Umbra undies: Fabric Options

You need a fabric with plenty of horizontal and vertical stretch (sometimes called “four-way stretch”) for this pattern. Preferably something that is comfortable, soft, and breathable.

Recommended fabrics include stretch jerseys, stretch mesh, stretch knits, or most fabrics with at least 5% spandex. For example, modal (rayon) jersey with spandex.

To test the stretch of your fabric, hold a known length of fabric against a rule. Then measure the length you can stretch that length of fabric to. For example if you can stretch a length of fabric from 10 cm to 18 cm, that fabric has a max stretch of 80%. If you can only stretch it to 13 cm, the stretch factor would be 30%. Measure the stretch in both directions (horizontal and vertical).

Good underwear fabric should have a max stretch of at least 60-80%. Store-bought underwear often has fabric with a stretch factor of around 150% (which means it can be stretched to 2.5 times the original length). If your fabric has a stretch of only around 30%, it’s probably pure cotton and less suitable for underwear.

If you have fabric with over 100% stretch, a good value for the horizontal stretch is 30%-50%. If you have fabric with around 80% stretch, a good default value for the horizontal stretch is 30%. If your fabric has 30% stretch, choose 15% horizontal stretch or less. If you want to use zipper pockets, go with a low stretch as the zippers can’t stretch themselves.

Fabric will wear out fast if you wear it near its maximum stretch, it will also be very uncomfortable.

Note that the thickness of the fabric also affects the stretch you should enter. Go with lower stretch for thicker fabric.

The vertical stretch can be much lower than the horizontal stretch. If you use a fabric that stretches more in one direction, use that for the horizontal direction. A good general value for the vertical stretch is around 5%. If it is too high, the fabric may press uncomfortably against your crotch area.