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When starting a new project, it's not so hard to purchase the number of balls or skeins of yarn indicated in the pattern (and maybe an extra ball or two, just in case). But, what about if you are substituting another brand of yarn that comes in different ball or skein sizes?
If you are substituting yarn, then calculate the total yardage required in the pattern (for example, the pattern says to buy 4 skeins of yarn, and each skein is 150 yards long, so the total yards required for the project is 600 yards) and divide by the number of yards contained in the substitute yarn (let's say you want to use a yarn that comes in a 200-yard skein). That will tell you how many balls or skeins of the new yarn to buy (in our example above, 600 divided by 200 equals 3 skeins, though, again, you might want to go ahead and buy an extra skein).
But, a word of caution, don't try to substitute a bulky number 5 weight yarn for a sport or baby weight number 2 yarn! The whole pattern would need to be reworked to account for the differences in gauge, so when substituting yarns, stay within the same weight of yarn originally called for in the pattern.