I had a similar curiosity and interest as well. I researched a bit on the web and searched through my sewing library and came up with following info.
The length of elastic is determined by the **percentage of stretch** you need for your application. The percentage of stretch depends on the application, width and type of elastic used. The total range of stretch used in many applications is between
10 - 30 %
length of elastic = (circumfrence of leg at the opening - percentage of stretch) + sew allowance
One method to determine percentage of stretch to use ... if you trust the swim wear manufacturer you can calculate the stretch they used in the swim suit and use that ? by measuring the relaxed diameter of the leg opening, then measure the stretched leg opening and then compare results to determine the percentage of stretch used.
well the gurus will chime in soon and i can see how close i was to the gurus advice :)
I always measure elastic directly on my body, and cut off the amount that feels right.
But then I always run elastic through a casing (can't stand the feel of elastic on my skin), so the stretch isn't changed by installing it. Don't forget seam or knot allowance, if needed.
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