Look for a tag or stamp inside the hat that tells what material the cap is made from. Caps made of cotton, polyester, nylon and twill can be washed in a washing machine. A wool cap should be washed by hand.
Wash the baseball cap in the washing machine in cold water on the gentle cycle. Use a mild detergent. If the cap is stained, use a stain remover or pre-wash cycle to help remove the marks.
Hand wash a cap in cold to warm water. Rub the hat gently using mild detergent to remove dirt. Wool materials require a special cleaning detergent. Rinse the hat thoroughly in cool water.
Never machine dry a washed baseball cap. Let the hat air dry on a can or on your head so that it retains its original shape.