Doing the Math

Yes, I know, this is not your favorite part--it's not mine either! One could probably skirt around ever having to do any sort of math and still produce beautiful quilts. You could use yardage and cutting instructions found in books, magazines, and websites (like this one) or wing it and not have to figure anything out and be perfectly happy.

But, what if you wanted to design something yourself and wanted to know how much fabric you'd need before you started? Or what if you wanted to submit your design to a book or magazine? Or what if you'd just like to have a better understanding of the process? Well, your gonna have to do a little math. It's not so bad really, just break it down into smaller parts and put it all together--just like making a quilt!

In Design Process Part 2: Drafting a Quilt I wrote about the Rush Hour quilt I designed and showed you a computer generated mock-up. I have since added a border to the top and bottom of the design (it needed a little something more) and recolored a few of the "blocks".

This is not a traditional block sort of quilt but will be made making horizontal strip rows and sewing the rows together to make the quilt, giving the illusion of blocks. For this lesson I will refer to the defined shapes in the quilt as blocks just to simplify things.

STEP 1: Time to make some decisions about the quilt that will help us in figuring our yardage and cutting information. Not counting the border, there are 72 defined blocks in this quilt. You could use 72 different fabrics (73 with the border) if you wanted to, but that's a lot of different prints! I originally was going to use 18 different prints, having each print appear 4 times in the quilt =72 but some of my fabrics were too small of cuts to make that happen. So, I dug through my stash and decided that if I used 36 prints then each one would be used 2 times in the quilt = 72.

STEP 2: Know your cutting sizes for your block or pieces. Cutting size and finished size are two different things. Cutting size is a measurement that includes the seam allowance and finished size is what the piece measures once sewn together into the quilt.

This quilt is made from cutting 2" strips. Listed below are the length cutting sizes for the half block strips and the full block strips. All 2" strips in this quilt will be sub-cut into various lengths.

STEP 3: Since I am using 36 prints, I will use that as my following examples. I will give yardage and cutting for 18 prints as well at the end. If you want to sew with a different number of prints, you will armed with math and can figure it all up on your own.

Know how many pieces are needed to make a block. Decide how many half and full blocks you will be making from the same print. There are 16 half blocks and 20 full blocks in this quilt.

For each print to be used twice I will have to make 2 different set of cuts. One series of strips cut will make 1 half block and 1 full block from 1 print and another series of strips cut will make 2 full blocks from 1 print. That means 16 prints will be cut one way and 20 prints will be cut another way = 36 prints.

STEP 4: Add up the total number of length inches needed to make 1 half block and 1 full block. You get 65". Know the width of your fabric. Most modern quilting cotton fabric is 42" wide. A fat quarter is (half that) 21" wide. How many 42" or 21" wide strips are needed to get at least 65" of useable strip? You will need 2 strips at 42" wide or 4 strips at 21" wide.

Now we know how many 2" strips we would need to make 1 half block and 1 full block using 36 different prints! To figure the yardage, add the height of the number of strips. In this case it is
2" + 2" = 4". So an eighth of a yard or a fat eighth will work for each of the 16 half and full block prints.

Same goes for figuring up 2 full blocks. Add the total number of length inches needed to make 2 full blocks (80"). How many 42" or 21" strips are needed to make 80"? You will need 2 strips at 42" wide and 4 strips at 21" wide. Cool huh? You'd need 36 eighth of a yard prints or 36 fat eighths to make the 2 full blocks of each print in this quilt. But, they are not cut the same way, we still need to cut 16 one way and 20 another way. Let's look at the diagram and cutting lengths again to figure out what cuts to make.

