Calculations for Race Quilt Variations

I have made three traditional jelly roll race quilts (here's one) using the tutorial from the Missouri Star Quilt Company.  I really enjoy the mindless sewing and the random placement of these type of quilts.  Basically, you make one really long strip by sewing all the pieces end to end.  Then you keep folding it in half and sewing down one side.  The only thing I do differently is iron the strips to one side after I sew each long seam.

I wanted to use up scraps and yardage from my friend, but had no idea how big it would end up.  So I made some calculations to make an educated guess.  Every time you fold the strip in half you are doubling the length of the quilt.  The number of rows grows in this order:  1, 2, 4, 8, 16, 32...*

My scraps were all cut 4 inches wide to finish at 3.5 inches.  So I multiplied 16 by 3.5 to get a length of 56 inches.  I decided that I wanted to make the width about 42 inches (making the backings easier to piece).  So I multiplied 16 by 42 to get 672 inches.  672 inches is about the length I want my very first super long strip to be.

My four inch strips ranged in length from 16 to 21 inches.  I guess that the average length was about 18 inches.  So I took 672 and divided it by 18.  This gave me  37.3 strips.  Through these calculations I estimated that I would need 37 or 38 of these four inch strips per quilt.

Happily my estimation was very close.  I made five of these quilts with 37 strips, and they all ended up between 42 and 45 inches wide.  I just trimmed one side to make it the size I wanted.  I was able to use what I trimmed as part of the backing.

One thing to note when using strips which are not all the same length is that the chunks will stagger in places and almost match in others.  

I think that is part of the charm.  I also love that although you may start with nearly identical quilt kits, the quilts are not identical.  

To see more of these quilts and the story behind them, check out this post.  :)

You can use these calculations to make a jelly roll race type quilt using any size of scraps or yardage.  I will use 6 inch cut, 5.5 inch finished width of fabric strips as an example to make a quilt top that is 70 x 88 inches.  

1. Start by multiplying the finished width of your strip (5.5 inches) by the number of rows (16) and you end up with  the length of your quilt (88 inches)  Remember that the number of rows doubles each time: 1, 2, 4, 8, 16, 32...*
2. Multiply what you want the width of your quilt to be (70 inches) by your number of rows (16).  This gives you the length of your very first strip (1,120 inches -- not too bad considering the length of the regular jelly roll race is 1600 inches)
3. Divide the length of the long strip (1,120) by the length of your strips (42 inches with straight seams) and you get the number of strips you need.  (26.6).  Just remember that first strip should be half the length of the others to make it stagger.  

I may just have to make one that size.  I think that would be a quick, fun stash busting quilt!  

*An exception to this doubling rule for the number of rows is if you stop after a few seams and divide the strip set into thirds.  Then you could multiply the number of rows by 3 instead of two.  For example, it could go: 1, 2, 4, 8.  Stop and cut the strips into thirds. Then sew the three sections together to get 24 rows.  Dividing it into thirds can be helpful with larger quilts like queen size.  You could even divide all your fabric strips into thirds before you start and sew each third individually.  It would help distribute the prints more evenly if you were using repeats.

Clear as mud?  Please let me know if you have any questions.  I LOVE quilty math!  

XX,

Jasmine

P.S.  I am linking up with Cynthia for Oh Scrap and Lorna for Let's Bee Social.