Craft Calculators

QuiltingNo. 304

Updated October 2026

Half-Square Triangle Calculator

Find what size squares to cut for half-square triangles made 2, 4 or 8 at a time, what to trim them to and how much of each fabric to buy.

Your half-square triangles
in
pcs

Finished size is how big the square looks once it is sewn into the quilt, without seam allowances.

Method

Two squares, sew 1/4 in on each side of the diagonal, cut on the line.

Room to trim

A little extra lets you square each HST up to the exact size after pressing.

Advanced
in
in

How to work out the square size for half-square triangles

A half-square triangle, or HST, is a square made of two triangles sewn along the diagonal. You rarely cut triangles directly. Instead you sew two squares together and cut them apart, which keeps the stretchy bias edge inside a seam. How big to cut the squares depends on how many HSTs each pair makes.

2 at a time square = finished + 7/8 in 4 at a time square = (finished + 1/2 in) × 1.414 + 1/2 in 8 at a time square = (finished + 7/8 in) × 2 Trim to finished + 1/2 in

Round the result up to the next 1/8 in, the smallest mark on most rulers. Many quilters add another 1/8 in to the 2-at-a-time size, cutting squares finished + 1 in, and trim every HST to finished + 1/2 in after pressing.

Example. For 24 HSTs that finish at 3 in, the 2-at-a-time method takes 12 squares of each fabric. Exact, they are 3 7/8 in. With room to trim, cut them 4 in: ten fit across a 42 in strip, so two strips of each fabric, 8 in or 1/4 yd. Trim each HST to 3 1/2 in.

HST size chart

Square sizes to cut for each method, exact and rounded up to the next 1/8 in, with a 1/4 in seam allowance.

Finished HSTTrim to2 at a time4 at a time8 at a time
1 1/2 in2 in2 3/8 in3 3/8 in4 3/4 in
2 in2 1/2 in2 7/8 in4 1/8 in5 3/4 in
2 1/2 in3 in3 3/8 in4 3/4 in6 3/4 in
3 in3 1/2 in3 7/8 in5 1/2 in7 3/4 in
3 1/2 in4 in4 3/8 in6 1/4 in8 3/4 in
4 in4 1/2 in4 7/8 in6 7/8 in9 3/4 in
4 1/2 in5 in5 3/8 in7 5/8 in10 3/4 in
5 in5 1/2 in5 7/8 in8 3/8 in11 3/4 in
6 in6 1/2 in6 7/8 in9 3/4 in13 3/4 in

The three methods

  • 2 at a time. Draw a diagonal on the lighter square, sew 1/4 in on each side of the line and cut on it. The most accurate method and the easiest to press.
  • 4 at a time. Sew all the way around the two squares, then cut both diagonals. Fast, but every outer edge of the finished HST is on the bias, so handle them gently and starch before cutting.
  • 8 at a time. Draw both diagonals, sew 1/4 in on each side of both, then cut through the middle both ways and along the diagonals. Also called magic 8. Good for many HSTs of the same two fabrics.

Why 7/8 inch?

The finished square needs a 1/4 in seam allowance on each of its four sides, which accounts for 1/2 in. The diagonal seam takes the rest: a 1/4 in seam measured across the diagonal uses 1/4 × 1.414, or about 3/8 in, of the square's width. Together that is about 0.85 in, rounded up to 7/8 in. The other formulas come from the same geometry, which is why the calculator can work them out for any seam allowance, including metric ones.

Questions

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

For the 2-at-a-time method, cut 3 7/8 in squares, or 4 in squares if you want room to trim. For 4 at a time cut 5 1/2 in squares, and for 8 at a time cut 7 3/4 in squares. Trim every HST to 3 1/2 in so it finishes at 3 in.

What is the formula for half-square triangles?

For 2 at a time, add 7/8 in to the finished size. For 4 at a time, add 1/2 in to the finished size, multiply by 1.414 and add another 1/2 in. For 8 at a time, add 7/8 in and double it. Round up to the next 1/8 in.

What is the difference between finished and unfinished size?

The unfinished size includes the seam allowance on all four sides, so it is 1/2 in bigger than the finished size with a 1/4 in seam. A 3 in finished HST is trimmed to 3 1/2 in and shows 3 in once it is sewn into the quilt.

How many half-square triangles can I make from a fat quarter?

A fat quarter gives about 18 × 21 usable inches. Cut into 3 7/8 in squares that is 4 × 5, or 20 squares. Paired with 20 squares of a second fabric, they make 40 HSTs that finish at 3 in using the 2-at-a-time method.

Which HST method is most accurate?

The 2-at-a-time method, because the outer edges stay on the straight grain and only the seam is on the bias. The 4-at-a-time method leaves all four outer edges on the bias, so the HSTs stretch more easily. Cutting a little oversize and trimming makes any method accurate.

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