In my sewing patterns created for stretch fabrics i often talk about the required fabric in terms of stretch percentage. For example, in the Briar tee pattern i say that you need to use a fabric with at least 20% stretch. For my maternity patterns i generally suggest fabrics with at least 40% stretch.
I feel like this is the best way of explaining exactly how stretchy your fabric should be to work with the design, but i know that if you’re not familiar with that measure it can be a little confusing. Thankfully it’s really really simple to calculate!
Your stretch percentage is just the width of your fabric when stretched to it’s maximum divided by the width of your fabric when unstretched (minus 1).
For example in the example shown below the stretch percentage would be 16/10-1 = 0.6 . This means the stretch percentage is 60%.
I find a very simple and quick way to calculate stretch percentage without a calculator is to keep the fraction simple. So I take a swatch of fabric and hold it width ways next to a ruler, tape measure or cutting mat, with my fingers placed at 0 and at 10 (it doesn’t matter if you’re using CM or inches, 10 is just a very easy number to use mentally). I leave my left hand static and stretch the fabric held by my right hand. That way i can very easily read off the number and mentally calculate the percentage.
So again with my example below, i can see at a glance that the stretch percentage is 60%.
One of the reasons i love talking about stretch percentage rather than the specific fabric content in our pattern requirements, is that it makes it easier to understand how you can manipulate the fit of your garment.
For example with the Briar, if you choose a fabric with a high stretch percentage, your garment is going to sit differently, more loosely. If you choose a fabric with a lower stretch percentage than required, it will fit more tightly and may be a little harder to get on and off. With the Briar this isn’t a big deal since it’s quite loose fitting. I often use sweater knits that have a lower stretch percentage and just accept the fact that i will either need to size up, or they will fit a little more snuggly. But on the other hand, one of my most upsetting makes ever was when i sewed up a pair of Virginia leggings with a very comfy fabric and forgot to check the stretch percentage before sewing. I couldn’t get them on! Considering i was pregnant at the time it was really really upsetting (cue me freaking out about pregnant weight gain) until i realised that the fabric had only 10% stretch when i needed 40% hehehe
Lesson here: Always check your stretch percentage babes!