Felt is made from animal fibres, such as wool, mohair or alpaca. When animal hair is exposed to hot water or a heated dryer, it shrinks. While a fedora or other lined, brimmed hat will require shrinking and reassembling by a milliner or dry cleaner, a handmade hat with no lining is simple to shrink at home. A variety of drying methods help ensure that your hat will fit your head, rather than shrinking down too much.
Place the hat into a pillowcase or lingerie bag. Tie or zip the bag shut.
Set your washing machine for the smallest load and the hottest water possible. Add the normal amount of washing powder recommended for the load size.
Add a few old towels and the bag containing your felt hat. Allow the machine to agitate until the load is halfway finished, then pull your hat out and check to see if it has shrunk enough. If it has, rinse the suds out by hand. If not, put it back in and allow the load to finish.
Try on the damp hat to see if it fits your head. If it's still too big, you can either wash it again or put it in the dryer for several minutes. Pull it out frequently to make sure it hasn't shrunk too much.
Wear the hat until it shapes to your head, then lay it flat or put it on a hat form the same size as your head and allow it to air dry.
Whenever possible, use a top-loading washing machine that you can interrupt in the middle of a cycle. Using an industrial or locking washing machine that forces you to wait for the load to finish can result in your hat shrinking too much.