Updated: Sep 23, 2021
Hello everyone, first a confession. I don't know much about Knit fabrics, it is new territory to me and probably most people in my neck of the world. We predominantly work with Cotton, linen, lace, and other non-stretch fabrics here. But, I have dabbled a bit. Do not ask me which is double knit or jersey or scuba or lycra or spandex right now, even though I can make a reasonable guess. This topic is specifically about the stretch properties of the Knit fabric and how to calculate the amount of ease your pattern might need for the fit to be right.
From my research, there are three main things to consider when choosing Knit or any other stretchy fabric for your project.
A) Percentage of stretch - How much the fabric can stretch
B) Recovery - How well the fabric returns back to its original form after being stretched.
C) Direction of Stretch -How many directions can the fabric stretch. Basically, 2 ways or 4 ways, which is only side to side, or both side to side and up and down, to put it plainly.
Percentage of Stretch
In order to calculate the Percentage of Stretch, you will need to cut or measure a control length of the Fabric. Maybe 4 or 5 inches (10 or 12 cm). Next, you will place this piece along a ruler and stretch it to as far as it can comfortably go without getting distorted.
The 4inch swatch above can comfortably stretch to 5inches.
Thus the extra length is 5 - 4 = 1inch.
Therefore, the percentage of 1inch out of 4inches is 1/4 X 100 = 25% stretch
Similarly, 4inch swatch stretched to 7inches is;
7 - 4 = 3, 3/4 x 100 = 75% stretch
Now, knowing the percentage of stretch is useful when trying to follow a pattern and a certain percentage is recommended for use.
Calculating the Ease for your Project
For the purpose of calculating the ease required in your sewing pattern, we need a multiplier to work it out.
For example, with the 4inch swatch that stretched to 5inches, which we calculated above as having 25% stretch if applied to the Bust measurement, for instance, it means that for every 5inches, we need 4inches of fabric assuming we want a skin-tight fit.
Therefore to get the multiplier, we calculate the fraction of 4 out of 5. which is, 4/5 = 0.8
With 0.8 as the multiplier, you can now apply it to anybody's measurement that you want to adjust. In the example below, Bust-Waist-Hip of 41-34.5-44 was reduced to 32.8-27.6-35.2 when multiplied by 0.8.