A pure cotton sweater will shrink more than one made of a blend of fabrics such as polyester. Check the label for the fiber content. The higher the cotton content, the more the sweater should shrink.
A sweater labeled "pre-shrunk" may still shrink in a hot, high-agitation wash. The term means that a manufacturer performed some activity to cause shrinkage but more may occur.
Many people have experienced pulling a favorite sweater from the dryer, only to find that it has shrunk. Experienced launderers know that tumble-drying can shrink both cotton and wool clothing. You can purposely shrink too-large clothing in the dryer, but the high heat may damage cotton by lowering the hardiness of the fibers. Using the washer alone may also allow you to shrink a sweater without these damaging effects, if hot water is used. The heat creates the shrinking effect in both types of fabric, and wool shrinks from dampness and agitation, as well.
Set the temperature on the washing machine as hot as possible, since heat increases the chances of shrinkage for both wool and cotton fabrics.
Use a setting with high agitation to increase the chances of shrinking your wool sweater, since such action causes the material to cling together more tightly.
Lay the sweater out flat to dry. Avoid stretching the garment, since that will stretch the fibers back out.
Kristie Sweet has been writing professionally since 1982, most recently publishing for various websites on topics like health and wellness, and education. She holds a Master of Arts in English from the University of Northern Colorado.