The economy block’s center sits on point, so its math runs through √2, the reason guessed cuts finish shy. Enter the finished block; you get the center square, the two corner squares to cut diagonally, and the trim-friendly oversizes.
Your Block
Cut
3 ½″center square
for a 4″ finished block · oversize-and-trim on
Sew opposite triangles first, press out, repeat, then trim to 4.5″ with the points ¼″ inside the edge.
- Center square3 ½″ (exact 3.33)
- Corner squares, cut 23″, slice each diagonally
- Triangles that makes4
- Square the sewn block to4.5″
The Formulas
center square = finished ÷ √2 + ½ in · corner squares (cut 2, slice diagonally) = finished ÷ 2 + ⅞ inThe center sits on point, so its side is the block’s diagonal ÷ √2. The four corners come from two squares cut once on the diagonal, the same ⅞ in triangle oversize half square triangles carry.
Worked Example
A 4 in finished economy block:
- Center: 4 ÷ 1.414 + ½ = 3⅜ in square (3.33 rounded to the ruler).
- Corners: 4 ÷ 2 + ⅞ = 2⅞ in: cut two, slice each corner to corner, four triangles.
- Trimming style: cut the corners at 3 in instead, sew, and square the block to 4½, the points survive every time.
Cut Chart
Exact cuts rounded up to the ruler’s ⅛, with the oversize column for trimmers.
| Finished block | Center square | Corner squares (cut 2) | Oversize corners |
|---|---|---|---|
| 2″ | 2″ | 1 ⅞″ | 2″ |
| 3″ | 2 ⅝″ | 2 ⅜″ | 2 ½″ |
| 4″ | 3 ⅜″ | 2 ⅞″ | 3″ |
| 5″ | 4 ⅛″ | 3 ⅜″ | 3 ½″ |
| 6″ | 4 ¾″ | 3 ⅞″ | 4″ |
| 8″ | 6 ¼″ | 4 ⅞″ | 5″ |
| 10″ | 7 ⅝″ | 5 ⅞″ | 6″ |
How to Sew It True
- Finger-press the centers: fold the center square and each triangle in half, pinch, and match the pinches, the triangle centers the side without measuring.
- Opposite sides first: sew two facing triangles, press out, then the other two. Pressing after each pair keeps the center square.
- The ¼ in cross: after sewing, the center’s points should float ¼ in inside the raw edge: that’s the seam allowance the next seam will use.
Assumptions and Defaults
- Cuts round up to the nearest ⅛ in; the exact decimal shows beside each, and rounding up plus a trim beats rounding down every time.
- Oversize adds ⅛ in to each piece for squaring after sewing.
- One round of corners. A double economy block (square in a square in a square) runs the math twice, the inner block is the next round’s center.
Economy Block Questions, Answered
What size do I cut square-in-a-square pieces?
For a square in a square block, cut the center at the finished size ÷ √2 + ½ in, and the corners from two squares at the finished size ÷ 2 + ⅞ in, each cut once diagonally. A 4 in block cuts a 3⅜ in center and 2⅞ in corner squares, and the chart lists the common sizes.
Why does the center math use √2?
The center math uses √2 because the center square is set on point, so the block’s width runs across its diagonal, and a square’s diagonal is √2 times its side. Dividing the finished size by 1.414 gives the center’s side, and skipping that step would cut the center more than an inch too large for a 4 in block.
Why do my square in a square points get cut off?
Square in a square points get cut off when the next seam crosses them, so each triangle point must be exactly ¼ in from the raw edge. Cut the corners ⅛ in oversize, sew, and trim the block square with the points ¼ in inside the edge, which guarantees the result careful sewing only aims for.
Is this an economy block or a snowball block?
This is the economy block, also called square in a square, which sets the center on point with four true triangles. A snowball block, by contrast, keeps the center square straight and folds small corner squares over it, which uses different math, is far more forgiving, and gives a different look.