Knitters frequently have leftover yarn when they complete projects, which can languish in their stashes. Because many projects have specific yardage requirements, knitters are loathe to use leftovers because they have no idea how many yards are left in the skeins. Few things in knitting are as frustrating as running out of yarn toward the end of a project. However, with the yarn's ball band and simple math, anyone can determine how much yarn is left, without having to unwind the ball and physically measure the yarn.
Find the yarn's ball band. If you are missing the ball band, look up the yarn's information on Ravelry.com, which maintains a database of information on thousands of different kinds of yarn. A free membership is required to access the information on the site. Note both the yardage and the weight in grams of the original skein.
Divide the yardage by the weight in grams with a calculator. This will tell you how many yards per gram there are in a skein. For instance, if your skein was originally 50 grams and had 100 yards, each yard of yarn weighs 2 grams.
Weigh your yarn. Set a kitchen scale to weigh for grams. Most kitchen scales will do this, but if it doesn't, you can use an instant ounces-to-grams calculator on the Internet or multiply the ounces by 28.349. Place your yarn on the scale and determine the number of grams.
Multiply the number of grams your yarn weights by the number of grams per yard you originally calculated. For instance, using the number calculated above, if your leftover ball of yarn weighs 43 grams, multiply that by 2. Your leftover ball of yarn has 86 yards of yarn left in it.