Quilt Math Part I - calculating binding

So let's start the Quilt Math series.  Calculating binding.  How much fabric do you need to bind your quilt?  Before we get started, here's a button for your blogposts or sidebar. 

<div align="center"><a href="http://lilysquilts.blogspot.co.uk/2012/09/quilt-math-1-2-3.html" title="Lily's Quilts Math"><img src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEifKp1cBaIWIYq-meiodx2ZnK56dvJPn_2EH26EgxUmgMp11kukqZotBoDY4nL7pjpWNRi6BfmHgzXHi4delMQqOe_zZISBAADO3JVOlSYB07znPKXbeqEdLwFPsOZ64yahMMlHvf9w9keP/s170/QMB.png" alt="Lily's Quilts Math" style="border:none;" /></a></div>

I make binding from WOF strips (i.e. cut from one selvedge to the other).  I make straight binding, not  bias binding.  I usually make 2 1/2" double fold binding but we can also calculate for narrower binding.  And the fabric I am using is usually 44" wide.  So this tutorial is based on 2 1/2" double fold WOF binding from 44" fabric.

1.   How long does the binding need to be?  The binding needs to be as long as the outside perimeter of your quilt plus a little bit for corners (see step 2).  So, if we have a quilt that is 70" x 90", we calculate 2 x 70" = 140".  Then 2 x 90" = 180".  Then add those two together.  So 140" + 180" = 320".  So we've added up the outside edges of the quilt.

2.   Then add 1" per corner since mitring corners does take a little bit of extra fabric.   So add on 4".  320" + 4" = 324".

3.   If you're using fabric that is 44" wide, I take off 2" to account for selvedges and pre-washing shrinkage.  I then take off an additional 4" to deal with the fact that, when making a diagonal join on a length of binding, some fabric is lost in those seams.  So we are making calculations with fabric that is 38" wide.

4.   To work out how many 38 WOF strips needed to make 324" of binding, you need to divide 324" by 38".  So 324"÷ 38" = 8.53.  Round 8.53 up to 9.  Always round up, never down or you will end up with less than you need.  So we need 9 WOF strips.

5.   If cutting 2 1/2" binding, you need to multiply the number of WOF strips (9) by the width of the binding (2 1/2" or 2.5").  So, 9 x 2.5" = 22.5".  So for a quilt 70" x 90", you need 22.5" of fabric.

6.   Since a yard is 36":

• 1/4yd is 9"
• 1/2yd is 18"
• 3/4yd is 27"
• 1 yd is 36"

So you will need 3/4 yd fabric for this binding.

7.   Since a metre is roughly 39 1/2":

• 1/4 metre is roughly 9 3/4"
• 1/2 metre is roughly 19 1/2"
• 3/4 metre is roughly 29 1/2"
• 1 metre is roughly 39 1/2"

So you will need 3/4 metre fabric for this binding.  

8.   So here is a shortened version of how you calculate your binding requirements:

• Calculate 2 x width.  
• Then calculate 2 x length.  
• Add those two together and add on 4" for corners.  
• Divide the whole lot by 38" (the width of 44" wide fabric after taking out selvedges and diagonal seams). 
• Multiply that figure by 2.5 for a 2 1/2" binding.
• OR multiply that figure by 2.25 for a 2 1/4" binding.
• OR multiply that figure by 2 for a 2" binding. 

9.   Or in algebra, where W = width and L = length:

2.5 x ((2W + 2L + 4) ÷38)   = the amount of fabric required to make a 2 1/2" double fold binding.
2.25 x ((2W + 2L + 4) ÷38) = the amount of fabric required to make a 2 1/4" double fold binding.
2 x  ((2W + 2L + 4) ÷38)     = the amount of fabric required to make a 2" double fold binding.

Your homework is as follows.  Calculate how much fabric is required to bind the following quilts with a 2 1/2" double fold binding cut from 44" fabric.  Answers at the bottom of the post in case you want to double check your calculations!

A.   50" x 50" quilt.
B.   60" x 80" quilt.
C.   40" x 40" quilt.

At the risk of sounding a wee bit of a #quiltmathnerd, I'd love to hear how you calculate binding.  And please comment or email if you're struggling with any of the posts in this series.  I know quilt math doesn't click with everyone so please let me know if you can't quite wrap your head around it yet.

Answers:

A.   15"
B.   20"
C.   12.5"