Although the traditional length of a skein of yarn is 560 yards, the number of yards varies depending on the type of yarn in question. Because a skein is based on weight, the level of weight and thickness of each strand directly affects the length of the yarn in a skein.
To accurately measure the length of a skein of yarn, an instrument called a yarn winder can be used. After the spinning wheel spins the yarn, the yarn winder works in intervals of 2 yards per revolution to accurately measure out 560 yards of yarn.
The process of measuring yarn can be more complicated, however, because yarns today are graded by weight or thickness. Often, different yarns are even mixed together on the same skein.
Other factors that come into play when evaluating yarn are needle count, coarseness and staple length.