PWS - How to Write a Quilt Pattern - Topic 5 - Quilt Math

Topic V– Quilt Math

And now for my favorite part of the pattern writing process, the quilt math! I have a simple, easy, and methodical way of determining all of the fabric and cutting requirements in a quilt pattern. The simplest way of teaching you this method is to take you through the math calculations for our sample quilt.

I typically do my calculations on a simple pad of paper but am recreating them here in a slightly neater and easier to read version (because no one would be able to read my awful handwriting). I calculate all of the quilt math before doing any illustrations or writing any of the pattern text.

Our sample quilt, Ninja Bears, contains 12 Friendship Star blocks, each measuring 12” square (finished) and a small border.


A. Quilt Block Fabric and Cutting Instructions

1. To start, break down the block into its individual components. The Friendship Star block is made up of a 3 x 3 arrangement of components:

2. We can further break down the fabric pieces needed for the block including the HST components.

This is a simple block so we are now at the individual fabric pieces. If the block was more complicated and had other components (such as flying geese, square in a square, etc.) I would continue breaking each one down until I reached the individual fabric pieces needed for one entire block.

I use the same methodology for paper pieced and appliqued quilts, breaking the block down into its individual fabric components.

3. Next, we multiply the fabric pieces per block by the number of blocks and put that in a column. The baby quilt has 12 blocks in a 3 x 4 block arrangement so the number per block is multiplied by 12. (If the pattern contained additional sizes, I would add an additional column on the right hand side of the table per quilt size). The right most column now contains all of the fabric pieces needed for the blocks in the quilt.

4. Now that we have all of the individual fabric pieces for the blocks in the quilt, we need to figure out the cutting instructions.

        A. The charm squares are easy: 24 are used as is for making the HSTs (5” x 5”) and 12 trimmed for the centers of each block (4 ½” x 4 ½”).

        B. To calculate the minimum background fabric required (and any other fabrics in your patterns being cut from yardage):

                a. First, figure out how many pieces can be cut from a single width of fabric (WOF) strip for each of the fabric pieces:

Dividing 42” (our assumed WOF) by 4 ½” you get 9.3333 so round down to 9. You can get 9 squares 4 ½” x 4 ½” from each 4 ½” x WOF strip.

Dividing 42” by 5” you get 8.4 so you round down to 8. You can get 8 squares 5” x 5” from each 5” x WOF strip.

                b. Second, divide the number of fabric pieces needed by the number of pieces you can get per strip to figure out how many strips are required.

The pattern requires 48 background squares (4 ½” x 4 ½”) and you can get 9 squares per strip so 48/9 = 5.333 which rounds up to 6. In order to cut 48 squares (4 ½” x 4 ½”), you will need 6 strips of 4 ½” x WOF.

The pattern requires 24 squares for the HSTs (5” x 5”) and you can get 8 squares per strip so 24/8 = 3. To cut 24 squares (5” x 5”), you will need 3 strips of 5” x WOF.

Note: Sometimes it will happen that you will wind up needing only a few inches of a strip. If that happens to me, I see if I can use the uncut portion of the strip to cut any other fabric pieces.


B. Border Fabric and Cutting Instructions

Now for the border. Ninja Bears has a 1 ½” finished (2” unfinished) border on all 4 sides.

To figure out the strips needed for the border I first figure out the number of strips for the sides of the quilt. The quilt top measures 48 ½” high so one and a half WOF strips can be pieced together to form each side border. That makes a total 3 strips needed for the side borders.

You can calculate the 48 ½” by taking:

# of blocks x height of blocks (unfinished) – ½ times [# of blocks -1]

Ninja Bears height is 4 x 12 ½ - ½ x (3-1) = 50 – 1 ½ = 48 ½” 

To determine the top and bottom border strips you add (all unfinished measurements):

The quilt top width + side border thickness + the side border thickness – 1” (for seam allowances). 

