Whenever I make a quilt, my sketchbook starts to look like a math book. I don't like to buy more fabric than I need for a project, and I hate to have to run to the store to buy more mid-project if I run out. So before I buy fabric to sash, bind, and for the back, I do the math.
Today I'm going to break down for you how I figure out how much fabric to buy to sash a quilt. This sample I'm using is the measurements and sketches for my DQS9 quilt... so it's doll size.
Here's the sketch. The shaded area is made up of my blocks, and will be sashed in a lattice style. The sashing will be strips that are cut 2" wide, and finished at 1.5"
To determine how much fabric is needed for the sashing, I first add up all the areas sewn horizontally to the sashing.
3+3+3+3+9+9 = 30
Then I add up the total height of the finished quilt (from the top down I've got: 3+1.5+12+1.5+3= 21
Then I add one inch (for wiggle room), and multiply this number by 2 (since I'm using two strips of this length). 21+1 = 22
22 x 2 = 44
Now add the two numbers together 30 + 44 = 74
So you'll need a strip that measures 74" x 2" to sash this quilt.
Since quilting cotton usually is printed on the bolt as 44" wide, we'll take our strip length (74") and divide it by the usable inches of the fabric (I usually bet on having 42" of usable fabric once the selvages are removed). So.
74 ÷ 42 = 1.76 (round up to the nearest number, 2)
Meaning you'll need two cuts, selvage to selvage to equal 74 inches.
Now if you've already got that fabric by the yard on hand, you'd go ahead and cut two strips that are two inches wide each. Sew them together, sash your project and go on your way.
If you have to buy fabric for this project, it's easiest to buy a 1/4th of a yard.
Here's how to figure out the yardage if you've got a larger project. For instance for this string quilt I'm working on, I'm doing just a basic border around all four edges (the string part is 45" x 31") I'm putting an 8" border around the string section.
For this project I'll end up needing 5 strips (selvage to selvage) that are 8" wide.
The math you'll have to do for this is 5 (number of strips) times 8 (width of strips)
5 x 8 = 40
I'll need 40" of fabric... divide this number by 36 (the number of inches per yard).
40 ÷ 36 = 1.11
I round up to the nearest quarter of a yard, so I'll be buying 1 1/4 yards of fabric for the sashing of my string quilt.