Tell the calculator how big your half square triangles should finish and how many you need. It gives the square size to cut for the 2-at-a-time, 4-at-a-time or 8-at-a-time method, how many squares of each fabric to cut, and how much fabric to buy.
Last updated:
Your half square triangles
in
Size in the quilt after seams (2½ = 2.5). Unfinished is ½" bigger.
Method
Oversizing gives a little extra to square up each block.
Fabric
%
Optional: make a few spares.
Enter your values to see the result.
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How to use the Half Square Triangle Calculator
Enter the finished size of each half square triangle, as it will measure in the quilt.
Enter how many HSTs your pattern needs, and an optional extra percentage for spares.
Pick the method (2, 4 or 8 at a time) and whether to cut exact or oversized squares for trimming.
Choose your fabric width and press Calculate HSTs.
Cut the squares listed for each fabric, sew as described, and compare methods in the table.
HST cutting formulas
A half square triangle (HST) is a square made of two right triangles sewn along the diagonal. The square you cut must be bigger than the finished HST to allow for the ¼" seam on the diagonal and the ¼" seams around the edges.
Method
HSTs from 2 squares
Exact cut square
Oversized (trim) square
Edges
2-at-a-time
2
finished + ⅞"
finished + 1"
straight grain
4-at-a-time
4
(finished + ½") × 1.414
(finished + ⅝") × 1.414
bias
8-at-a-time
8
2 × finished + 1¾"
2 × finished + 2"
straight grain
Example: 48 HSTs that finish at 3" with the 8-at-a-time method need 48 ÷ 8 = 6 squares of each fabric cut at 2 × 3 + 1¾ = 7¾". Five 7¾" squares fit across 40" of usable width, so you cut two strips (15½") of each fabric, which rounds up to ½ yard of each.
Tips for accurate half square triangles
Trim if you can. Cutting oversized squares and trimming each HST to the unfinished size (finished + ½") makes the points line up far better than relying on an exact cut.
Use a scant ¼" seam on each side of the drawn line, and press the seam before trimming.
Starch the squares before cutting for the 4- and 8-at-a-time methods: the diagonal cuts expose bias that can stretch.
Square up on the diagonal. When trimming, line up the 45° line of your ruler with the seam so the diagonal runs exactly corner to corner.
Directional prints: 4- and 8-at-a-time methods rotate the print in different units, so choose 2-at-a-time for stripes or one-way prints.
Frequently asked questions
What size square do I cut for half square triangles?
For the 2-at-a-time method add ⅞" to the finished size: a 3" finished HST needs 3⅞" squares. Many quilters add 1" instead (4" squares) and trim the sewn unit down to 3½" for more accurate blocks.
How does the 8-at-a-time HST method work?
Place two squares right sides together, draw both diagonals, and sew ¼" on each side of both lines. Cut the square in half across and down, then along both diagonals to get eight HSTs. The exact square size is 2 × (finished + ⅞"), so 3" finished HSTs need 7¾" squares (8" if you want to trim).
What size square for 4-at-a-time half square triangles?
Sew ¼" around all four edges of two squares, then cut both diagonals. The square must be (finished size + ½") × 1.414, rounded up to the next ⅛": 3½" × 1.414 = 4.95", so cut 5" squares for 3" finished HSTs. The edges of these HSTs are on the bias, so handle and press them gently.
What is the difference between finished and unfinished HST size?
The unfinished size includes the ¼" seam allowance on every side, so it is ½" bigger than the finished size. A 3" finished HST measures 3½" before it is sewn into the quilt.
How much fabric do I need for half square triangles?
Work out how many pairs of squares you need (HSTs ÷ HSTs per pair), how many squares fit across the fabric width (usually 40" usable), then strips × square size ÷ 36 gives yards. You need that amount of both fabrics.
Which HST method is best?
2-at-a-time is the most accurate and the easiest for beginners. 8-at-a-time is fastest for many identical HSTs and keeps the straight grain on the outer edges. 4-at-a-time is quick but leaves bias edges, which can stretch.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.