3 7/8 = 7 3/4 inches. Two squares give eight units. Because the final cuts include bias edges, handle these gently and press carefully.

Quick reference

Cutting oversize and trimming

The 7/8 inch allowance is exact, which leaves no room for error. Many quilters cut squares a little larger, for example adding 1 inch instead of 7/8, sew as usual and then trim each unit down to the unfinished size (finished size + 1/2 inch). It costs a little fabric and time but makes points line up far more reliably.

Pressing

Press seams towards the darker fabric or open, depending on how the units will nest in your block. Press, don't iron: pushing the iron back and forth stretches the bias seam.

Frequently asked questions

How many squares do I need for a given number of HSTs?

Each pair of squares gives 2 units with the 2-at-a-time method, so divide the units you need by 2 and round up to get pairs. With 8-at-a-time, each pair gives 8 units: divide by 8 and round up. For 24 units at 3 inches finished, that is 12 pairs of 3 7/8 inch squares, or 3 pairs of 7 3/4 inch squares.

Which method is more accurate?

The 2-at-a-time method has straight-grain outer edges, so it stretches less. The 8-at-a-time method is faster but leaves bias edges. If accuracy matters most, use 2-at-a-time and trim.

What is the unfinished size?

The unfinished size is the size of the HST before it is sewn into the block: finished size plus 1/2 inch. A 3 inch finished HST measures 3 1/2 inches unfinished, which is the size to trim to.

Calculate any size

The half-square triangle calculator gives both cut sizes for any finished size. Keep a list of your cut sizes and counts on the cutting chart pages in The Quilt Planner.

Related tool: open the calculator.