Fat Eighth Option

From each of the 16 prints:

To make 1 half and 1 full block from each of 16 fat eighths

Cut (4) 2" x 21" strips then cut into (1) 2" x 14" strip, (1) 2" x 11 strip, (2) 2" x 8" strips, (1) 2" x 6-1/2" strip, (2) 2" x 5" strips, (1) 2" x 3-1/2" strip, and (2) 2" squares

From each of the 20 prints:

To make 2 full blocks from each of 20 fat eighths

Cut (4) 2" x 21" strips then cut into (2) 2" x 14" strip, (2) 2" x 11 strip, (2) 2" x 8" strips, (2) 2" x 5" strips, and (2) 2" squares

NOTE: I amended steps 5 - 7 on August 22 because my math was wrong! I had the width of the quilt at 60-1/2". Yardage does not change, just the numbers in the example have been adjusted. My blog program doesn't have a cross text out feature or highlight feature so it was easiest to just make the changes so it reads correctly. Ooops!

STEP 5: Just about done with the math, we still need to figure yardage for the border, backing, and batting. If you look back at the Quilt Diagram, you can figure out the length you will need to add a border to this quilt. Here you will want to know and use the finished sizes of the block pieces. Looking at the strip row that touches either the top or bottom border you know the longest strips in that row will finish out to 13-1/2" and the squares will finish at 1-1/2". Add them all up and get 61-1/2". Add an additional 1/2" to this number to include the un-sewn edges (seam allowance) = 62" times 2 (because we are adding 2 border strips) = 124". How many 42" strips will it take to go into 121"? You will need 3 strips.

To figure your yardage, know how wide the border will be. I want my border to finish at 6" so I am cutting a 6-1/2" strip. Times 6-1/2" x 3 strips to know how much fabric to use. You will need 19-1/2" but I am rounding that up to 5/8 of a yard

From the border print:

Cut (3) 6-1/2" x 42" strips. Sew the strips short ends together to make 1 long strip. Measure the top and bottom of the quilt and average those numbers. Cut 2 strips to this measurement.

STEP 6: With all the information you know about the quilt pieces and border size, you can figure up the finished size of the quilt to know how much backing is needed. This quilt (on paper) will finish out at 62" x 72". Sewing varies and your seams could be bigger or smaller depending upon your actual seam allowance. Add 8" to each of these finished sizes to allow a generous backing to work with when quilting the quilt = 70" x 80". (This will also be the size needed for batting.) Working with 42" wide fabric you could piece (2) 42" x 70" panels together and have a 70" x 83-1/2" backing.

Backing Diagram - NOTE: Sizes should read 42" x 70" on both panels with correction to quilt width

So times 70" time 2 = 140". How many 36" yards go into 140"? You will need 3-7/8 yards for backing. (NOTE: This is just enough but with the corrections 4 yards has more wiggle room for squaring up if needed).

STEP 7: Last thing is binding, hang in there! Knowing the finished size of the quilt, 62" x 72", add up all 4 sides = 268". Add 12" to that for seams = 280". For regular, straight binding, you will need to know how many times 42" goes into 280". That is just a little over 6-1/2 times so you will need 7 strips at 2-1/4" wide. So, 1/2 yard of fabric will work just fine for binding. I figure bias binding the same way, but cut my 2-1/4" strips on the diagonal (45° angle).

Okay, that was painful for me too, but now it's over and we can get to the fun part--putting it together!

Here are the cutting options for using 18 different prints:

Fat Quarter Option

From each of the 16 prints:

To make 3 half and 1 full block from each of 16 fat quarters

Cut (7) 2" x 21" strips then cut into (3) 2" x 14" strip, (3) 2" x 11 strip, (4) 2" x 8" strips, (1) 2" x 6-1/2" strip, (4) 2" x 5" strips, (1) 2" x 3-1/2" strip, and (4) 2" squares

From each of the 2 prints:

To make 4 full blocks from each of 2 fat quarters

Cut (8) 2" x 21" strips then cut into (4) 2" x 14" strip, (4) 2" x 11 strip, (4) 2" x 8" strips, (4) 2" x 5" strips, and (4) 2" squares

Border, binding, backing, and batting stay the same in either option.