Half-square triangle calculator

For a 3″ finished HST, cut 3 7/8″ squares to make 2 at a time, 5 1/2″ squares for 4 at a time or 7 3/4″ squares for 8 at a time, then trim each unit to 3 1/2″. Pick a method and type your size below; quarter-square triangles and flying geese are in the list too.

HST size chart

Square to cut, in inches with the centimetre size under it. Every size is rounded up to the next ⅛″; trim each finished unit to the size in the second column.

Which method to use

• 2 at a time (two squares, one diagonal line, sew ¼″ each side): the simplest and the most accurate. Square = finished + ⅞″.
• 4 at a time (sew all the way around two squares, cut both diagonals): fast, but every edge of the unit is on the bias, so press gently and starch first. Square = finished × 1.414 + 1¼″.
• 8 at a time (draw both diagonals, sew ¼″ each side of both, cut in half both ways and along the lines): quickest for a big batch. Square = finished × 2 + 1¾″, which is twice the 2-at-a-time size. Published charts disagree on this one: some add another ½″–1″ so there is room to trim, which is worth doing if your ¼″ seam tends to drift.

Quarter-square triangles

A QST (hourglass unit) starts as two HSTs: cut squares of finished size + 1¼″, make 2 HSTs, cut them on the other diagonal and re-sew. For a 3″ finished QST cut 4 1/4″ squares; for 4″, 5 1/4″; for 6″, 7 1/4″. Choose "Quarter-square triangles" in the calculator for any size.

Setting blocks on point? The setting triangle calculator gives the side and corner triangles. All the formulas are on how we calculate.

Learn it by doing: How to make half-square triangles, animated.

Questions quilters ask

What size square do I cut for a 3 inch half-square triangle?

For a 3 inch finished HST, cut 3 7/8 inch squares to make 2 at a time, 5 1/2 inch squares to make 4 at a time, or 7 3/4 inch squares to make 8 at a time. Trim each unit to 3 1/2 inches.

What is the formula for half-square triangles?

Two at a time: finished size + 7/8 inch. Four at a time: finished size × 1.414 + 1 1/4 inch. Eight at a time: finished size × 2 + 1 3/4 inch. Round up to the next 1/8 inch, then trim each unit to finished size + 1/2 inch.

How do you make 8 half-square triangles at once?

Place a light and a dark square right sides together, draw both diagonals, and sew 1/4 inch on both sides of each line. Cut the square in half both ways, then along the drawn lines: you get 8 HSTs. Press and trim each one.

Should I trim half-square triangles?

Yes, if you want exact points. The chart gives the exact size; cutting the squares 1/4 to 1/2 inch larger and trimming each unit to finished size + 1/2 inch fixes small errors in the seam allowance and pressing.

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Updated 2026-11-14.