It also checks the block grid. A nine-patch is a 3 x 3 grid, so a 9 in block has 3 in units that are easy to cut, while a 10 in block has 3 1/3 in units that no ruler marks. The calculator suggests nearby block sizes that divide cleanly.
Worked example: shrinking a 12 inch block to 9 inches
A 12 in nine-patch-based block with 4 1/2 in squares and 4 7/8 in HST squares, remade at 9 in.
- Scale: 9 / 12 = 75%.
- Square: 4 1/2 - 1/2 = 4 in finished; 4 x 0.75 = 3 in; + 1/2 = 3 1/2 in to cut.
- HST square: 4 7/8 - 7/8 = 4 in finished; x 0.75 = 3 in; + 7/8 = 3 7/8 in to cut.
- At 10 in instead, each grid unit would be 10 / 3 = 3.333 in, which is not on a ruler line. Nearby sizes with whole or half inch units: 7 1/2 in, 9 in, 10 1/2 in, 12 in.
Result: Cut 3 1/2 in squares and 3 7/8 in HST squares for the 9 in block.
How block resizing works
- Finished piece = cut size - seam allowance. The allowance is 1/2 in for squares and rectangles, 7/8 in for HST starter squares and 1 1/4 in for QST starter squares.
- New finished piece = finished piece x new block size / original block size.
- New cut size = new finished piece + the same seam allowance.
- Grid unit = block size / number of grid units across. If the unit is not a whole number of sixteenths, the block will be hard to cut accurately; choose a block size that is a multiple of the grid count x 1/2 in.
- For templates, applique or foundation patterns, print at the scale percentage (new / original x 100) and check the printed seam allowance is still 1/4 in.
Assumptions
- Seam allowance is 1/4 in on every edge.
- Results that do not land on a 1/16 in ruler line are rounded to the nearest 1/16 in and marked as approximate.
- Example rounding: a 10 in nine-patch unit is about 3 5/16 in, so a better choice is 9 in or 10 1/2 in.
Frequently asked questions
How do I resize a quilt block?
Subtract the seam allowance from each cut size, multiply what is left by new size / old size, and add the allowance back. Never scale the cut size directly, or the seam allowance shrinks or grows with it.
What block sizes work for a nine-patch?
Sizes where each of the 3 units is an easy measurement: 4 1/2, 6, 7 1/2, 9, 10 1/2 or 12 in all give units on whole or half inches.
Can I enlarge a paper-piecing or applique pattern by percentage?
Yes, print it at the scale percentage the calculator shows. Check the seam allowance afterwards: on foundation patterns it should stay 1/4 in, so you may need to redraw it after scaling.
Why does my resized block need odd fractions?
Because the new size does not divide evenly by the block grid. Choose one of the nearby sizes the calculator suggests, or round to the nearest 1/8 in and trim the block to size at the end.