How to make half-square triangles

Stack a light and a dark square, draw the diagonal, sew ¼″ either side, cut on the line. For 2″ finished units cut 2 7/8″ squares, then trim each unit to 2½″. Pick your own size and method below and play the steps.

Half-square triangle simulator

Units Inches & yards Centimetres & metres

Finished size

Method 2 at a time4 at a time8 at a time Units you need

Try: 2″ finished · 2 at a time3″ finished · 4 at a time1½″ finished · 8 at a time

▶ Play the steps ‹ Back Next ›

1. Cut two squaresOne light and one dark, bigger than the finished size.
2. Draw the lineStack right sides together and mark the diagonal(s) on top.
3. SewStitch ¼″ either side of the line (or round the edge for 4 at a time).
4. CutCut on the drawn lines with a ruler and rotary cutter.
5. Press openPress the seam toward the dark fabric.
6. TrimSquare each unit up to the finished size plus ½″.

Step by step (2 at a time)

1. Decide the finished size of the unit as it will look in the quilt, for example 2″.
2. Cut one light and one dark square of finished size + ⅞″ (2 7/8″ for a 2″ unit). Each pair makes 2 units.
3. Draw a diagonal line corner to corner on the back of the light square and put the squares right sides together.
4. Sew ¼″ on both sides of the line, or sew on the line itself and again ½″ away.
5. Cut on the line to get two units.
6. Press open toward the dark fabric, then trim each unit to finished size + ½″ (2½″ for a 2″ unit).

Cut-size chart

The same chart, with quantities, is in the half-square triangle calculator. Math and rounding: how we calculate.

Questions quilters ask

What size do I cut squares for half-square triangles?

For the two-at-a-time method, cut squares 7/8 inch bigger than the finished size: 2 7/8 inch squares for 2 inch finished HSTs, 3 7/8 inches for 3 inch finished. Trim each unit to the finished size plus 1/2 inch.

How do I make 4 half-square triangles at once?

Stack a light and a dark square (finished size x 1.414 + 1 1/4 inches, so 4 1/8 inches for 2 inch HSTs), sew a quarter inch from every edge all the way round, then cut on both diagonals. You get four units, each trimmed to the finished size plus 1/2 inch.

What is the 8-at-a-time half-square triangle method?

Stack two squares of finished size x 2 + 1 3/4 inches (5 3/4 inches for 2 inch HSTs), draw both diagonals, sew a quarter inch either side of each diagonal, then cut on both diagonals and across the middle both ways. You get eight units to trim.

Why do you trim half-square triangles?

Pieced units drift by a few sixteenths of an inch from stitching, pressing and the width of the pencil line. Trimming to the exact unfinished size, which is the finished size plus 1/2 inch, makes the blocks join up cleanly.

Which side do I press half-square triangles to?

Press the seam toward the darker fabric, so it does not show through the lighter one, and so that the seams of neighbouring units can nest against each other when you join them.

Keep learning

• Interactive How to calculate quilt backing Drag the quilt size and see the panels, the seams and the yardage change.
• Interactive How to calculate and make quilt binding Watch the strips get cut, joined at 45° and sewn around your quilt.
• Interactive What size quilt for a bed? Choose a mattress, a drop and a pillow tuck and see the quilt fall over the bed.
• Interactive How many squares from a fat quarter? See exactly how a fat quarter cuts up, and how much is left over.
• Interactive How to set a quilt on point Turn blocks 45° and watch the side and corner triangles fill in.
• Free calculator HST calculator Just want the number? Type your size and get it straight away.

Updated 2026-11-14.