Loom Mosaic

Quilt block designer & quilting calculators

Half-square triangle calculator

What size squares for half-square triangles?

Enter the finished size and how many you need. You get the square to cut for each batch method, how many squares of each fabric, what to trim to and how much fabric it takes.

Units
in
Cut squares
in

Squares to cut, by method

Two at a time

37/8″
9.8 cm squares
  • Cut 12 squares of each fabric (12 pairs).
  • Makes 24 HSTs; 2 from each pair.
  • Draw one diagonal, sew 1/4″ either side of it, cut on the line.
  • Each unit should measure 31/2″: check with a square ruler.
  • Fabric, per colour: 2 strips 37/8″ wide (10 squares per strip), 1/4 yd (0.2 m), or 1 fat quarter (20 squares each).
  • Outer edges on the straight grain; only the seam is on the bias.

Four at a time least fabric

51/2″
14 cm squares
  • Cut 6 squares of each fabric (6 pairs).
  • Makes 24 HSTs; 4 from each pair.
  • Sew 1/4″ round all four edges, then cut on both diagonals.
  • Each unit should measure 31/2″: check with a square ruler.
  • Fabric, per colour: 1 strip 51/2″ wide (7 squares per strip), 1/4 yd (0.2 m), or 1 fat quarter (9 squares each).
  • All four outer edges are on the bias: handle gently, starch before cutting, press without pulling.

Eight at a time

73/4″
19.7 cm squares
  • Cut 3 squares of each fabric (3 pairs).
  • Makes 24 HSTs; 8 from each pair.
  • Sew 1/4″ either side of both diagonals, then cut across the middle both ways and on both diagonals.
  • Each unit should measure 31/2″: check with a square ruler.
  • Fabric, per colour: 1 strip 73/4″ wide (5 squares per strip), 1/4 yd (0.2 m), or 1 fat quarter (4 squares each).
  • Outer edges on the straight grain; only the seam is on the bias.

Exact sizes are rounded up to the next 1/8″ (or millimetre). “To trim” adds a little so every unit can be squared up to size.

Choosing a method

A half-square triangle (HST) is a square made of two right-angled triangles. All three methods start with two squares of different fabrics placed right sides together.

  • Two at a time: the classic finished size + 7/8″. The outer edges stay on the straight grain, so units keep their shape. Best for small batches and directional prints.
  • Four at a time: sew round the edges, then cut both diagonals. Quick, but every outer edge is on the stretchy bias, so press gently and trim.
  • Eight at a time: sew either side of both diagonals, then cut into eight. The fastest for large numbers, with straight-grain edges.

Where the numbers come from

Each unit must measure the finished size plus two seam allowances before it is sewn into a block. The diagonal seam uses up more: with 1/4″ seams the triangle legs need to be the finished size + 0.854″, which rounds up to 7/8″. Four at a time uses the square's half-diagonals as those legs, so the square is √2 (about 1.414) times bigger; eight at a time cuts the square in half both ways first, so it is twice as big.

Worked example

For 24 HSTs finishing at 3″, two at a time needs 12 pairs of 3 7/8″ squares, four at a time 6 pairs of 5 1/2″ squares and eight at a time 3 pairs of 7 3/4″ squares. Every unit is trimmed or checked at 3 1/2″.

Sources

Trimming half-square triangles

  1. Press the seam (to the darker fabric, or open) before trimming.
  2. Lay a square ruler on the unit with its 45° line exactly on the seam.
  3. Line up the trim size (for 3″ finished, 3 1/2″) so the unit sticks out a little on all sides, and trim the two edges at the top and right.
  4. Turn the unit round, line the trimmed edges up with the ruler's trim-size marks and the 45° line with the seam, and trim the other two edges.
  5. Snip the dog ears, the little triangles of seam allowance at the corners, so they do not add bulk.

Half-square triangle questions

Why add 7/8″ and not 1/2″?

A plain square only loses two 1/4″ seam allowances. A half-square triangle also has the diagonal seam, which takes about 0.354″ more along each edge. Added together that is 0.854″, so quilters round up to 7/8″.

Should I cut exact or oversized squares?

Exact squares give the right size if your 1/4″ seam is accurate. Oversized squares (two at a time: finished + 1″) leave a little to trim, which makes every unit the same size even if seams wander, at the cost of a few minutes trimming. For four at a time, trimming is strongly advised because of the bias edges.

Which method uses the least fabric?

It depends on the size and how the squares fit across your fabric, so the calculator works out all three and marks the one with the shortest length of fabric. For small numbers the difference is usually only a strip or two.

Can I use metric seam allowances?

Yes. Choose centimetres and a 0.75 cm seam allowance if your pattern uses one. The same geometry applies, and sizes are rounded up to the next millimetre. With a 1/4″ presser foot, keep the 1/4″ option and read the sizes in centimetres.

How many HSTs do I get from a fat quarter?

The calculator assumes a usable fat quarter of 18 × 21″ and counts how many whole squares fit. For two at a time with 3 7/8″ squares that is 20 squares, enough for 40 HSTs when paired with another fat quarter.