I am so on my soapbox about swatching. It not only crucial for ensuring your projects fit. It is also a truly knitterly thing to do. It allows you to get to know your yarn, your gauge and the type of needle you want to use. Finally, you can use a gauge swatch to estimate the yarn requirements of your project.
First knit your swatch and measure it. In addition to gauge, you will want to calculate the area of your swatch. For those of you who haven’t studied geometry lately, it is length x width = area (in square inches in this case). Then weight your swatch on an accurate digital scale, such as the type bakers use. When you know how much the swatch weighs, you can calculate how much yardage you have used, based upon the weight of the swatch. If the skein of yarn weighs 50 grams and it has 87 yards, a 4 gram swatch uses 7 yards of yarn.
How did I come up with this number? I like to say that knitting requires a great deal of lower mathematics – algebra, geometry and the occasional dash of trigonometry may be called on to help you complete your project
In this instance, I figured out that:
4/50 = x/100, or 8%.
Then I multiplied 87 yards by .08 (8%) and got 6.96, rounded up to 7 yards. Ok, so what? Well, our mythical swatch also had an area of 6 x 6 inches or 36 square inches. So, 36 sq. in. of knitting uses 7 yards of yarn and 5 sq. in. uses 1 yard. If I were planning a 60 inch x 60 inch blanket, (3600 sq. in.) I would divide 3600 by 5, getting 720. Therefore, my blanket would require approximately 720 yards of yarn.
I would go ahead and buy at least 800 yards in order to ensure that I have enough of a matching dye lot to complete my project (and that is a sermon for another day).