Most of today's cotton clothing is sold pre-washed or preshrunk. The garments are washed prior to being offered for sale so that the consumer does not have to worry about shrinkage. Many cotton garments are also blended fabrics which are designed for minimal shrinkage. 100 percent cotton will shrink if not handled correctly. You should preshrink any 100 percent cotton fabric prior to constructing anything from the fabric. You may also want to shrink a cotton garment which is a slightly larger fit than desired.
Wash in a washing machine using a hot water setting. You can also place the garment in a large pot of boiling water. Let the fabric run through a complete wash cycle or boil for 15 to 20 minutes.
Place the wet fabric in the dryer and tumble until dry. According to Cotton Incorporated, you don't have to use a high heat setting to achieve shrinkage. The tumbling action of the drying cycle will cause the fabric to lose moisture and shrink.
Repeat if necessary. Some fabric may take a few wash and dry cycles to shrink to the desired size.