When choosing the best sweater yarn, a knitter should consider what kind of sweater she would like to make and who will be wearing the finished sweater. Accurate knitting gauge is especially important when knitting a sweater, in order to ensure the sweater fits properly. Gauge is the number of stitches that can be counted in an inch of knitted fabric. Yarn weight should also be considered, as not all people can wear garments knitted with super bulky yarn. The fiber content of the sweater yarn is also important, as a light sweater for spring would be hot and uncomfortable knitted in wool yarn, while a winter sweater knitted in cool bamboo fiber would not be warm enough.

If a knitter's gauge is off when making a sweater, the sweater will almost certainly turn out too large or too small. To ensure accurate gauge, the chosen sweater yarn should be as similar as possible to the yarn suggested by the sweater pattern. If the pattern calls for fingering weight yarn that knits at a gauge of eight stitches to the inch, for instance, then the knitter should attempt to purchase sweater yarn that knits at a similar gauge, if not the same gauge.

A knitter may also want to consider the body shape of the intended sweater recipient before beginning to knit a sweater in heavier yarn weights. Lace, fingering, sport, and double knitting (DK) weight yarns all have thin strands that knit up into a thinner fabric. Worsted and aran weight yarns are usually the most common weights chosen for use as sweater yarn, and knit up into a fabric of medium thickness. Bulky and super bulky weight yarns, however, create a very thick fabric that can add mass to the person wearing it, and therefore might not be good choices if the sweater recipient is somewhat overweight.

Fiber content is also an important consideration when selecting a sweater yarn. Wool is relatively inexpensive, warm, and durable, and produces a fabric that has good memory so it won't stretch out of shape, making fiber blends with high wool content a good choice for almost any warm winter garment. For those who are allergic to wool but still wish to knit a warm sweater suitable for cold weather wear, hypoallergenic alpaca will knit up into a very warm, soft garment with lots of drape and a fuzzy texture that resembles the halo characteristic of mohair. Cotton is usually preferable for a garment intended for warm weather wear, though because cotton lacks memory and durability, a blend of cotton and wool, or cotton and linen may be the best choice. Acrylic or superwash wool would be appropriate choices for a sweater meant for a baby or a child, as both types of yarn can be machine washed.

As an adult-sized sweater requires a lot of yarn yardage and takes time to knit, the knitter should keep in mind that she will be working with a relatively large amount of the same sweater yarn for an extended period of time, perhaps as long as several months. Therefore, the best sweater yarn is the yarn that makes the individual knitter feel comfortable while knitting with it. Many knitters dislike blends that contain acrylic or plant fiber, for example, as both types of fiber can be tough on the hands. Other knitters object to yarn that has knots or vegetable matter in the skein, or to yarn that has a tendency to pill or break. For some, cost is no object and the enjoyment of working with cashmere or other luxury fiber is its own reward, while other knitters prefer to be more conservative and choose a so-called workhorse wool to complete large projects like sweaters.