For our quilt this is 36 ½” + 2” + 2” = 40 ½” – 1” = 39 ½”. 

We will need one strip to make each of the top and bottom borders.  Therefore 5 strips 2” x WOF are needed to make all of the border pieces.

C. Yardage for the Quilt Top (Blocks and Border)

To calculate the yardage of background fabric needed, we multiple the number of strips times the thickness of the strips for all of the strip sizes we use (for Ninja Bears this will be for the background pieces within the block and the border).

We need a total of 52” x WOF of the background fabric for this quilt. I typically like to add about 5” (the width of the largest cut strip) to this total to give quilters a little wiggle room for cutting mistakes and then round to the nearest 1/8 of a yard.

Exact yardage needed: 52” = 1.44 yards

Yardage plus wiggle room: 52” + 5” = 57” = 1.58 yards

Yardage plus wiggle room rounded to the nearest 1/8 yard: 1 ⅝ yards (which is 58 ½”)

So Ninja Bear will need 1 ⅝ yards of background fabric.

D. Binding Fabric 

To calculate the binding fabric (for a straight grain, not bias binding), I add up the lengths of the 4 sides, add 10” (wiggle room) and divide by 40” (width of fabric – a little due to the fabric join).

For Ninja Bears we have 39 ½” + 39 ½” + 51 ½” + 51 ½” + 10” = 192”

192”/40” = 4.8 so rounding up we need 5 strips for the binding (each strip being 2 ½” x WOF)

Binding yardage = 5 strips x 2 ½” wide = 12 ½” or about ⅜ yard.

E. Backing Fabric

For a backing fabric, I usually assume an 8” total overhang (4”on each side) of the backing as compared to the front of the quilt. This is what longarm quilters will typically require.

Occasionally, like in our Ninja Bears example, you might be able to get away with a single width of fabric for a baby quilt but you will have less than the minimum 4” overhang. In that case, I usually add in some language under the fabric requirements chart that says,

“You may use a single width backing piece (1 ½ yards x WOF) if you can tolerate having a small overhang of the quilt backing compared to the quilt top (1 ½”) on all sides.”  When I made my Ninja Bears quilt, I quilted on my domestic machine and was able to use a single width of fabric for the backing.

For the regular 4” overhang, the width of the Ninja Bears quilt is 39 ½” so the backing will need to be 47 ½” wide.  We will need two widths of fabric because the quilt is taller than the WOF (42”). Here is our quilt top superimposed over the backing fabric.

Backing = 2 lengths of fabric, 47 ½” x WOF times 2 = 94 ½” x WOF needed

95” / 36” per yard = about 2 ⅝ yards for backing

F.  Conclusion

So we have now calculated all of the fabrics and cutting instructions needed for the quilt! I hope that taking you through the calculations of an actual quilt pattern helped make the math easier to understand.  Below is a summary of the quilt fabric requirements that appear in the quilt pattern.

There are many programs available on the computer and phone that we calculate a some of these fabrics and cutting instructions for you. My favorite is QuiltingCalc by Robert Kaufman which is available as a free phone app. It will calculate borders, binding, backing, and some other calculations.

If Ninja Bears was a full, for purchase, quilt pattern, then I would probably made the quilt pattern contain 5 sizes (baby, lap, twin, queen, and king) and also include additional options such as using 3 different yardages for the stars instead of pre-cuts. I would use the method described above to calculate and keep all of the numbers for all of the different options organized.

I really like this systematic approach (probably because I am an engineer) because it will keep your math organized and minimize the chance of forgetting a piece of the block or cutting or yardage calculations and works for very simple quilts all the way up to very complex quilts.

Christa from Christa Quilts has a great post about Quilty Math including some typical quilt sizes and fabric piece size calculations. She also did a Modern Quilt Guild webinar on quilt math that if you are a member of the MQG you can view here.

Join me this Wednesday (the 17th) for Topic VI – Computer Programs :)