Stretch and Knits
I do not make clothes that are tight enough to need measured stretch, so I rarely run a stretch test. I use the stretch of a knit for necklines, so a garment goes over the head without a neck fastener.
If you draft close-fitting knits, though, stretch is the property that changes the most. A pattern for stretchy fabric is smaller than the body, on purpose, so you need to know how much your fabric really stretches and whether it springs back after.
Woven Stretch vs Knit Stretch
- A knit is made of loops. Loops pull open, so knits stretch.
- A woven stretches only when elastic fiber such as spandex is added, usually a few percent. Woven stretch eases fit, but it is not a knit. It still needs darts and shaping.
Spandex is the generic name for the fiber. Lycra is a brand name.
Measuring Stretch
Mark two points on a piece of fabric, pull it as far as it will go without distorting, and measure the new distance.
Stretch percentage = (stretched length - original length) / original length x 100
Example: 4 inches (10 cm) marked, pulled to 5.6 inches (14 cm). That is (14 - 10) / 10 x 100 = 40 percent.
Even if you only use stretch for a neckline, a quick pin test tells you whether the knit will go over a head before you cut.
Test lengths vary. Common choices are 4 inches (10 cm), 5 inches (13 cm) and 10 inches (25 cm). It does not matter, as long as you use the same math every time. A longer test length is a little easier to read.
Stretch Categories
Stretch is often sorted into groups. These are typical numbers:
| Group | Stretch |
|---|---|
| Stable | up to about 20% |
| Moderate | up to about 50% (a tee shirt is here) |
| Stretchy | up to about 75% |
| Super stretch | 100% or more |
The cut-offs shift a little from one source to the next, and some put the stable group at up to 25%. The groups are a guide, not a law.
Recovery
Stretch is only half of the story. Recovery is how well the fabric returns to its size after you let go. Pull, release, wait a moment, and measure again. A knit that sags and stays stretched is a poor choice for a fitted garment. Heavy, loosely knit fabric is more likely to sag.
Fabric Weight Does Not Predict Stretch
A heavy knit can have lots of stretch. A light one can have very little. Test every fabric.
For test-wearing swimwear and anything that will be worn wet, test the fabric wet and dry.
Which Way Does It Run?
Some knits run, like pantyhose, and they usually unravel from one end only. To find out which, pick out a loop at each cut end and gently pull the yarn. One end will start to unravel upward.
Cut with the unraveling end at the hem. The neckline gets the most stretch when you pull a garment on, so a run is likely to start there, and if the loose end is at the neck the run travels down the garment. The hem gets the least stress, so a run is unlikely to start there, and if one does it travels up. If your knit runs, mark the grainline with arrows pointing one way, so every piece is cut in the same direction.