LeMoyne Star Calculator
The eight-point star every quilter eventually meets. Type the finished block size, get the strip to cut for the diamonds and the squares for the background, then follow the construction step by step, with Y-seams the traditional way or with straight seams only.
Type the finished block size or tap a common one. The summary tells you the three cuts and whether the size is a friendly one. Then pick a method below and follow its pictures. Every Y-seam on this page starts or stops at a marked ¼″ dot; that dot is the whole technique.
What a LeMoyne Star actually is
Eight identical diamonds with 45° tips meet at the center; eight times 45° is a full circle, which is why it works. Where two diamonds’ wide corners meet, a 90° notch is left over, and the background alternates between a square at each block corner and a right triangle at each side. The star’s tips reach the block edge where a square meets a triangle, so no tip points straight up.
Every seam in the block runs at 0°, 45° or 90°, and every piece is a rotary cut: the diamonds come off a strip subcut with the ruler’s 45° line, the corner squares are squares, and the four side triangles come out of one square cut on both diagonals so the straight grain lands on the block’s edge. The Lone Star is this same block with each diamond pieced from a grid of smaller ones; it has its own calculator, because there you choose the strip width and the block size follows.
Questions people ask
Why is the strip 3″ for a 12″ block? Where does that come from?
The block edge is corner square + side triangle + corner square, and the geometry of a 45° star makes the side triangle’s long edge exactly √2 times the corner square. So the diamond side is the block size divided by (2 + √2), about 0.29 of the block: 3.515″ at 12″. A 45° diamond is its side divided by √2 tall, 2.485″, and the strip is that plus two ¼″ seams: 2.985″, which every ruler calls 3″.
The numbers never land exactly on the ruler. Is something wrong?
No. The diamond side is the block size divided by (2 + √2), and √2 is irrational, so no block size gives a strip that sits on a ruler mark. Every published chart rounds. What matters is that the background pieces match the diamond you actually sewed, so this page figures them from the strip you cut, not from the ideal. 12″, 15″ and 18″ round almost perfectly; 8″ and 10″ are the awkward ones and the page says so.
What is a Y-seam, really?
A seam that stops ¼″ short of the edge instead of running off it, so a third piece can be set into the corner it leaves open. On a LeMoyne Star the corner squares and side triangles sit in notches between two diamonds, and the two seams that hold each one meet at the notch’s inner corner. The trick is only ever this: mark the dot where the seam lines cross, sew to the dot, stop, back-stitch. Eight times.
Which method should I pick?
If you have never sewn a Y-seam, the no-Y-seam version makes the same star with nothing but straight seams; the only cost is eight extra seams in the background, which vanish on a print. If your background is a solid, or you want the block the way it has been made for two hundred years, sew the Y-seams. They are one technique, learned once.
Why do the diamonds have to be starched?
Two of a diamond’s four edges are cut on the bias, at 45° to the grain, and bias stretches if you look at it wrong. The strip edges are on grain and behave; the subcut edges do not. Starch before the 45° cuts, handle the pieces by the on-grain edges, and let the feed dogs move the fabric instead of pulling it.
Is the math the same as in TideStitch?
Yes. The app names a star’s diamonds as strip-and-subcut pieces and its side triangles as quarter-square triangles, at whatever seam allowance you set; this page assumes ¼″. In the app you also see the star in your own fabrics before you cut, and the cut list comes out per fabric.