The time will come when you need or want to substitute a yarn in a pattern. The reasons for this are varied. Perhaps the yarn recommended is discontinued; perhaps it's too expensive, or perhaps it's a fiber you don't wish to use.
First, what is the weight of the yarn? Worsted? Bulky? You need to select a yarn in the same weight class. If you don't, you won't be able to obtain the correct gauge and your project will not be the correct size. You may eventually be able to obtain the gauge of the pattern but it will be as stiff as cardboard or very loose (depending on if you selected a lighter or heavier weight). For a project such as a sweater, this will have a huge impact.
For a listing of Lion Brand yarns by weight click here.
You can often combine multiple strands of a lighter weight yarn to achieve the same weight as a heavier one. Be sure to check your gauge very, very carefully when doing this. Yarns within the same weight class still work to various gauges. For example, a worsted weight yarn is usually classified as one that works between 16 and 20 sts per 4 inches. These are approximate equivalents:
2 strands fingering = one strand sport.
2 strands sport = one strand worsted.
2 strands worsted = one strand super bulky.
Second, think about the fiber content and construction of the original yarn. Although you may your project turns out perfect if you substitute a polyester chenille for a 4-ply cotton, these two fibers/yarns produce a knitted garment with entirely different looks and feel. The way the knitted fabric will drape is another consideration in terms of fiber. You should also be aware of the look of the yarn. A complex cable pattern will look entirely different in a textured yarn that it will in a smooth yarn that provides good stitch definition.
Third, make sure you purchase sufficient yarn from the same dye lot to complete your project. The pattern may call for 8 balls of X yarn but this does not mean the equivalent amount is the same for Y yarn which you are going to substitute. Do not substitute by buying the same number of balls or by the total number of ounces or grams.
You must substitute by making sure you have bought the equivalent number of yards or meters (plus extra to be safe). Even in closely equivalent yarn, there are differences in the number of yards per skein; if the fiber content is different, you will likely find a big difference in yardage because some fibers are heavier than others (cotton is heavier than wool and will have fewer yards, all things being equal).
Determine how many yards per skein the original yarn contained. Let's say it had 100 yards per skein and you needed 8 skeins. That is a total of 800 yards. The yarn you selected as a substitute has 85 yards per skein. Divide 800 by 85 and you'll find you need 9.4 balls of the new yarn. Round this up to 10; do not round down. It's always a good idea to purchase one extra skein in addition to the number you've come up with.
If you are substituting by double stranding, you must double the yardage you will need to purchase.
By considering all of these different factors, you will be able to make good buying decisions when substituting yarns, so good luck and happy knitting!