Making a stuff sack to a specific size can be a challenge. It’s pretty easy if you make a round bottom stuff sack, but can be confusing if making a square/rectangular bottom one. It’s also difficult to determine the size of a square bottom sack you can make with a piece of scrap material.
In this tutorial, I will hopefully demonstrate how to make a square/rectangular bottom stuff sack to a specific size. I typically make square bottom stuff sacks, but will show a rectangular version in an effort to make the example less confusing. I am assuming that you already know how to sew a square bottom stuff sack and you have a preferred method of installing the drawstring channel. Whatever method you use, just account for it in the CSA (channel seam allowance). This method use the following process which is similar to the one on thru-hiker.net:
-Fold the fabric in half length wise.
-Stitch the sides together (plan for your preferred drawstring channel). The fold becomes the bottom of the sack. The stitch lines are the sides.
-Sew the drawstring channel.
-Form and stitch the square/rectangular bottom, by pulling out the “corners”
Let’s assume we want a 200 cubic inch stuff sack. Its finished size will be 10” tall with a 5”x4” rectangular bottom. It will have a draw string channel to close the bag.
To calculate the blank of material you need to start with, I use this formula.
Height of Fabric Blank = 2*H + y + 2*CSA
Width of Fabric Blank = x + y + 2*SA
In this example:
H=10” (height of bag)
SA = ˝” (side seam allowance)
CSA = 1-1/2” (draw string channel seam allowance) (I should have used larger, but didn’t want to redraw the picture)
Blank should be 27” tall x 10” wide
For a square bottom sack, x = y
What if I just have a scrap piece and want to know if it's big enough?
If you already have a scrap of fabric and you want to know what size you can make, solve it backwards and make it a square bottom. For example, a 24x9” piece.
Again, for a square bottom
Max width (x) = (Fabric width - 2*SA) / 2.
Sack height (H) = (height of fabric blank – y – 2*CSA) / 2
x = y = 4" for a square bottom.
Sack height (H) = 8.5"
Also, if you always use 1/2" seam allowance, x = (Fabric width - 1") / 2
height of blank is 24”
width of blank is 9”
CSA = 1.5”
So you can make an 8.5” stuff sack with a 4x4 bottom
I like a 4x4 square bottom for things like hammocks, tarps, etc.. For larger items, figure out the rectangular bottom size you need.
A discussion on a confusing part.
Why is the bottom height (y) included in the sack width? (y = 4 for the example)
Because it’s folded on the center, you have 2” on each side of the centerline fold. When you pull out the corners to form the bottom, it forms a 45 degree triangle. From geometry of a 45 degree triangle. Base = Height. Hypotenuse doesn’t matter. I know the triangle height is half of the y dimension, so the base is also ˝ of the y dimension.
Clear as mud?