Annie Modesitt is one of my favorite pattern designers because she usually gives an estimate of the yardage used when she uses only a small portion of a skein. Most patterns indicate that you need an entire skein even if you only need a few yards. In designs like Fair Isle where multiple colors are used, buying an entire ball of each color can get expensive. If you know the yardage of each color used, you can figure out if your partial skein has enough yardage left, or dye the yardage needed from only a few skeins.
Even if you are not using one of Annie's patterns it can still be useful to estimate the yardage you have used, and the yardage you have left. You may want to add up the yardage in several partial skeins, see if you have enough yarn left to make the same project again, or detail the yardage used in your own design project. You can even find out if you have enough yarn left for an additional pattern repeat.
1) Weigh your full ball/skein on a scale accurate to at least 1/10th of a gram (A good kitchen scale should do), and take note of the humidity.
---> The weight of yarn will change based on the amount of water in the air. If there is a big moisture change during shipping, or even in your home, there can be a measurable weight change.
2) Check the yardage of your full ball/skein by checking the ball band or another reliable source, such as the manufacturer's website. We must assume that this measurement is correct, but it may not be. Most wool mills measure yarn by weight only, and again, if the humidity has changed the weight will change, and therefore the yardage as well. For example, in high humidity yarn will be heavier because it contains more water. So when the mill measures out a 100 gram skein of sock yarn, the skein might contain 430 yards. On the other hand, if the skein is measured in low humidity, where the same amount of yarn weighs less, a 100 gram skein might contain 440 yards. The yardage on the ball band is a good estimate of how many yards the skein actually contains.
3) Calculate the weight per yard (the amount each yard of yarn weighs) by dividing the weight of the skein by the stated yardage.
------- = weight per yard
For example: If a skein of Cascade Heritage weighs 100 grams, and is 437 yards long, the weight per yard would be 100 grams divided by 437 yards, or .229 grams per yard.
4) Use as much yarn as you need.
5) Calculate the yardage of the left over yarn. Weigh the partial skein. Then divide the weight of the partial skein by the weight per yard. In our example, if I had 50 grams left in my partial skein, I would divide 50 grams by .229 grams indicating that I had 218.341 yards left.
grams in partial skein
---------------------- = yards in partial skein
(grams per yard)
6) Calculate the yardage of the yarn used. Subtract the weight of the partial skein from the weight of the full skein, to find the weight of the yarn used.
weight of full skein - weight of partial skein = weight of the yarn used
Then divide the weight of the yarn used by the weight per yard.
Note 1: This is an estimate. Its accuracy relies on factors outside our control, and therefore cannot be considered an accurate measure of yardage. I recommend rounding up measures of yardage used to avoid problems of too little yarn.
Note 2: Meters can be substituted for yards.