How it works
Center square = finished ÷ 1.414 + ½" (rounded up to ⅛")
Corner squares = finished ÷ 2 + ⅞", cut once diagonally (2 per block)
Stitch and flip: center = finished + ½", 4 corners = finished ÷ 2 + ½"
In a square-in-a-square block, a center square sits on point and its corners touch the middle of each block edge. Each side of the center square runs from the middle of one block edge to the middle of the next, so its finished side is half the block diagonal: the finished block divided by 1.414. Add 1/2" for seams and round up to the next 1/8" for the center square.
The four corner triangles have their short sides along the block edge, each half the block. That makes them half-square triangles, so you cut squares half the finished block plus 7/8" and cut them once on the diagonal. Two squares give the four corners for one block, and the straight grain stays on the outside edge.
Rounding the center up means your center is up to 1/8" big, never small. That keeps the points from being cut off; you can trim the block to the unfinished size if you need to.
The stitch and flip method skips the math. Cut a center square the size of the unfinished block, and four corner squares half the finished block plus 1/2". Sew each corner square on the diagonal, trim and press. It is accurate and fussy-cut friendly, but you throw away four triangles per block.
Worked example
For 12 blocks finishing at 6": 6 / 1.414 = 4.24", plus 1/2" is 4.74", so cut 12 center squares at 4 3/4". Corners are 3" + 7/8" = 3 7/8": cut 24 squares and cut each once diagonally. That is 3/8 yd of center fabric and 3/8 yd of corner fabric at 40" wide. For stitch and flip, cut 12 squares at 6 1/2" and 48 squares at 3 1/2".
Common mistakes
- Cutting the corner squares twice. Cutting corners into quarters puts the bias on the outside of the block, which stretches. Cut once, corner to corner.
- Rounding the center square down. A center that is too small loses its points in the seams. Round up and trim if needed.
- Using the center square formula backwards. 1.414 links the center square to the block: block / 1.414 gives the center, center x 1.414 gives the block.
- Sewing triangles off-center. Fold the center square and the triangle in half and match the creases before you sew.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.