Ever bought a pattern that you couldn't wait to start knitting only to find out that the suggested yarn has been discontinued or is way out of your price range? Maybe you want to use a cotton blend to make a lighter version of a winter pattern. These are all appropriate situations to substitute yarn. When going about this task, be sure to choose a yarn with similar weight. Occasionally, you may be able to substitute a lightweight yarn for a heavier one by knitting with a double strand of the lighter yarn, but you must be sure to check your gauge very carefully. In fact, the only way to ensure any yarn substitution is accurate is to knit a swatch and compare its gauge with the gauge given for the original yarn, so whatever you do, do not skip this vital step!
HOW MUCH IS ENOUGH?
How many balls of your substitute yarn will you need to complete your chosen project? The answer is just a few easy calculations away.
Step One:
Determine the quantity of yarn needed to complete the project using the original yarn listed in the pattern.
 Find the number of balls required for each color.
 Find the number of yards (or meters) per ball or skein of that yarn. This information is included in the Materials section of all KnitSimple patterns.
 Multiply the number of yards (or meters) in each ball times the number of balls. Example: Let's say the pattern requires 9 balls of yarn, and each ball is made up of 200 yards (182 meters). Multiply 9 times 200. 9 balls x 200 yards = 1,800 yards (or 9 balls x 182 meters = 1,638 meters) 1,800 yards (or 1,638 meters) is the total yardage required.
Step Two:

Figure out the quantity of substitute yarn needed to complete the project.
 Determine the number of yards (or meters) per ball or skein of your new yarn. The yardage should be listed on the ball band or skein tag.
 Divide the total yardage required (from Step One) by the yardage of your new yarn. Example: Let's say your new ball of yarn has a yardage of 109 yards (100 meters). 1,800/109 (or 1,638/100)=16.5 balls. Since you can't buy half of a ball, you need 17 balls of your replacement yarn. If more than one kind of yarn is used in the pattern, repeat steps 1 and 2 for each type.