Often times people will contact me about the percent of stretch to a fabric. I am always caught off guard, so thought I would write a little about how we can determine percent of stretch, and how those numbers can be misleading, but helpful all at once.
First, there are a variety of methods of determining the percentage of stretch in a fabric. Even when you stretch the fabric from Point A to Point B, there is a matter of opinion about how far is 'correct'. The easiest way I have seen is my favorite and in keeping with my "Keep It Simple" approach. Fold your fabric first - do not use a cut edge to stretch. Using a ruler, carefully note 10" of fabric. Now stretch the fabric out as much as you would ever want it on your body (plus a tiny bit more if you wish for putting on). Remember, this is your opinion :-) If 10" stretches to 13", this is 30%. If 10" stretches to 16", this is 60%, and so on.
Next, is the personal wearing ease that each individual prefers. If I design a pattern and -- in my opinion -- the optimum percentage of stretch is 30% -- the next person may not like that look and might prefer a 40% or even a 50% fabric as the final result would be less structured and have more drape in the final garment. If a pattern states it is designed for a 30% stretch, you can make it out of anything all the way up to 100% and simply plan on adjusting the final fit before you finish the hems. The armholes might be a little softer than a garment made from the 30%, but the final garment might be exactly what you desire -- it's all up to you. You can even (yes really!) make such a pattern from a 2-way stretch fabric (such as swimwear or exercise, etc.) Just know when you start that the finished garment will be slightly different than one made from a 30% stretch -- and also the lengthwise stretch might cause some sagging (would depend on the weight of the fabric), so keep your pattern choice very simple. Pretty much the only time it gets real critical is if you want to purchase a fabric with LESS stretch than the one specified in the pattern. Even this is easily accounted for by simply cutting the side seams and perhaps the underarm seams a tad wider and then adjusting the final fit before hemming.
All being said, however, it is important to be aware of the 'optimum' percentage that a pattern is designed for and also for the percentage of stretch in a fabric you may be considering purchasing. Knowledge is a wonderful thing and it's much nicer to make an informed decision rather than finding out later after you've made your purchase. But . . . if I like a fabric I do not let the percentage of stretch sway my decision to purchase the fabric. I can easily cut the side seams (for example) a tad wider and tweak the final fit after I have the garment together and before I finish the hem, etc. if the fabric does not have quite the amount of stretch that the pattern specified.
Oh, I'm not done yet . . . To complicate things even further, the amount of retention (i.e. when you stretch the fabric out, does it return to the original size?) is a factor -- and a very important factor to me personally. I want the fabric to go back where it came from, because if it stays stretched out - at all - it will stretch out with wear and not retain the desired fit throughout the day. That . . . is just plain Yuk!
In closing, if I like a fabric, I buy it and worry about the stretch later.