When it comes to stretching elastic when sewing a bra, there are many different theories on how much to pull. Some rely on mathematics, reducing the length 10-15% or 1-2″ to get the correct/even amount of stretching. Because elastics come in different widths, qualities and amounts of stretch, I do not calculate the amount I should pull. Surprising, considering my very particular nature. Over time, I have developed a hand where I know how much tension the elastic needs and that’s usually a slight pull. The elastic should create a snug fit, but not ruche or gather the fabric like a skirt. In my bra making class, I tell students to stretch the elastic on one of their ready-to-wear bras several times before taking sewing their first pass. Just like muscle memory, the goal is to develop a “feel” through repetition.
Also to note is that the elastic is stretched different amounts at different places in a bra. While I stretch the elastic the “normal” amount on the top and the bottom band, I stretch it slightly more in the underarm (to encourage it to shape up towards the straps point and to cover that little flab of underarm flesh) and almost don’t stretch it along the neckline (so that is doesn’t cut into breast tissue).
This is very hard to show in pictures, so in the video below, I demonstrate the “normal” amount of pull. You’re probably wondering why I don’t start stretching further back from the presser foot, and that’s because I have found that if I do, I lose the original amount of stretch while I sew; I let go of the elastic slightly as I sew.