Hatter's felt is different than the felt you find in most craft stores. Craft store felt is made from synthetic materials and comes in a variety of qualities. Hatter's felt is made from wool or fur, which is woven and treated with hot water or steam. Moist heat causes the fur or wool fibres to tightly bind together. Because it is moist heat that forms the felt during the hat making process, you can use steam to adjust the size of your hat at home.
Measure the hat along the headband line (the part that touches your forehead during wear). Write down the length of the headband line.
Fill the kettle completely with water and heat to boil until it produces steam. Filling the kettle completely ensures there will be enough water to produce steam for several minutes.
Hold the hat over the steam and rotate it so the steam moistens all of the headband; it should be moist but not wet.
Pull gently and stretch the headband outward. Rotate the hat as you stretch so the headband is expanded evenly. Continue to stretch the hat, stopping after every rotation; measure if it is the desired size.
Allow the hat to air dry. Do not use a blow dryer or clothes dryer, as these can shrink the hat. Make sure the hat is in the shape you want before it dries.
Hat stretchers are available at millinery supply shops to help you resize your hats.
Do not stretch your hat more than ¾-inch.