Cotton is a common fiber used to make sweatshirts, and fortunately for you clothing made of cotton is one of the easiest fabrics to shrink. According to Cotton Incorporated, cotton clothing shrinks because the tension applied to the fabric is released when it is washed and dried. You will have the most success shrinking your cotton sweatshirt the first time it is washed and dried. The key to shrinking your sweatshirt is simple: lots of heat.
Check the tag on your sweatshirt to determine if it susceptible to shrinking. If it is not 100 percent cotton, or if the tag says preshrunk, you probably will not be able to shrink your sweatshirt very much. Cotton blends and preshrunk fabrics may shrink slightly, but not enough to make a very noticeable difference.
Wash your sweatshirt in the washing machine with hot water and color-safe detergent. Color-safe detergent helps maintain the color of your sweatshirt as much as possible throughout the shrinking process. However, the high heat needed to shrink your sweatshirt may fade the colors as well.
Dry your sweatshirt in the dryer at the hottest heat setting possible until it is completely dry. After the drying cycle ends, let your sweatshirt cool in the dryer for 15 minutes.
Repeat the washing and drying process one or two more times to encourage more shrinkage.
Take your sweatshirt to the dry cleaner if it still has not shrunk sufficiently. According to Cotton Incorporated, steam pressing can also shrink cotton fabrics. Ask your dry cleaner if they can steam press your sweatshirt, and mention that you want it to shrink.