Wednesday, March 14, 2012

Thursday Tidbit - Math for Quilters, Part 3

Are ya'll still with me on this math thing?  Has your head exploded yet?  Are things, heaven forbid, starting to actually make sense to you?  I hope so!  It's a little bit harder doing this online (in a one-way conversation) instead of face to face, and I appreciate you hanging in there.  Again, if there's something that's not making sense, just leave a comment and I'll try to answer as best I can.

Let's review what we've covered so far:  determining the underlying grid base of blocks, resizing blocks by changing a grid unit size, calculating sizes of finished quilts by deciding on block, sashing and border sizes, and determining the number and sizes of blocks, sashes, and borders we need to cut to finish our quilt.

Here's a graphic of our sample quilt, along with the chart we made up of how many of each part we need as well as the cut sizes of each part.
Quilt Size:  50" x 63" Finished

If you were making the quilt in this example, it's time to raid your stash or go to the quilt shop for yardage for each component to set the quilt blocks together.  I'm assuming the (12) 12" finished blocks are already made - we're just working on setting the blocks together.  There are tons of reference books and online tutorials for making quilt blocks, so I'm not getting into that here. This is more on how to actually put a grouping of blocks together into a quilt.

How much linear (or cut) yardage for each setting component do you need?  You really need to know these figures, or you'll run short, and we all know how frustrating that can be!

Helpful Hint:  Although today's fabric is manufactured to be 44"/45" wide, you should only count on 40" of usable width when calculating necessary yardage due to varying selvedge sizes, etc.  When working with fat quarters, use 20" as your guide for width.  A linear yard of fabric measures 36" long.

Sashing:  We know we need 17 pieces cut 1-1/2" x 12-1/2" by using the chart above.  We can either cut our strips 1-1/2" across the width of the fabric (WOF) or 12-1/2" WOF.  Which way is the most efficient use of fabric?
 
If we cut one strip of fabric at 1-1/2" by width of fabric (WOF), how many pieces at 12-1/2" can we subcut?
WOF / subcutting measurement = #pieces
  40" / 12.5" =3.2
We can get 3 sashing pieces from a 1-1/2" by WOF strip. 

How many strips at 1-1/2" each would we need to cut to get 17 sashing pieces?
Qty of Pieces Needed / # Pieces per Strip = # Strips Needed
17 pieces total / 3 per strip = 5.6 strips
We can get 17 sashing pieces from (6) 1-1/2" x WOF strips.

How many linear inches of fabric would we need if we cut our sashing using 1-1/2" x WOF strips?
Total # Strips Needed x Width of Cut Strip = Linear Inches of Fabric Required
6 strips x 1-1/2"  wide = 9" Linear inches of fabric (1/4 yard)
I would purchase 1/3 yard (12") so that I was sure that I had room to square up the fabric.  If using fabric from my stash, I would make sure I had at least 10" on hand.

How many sashing pieces can we cut from one 12-1/2" x WOF strip?
WOF / subcutting measurement = # pieces
40" wide / 1-1/2" subcut =26.6 sashing strips

So, one cut of 12-1/2" across the width of the fabric will yield more than enough sashing strips if we subcut at 1-1/2" wide.  But, how much yardage would you need to buy for a 12-1/2" cut?
Linear inches needed / 36" per yard = yardage required
12-1/2" / 36" =.347 yard
This rounds up to 3/8 (.375) yards or 13-1/2".

Now we know that we will need less fabric (1/4 yard) if we cut (6) strips 1-1/2" wide than if we cut (1) 12-1/2" wide strip (3/8 yard).

I suggest that you use these calculations (both ways) any time you need to cut rectangles to determine the most efficient use of fabric.

POSTS (or CORNERSTONES):  According to the chart, we need 6 squares cut 1-1/2" x 1-1/2".  We can probably use scraps for these, or purchase the smallest amount sold by the quilt shop (an 1/8th yard or a fat quarter).

INNER BORDERS:   If you are going to cut your borders length of grain (parallel to the selvedge), you would need to buy yardage a couple of inches longer than your longest border to allow for shrinkage or squaring up.

Here is the calculation for determining cross-cut (or pieced) borders.

We need (2) inner borders for the sides of our quilt cut at 1-1/2" x 51-1/2" and (2) borders for the top and bottom cut at 1-1/2" x 40-1/2".  We also know we need to cut the strips at 1-1/2" x WOF.

How many strips do we need to cut for the side borders?
If the border measurement is greater than 40", you will need to piece your border strips.
(Border Length x 2) / 40" usable width of fabric = Number of Strips Needed
(51-1/2" x 2) / 40" = 2.575       (3 strips of fabric are needed for side borders)

How many strips do we need to cut for the top and bottom borders?
If the border measurement is roughly 40" or less, you can get one border cut from each strip.
(2) 40-1/2" borders = 2 strips of fabric needed for top & bottom borders

Total number of Inner Border Strips:  (3) for sides + (2) for top and bottom = 5 Strips cut 1-1/2" x WOF

**IMPORTANT!!!!  If you are going to JOIN your border strips (for length) using a bias/diagonal seam, you need to allow additional inches for that!  You will lose inches in length by seaming this way.  I recommend purchasing extra fabric for an additional strip to accommodate the diagonal seam.

Yardage for Inner Borders:
# Strips Needed x Width of Cut Strip = Linear Inches of Fabric Needed
5 Strips x 1-1/2" wide = 7-1/2" Inches

7-1/2" / 36 inches per yard = .208 yards (Round up to 1/4 or .25 yards)

OUTER BORDERS:  The process is exactly the same as the inner borders.
Border Measurements:
    Side Borders:                              (2) 5-1/2" x 53-1/2"
    Top/Bottom Borders:                  (2) 5-1/2" x 50-1/2"

Number of strips needed for side borders (use the formula above):
(53-1/2" x 2) / 40" wide = 2.675 strips    (Round up to 3 strips)

Number of strips needed for top and bottom borders:
(50-1/2" x 2) / 40" wide = 2.525 strips     (Round up to 3 strips)

Total number of Outer Border Strips Needed:  6  (3 + 3)

Linear Inches of Fabric Required for Outer Borders:
6 Strips x 5-1/2" wide - 33 Inches

Yardage Required:
33" Needed / 36" Inches per Yard = .916   (Round up to 1 yard)

Wow.  That was a lot for one post.  I hope you stuck with me up to this point.  I hope you are not too overwhelmed; just think about each step as you go through the process, and it will all begin to make sense.  If it doesn't, just holler!  I'll do my best to clarify it for you!

Next Week:  Figuring yardage for Backing and Binding!

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