How to calculate denim shrinkage

Here’s today’s guest entry from Maria Santiago from Tucson AZ who shares what she’s learned about calculating shrinkage for denim. This first part was the email which I included because I thought it was funny, followed by her entry.

Kathleen, here is the article, it’s not super-long, but I think it says everything. I am currently undergoing similar difficulties with buttons. ARRGH!!! I remember when I was still in school, and I bought some books you recommended, “Quality Management in Production” and other stuff like that. I should have been buying “All the Little Intricacies of Buttons” and “A Million Things About Denim You Never Would Have Ever Imagined”. That would have been WAAAAAY more appropriate. Anyway, I am surely not having a unique experience in trying to make a denim company; certainly, other companies exist that are considering making one jean part of their line. I’m sure they could use all my experience to their advantage.

Once I figure out the deal with buttons, I would not mind writing one about all the different things about buttons-for-jeans (shank buttons vs. snap buttons vs. rivets). In fact, I would not mind writing an entire tutorial (in bits and pieces of course, as I figure it out) of what I’ve learned.

How to calculate denim shrinkage
I graduated college a year ago after having spent a semester writing a business plan for a small apparel manufacturing company -a jean company specifically. An economics major, I spent 50 pages talking about employee compensation and incentive; about supply-chain management and benefits above cost; things an economics major would consider the forefront of Important Things in the creation of a company. Well, I was wrong.

Instead of writing about conceptual matters, I should have been learning about practical ones. As a result, everyone at Patternworks is schooling me in the practical matters in making jeans.

Challenge number one involved shrinkage. After I had acquired my denim, my wash-house guy and I set about finding the different colors and shades we could get from the denim with different washes and other processes; when Patternworks asked for shrinkage, I figured I could kill both birds with one stone. With Wash-House Guy, I cut the denim into 20-inch by 20-inch sections, stapled them together into the shape of a pant leg, and washed them (Kathleen describes how to test for torquing and shrinkage in her book) .

I took the raveling squares of denim to Patternworks so that they could calculate the shrinkage from the denim. For many reasons, this was completely unacceptable. First, shrinkage is traditionally calculated with 12.5-inch axes (make a 90-degree angle out of two lines, both 12.5 inches long) because it’s easy to calculate percentage-shrinkages with those measurements. So, the length of my denim squares was incorrect. Note that “length” is defined as along and parallel-to the selvage edge; “width” is perpendicular to the selvage edge.

Then, denim typically shrinks a little bit- not a ton; but approximately 2% – 5%. When you have small shrinkage percentages like these, “a little bit of raveling here and there” actually translates to “we can’t figure out your shrinkage because the raveling is too large relative to actual shrinkage”. I should have made my axes far, far away from any un-sewn edge, so that the raveling would not interfere with my 20-inch by 20-inch (which, of course, should have been 12.5-inch by 12.5-inch) axis.

Denim is composed of little fibers that run both lengthwise, vertical (“warp” yarns) and horizontal, width-wise (“weft”, or “filling”) yarns. These yarns shrink in different ways- due to their weave, the selvage edge, and other factors. Thus, when you calculate shrinkage, you have to account for this and note where the selvage edge is. So, a length of denim, freshly-unrolled, will shrink differently in different sections. Sections closer to the selvage edge will shrink less; sections closer to the middle of the length (between the selvage edges) will shrink more. So, any given length of denim, after having been washed, will have shrunk (minutely) differently, given the “section” it is in, relative to the selvage edge. This is important when you consider the marker that will be made for the pattern. To overcome this, you have to average out the shrinkage.

These are the steps to calculate denim shrinkage:

  1. Lay out a length of denim; 1 yard in length (note: “length” is defined as along and parallel to the selvage edge) should be sufficient.
  2. Make an axis maybe 4 inches away from the top selvage edge; make the vertical axis perpendicular to the selvage edge, and the horizontal one parallel to the selvage edge.
  3. Make sure to label in permanent ink “12.5 inches” on both axis, just in case.
  4. Repeat with the “middle” axes, which should be in the middle of the length of cloth.
  5. Repeat again with the “bottom” axes, which should be at the bottom, maybe 4 inches up from the selvage edge.

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