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Cut sizes for the center square and corner triangles of any square-in-a-square unit — with the trim-to-size method that keeps your points.
| Finished Center | Cut Center | Corner Squares (cut 2, slice ↘) |
|---|---|---|
| 2" | 2-1/2" | 2-3/8" |
| 2-1/2" | 3" | 2-3/4" |
| 3" | 3-1/2" | 3" |
| 3-1/2" | 4" | 3-3/8" |
| 4" | 4-1/2" | 3-3/4" |
| 4-1/2" | 5" | 4-1/8" |
| 5" | 5-1/2" | 4-1/2" |
| 6" | 6-1/2" | 5-1/8" |
| 8" | 8-1/2" | 6-5/8" |
Cut two corner squares and slice each once diagonally for the four corner triangles. Corner sizes are rounded up to the nearest 1/8″ — trim the finished unit to size, leaving 1/4″ beyond the center's points.
High contrast between center and corners makes the on-point square pop — it's also a favorite frame for fussy-cut prints.
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Divide the finished center square size by 1.414 and add 7/8", then cut two squares that size and slice each once diagonally to get your four corner triangles. For a 4" finished center: 4 ÷ 1.414 + 0.875 = 3.7" → cut two 3-3/4" squares. Round up and trim for a more forgiving unit.
The finished unit is the finished center square × 1.414 (the diagonal of the center). A 4" finished center makes a unit that finishes at about 5.66" — which is why square-in-a-square units usually come out at 'odd' sizes unless the designer works backward from the center.
Work backward: finished center = desired block size ÷ 1.414, and corner squares are cut at (block size ÷ 2) + 7/8". For a 6" finished unit, the center cuts at 4.74" (awkward — cut 4-3/4") and the corner squares at 3-7/8". Because the center lands on awkward fractions, most quilters oversize everything slightly and trim the finished unit down to exact size.
An economy block is a square in a square done twice — the center square gets one round of corner triangles, then a second round, putting the fussy-cut center back to straight orientation. The same math applies to each round: the finished size multiplies by 1.414 every time you add corners.
Cutting the corner squares once diagonally puts the straight grain along the two short sides — which become the outer edges of the unit. That keeps the unit's edges stable. The bias edge (the hypotenuse) gets sewn to the center square first, where it's controlled by the seam.
It's the most reliable method: add 1/8" to 1/4" to the corner square size from the table, sew all four corners on, then trim the unit to its exact unfinished size, keeping 1/4" of seam allowance beyond the center square's points. You sacrifice a little fabric for perfectly sized units with no cut-off points.