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.
For Example;
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.
(Note: Other adjustments are required to make the sample above usable, such as the chest, neck, armhole depth, and shoulder. This is just for illustration purposes)
Now, just because your fabric stretches, doesn't mean you also need the garment to be skintight. You can use the methods above even when you need some ease. For instance, instead of using the max stretch of 25% as above, I could decide to create my multiplier based on half of that, 4/4.5 = 0.89 or 0.9.
Direction of Stretch
As regards the direction of stretch, it only matters up to the extent of what your project is and how the stretchiness will affect wear. For a swimsuit, bodysuit, leotard, 4-way stretch is ideal, but for every other thing, 2-way stretch can usually cut it. I have used 2-way stretch for a bodysuit and it fits just fine.
Fabric Recovery
A quick note on the purpose of Fabric recovery. If a garment doesn't need to be stretched out while worn, such as with a loose t-shirt, the fabric doesn't need to have great recovery. But if the garment needs to be stretched out while worn, such as with a bodysuit, or swimsuit, the fabric needs to have great recovery. There are other reasons why this is important, like wear and tear, or suitability for children's clothing, etc.
That brings us to the end of this quick lesson. I hope it offers some assistance. It is definitely not a foolproof method, and I will update as needed over time.
This was helpful and easy to understand. Thanks.