Nothing is more frustrating than buying a hat and realising it is either too big or too small. One way to avoid this is to know the circumference of your head and how it translates to hat sizes. Hat sizes are based on a range of head circumferences. For example, a hat may be displayed as a size 20, which is said to fit head circumferences of 135 cm to 140 cm (54 inches to 56 inches). In many cases, though, that sizing is not accurate enough to buy a hat that fits well. Generally, hats are much easier to stretch than they are to shrink. Therefore, it is better to buy a hat that might be slightly too small than to buy one that is too large. Some hats can be shrunk, however, if made of cotton or other material that is prone to shrink.
Making a hat bigger
Soak the hat in water. That allows the hat material to be stretched.
Wear a wet hat and allow it to dry on your head. This lets the hat stretch and form to the circumference of your head. As it dries, it will remain permanently stretched to the new size.
Stretch a wet hat on an hat stretcher. Set the stretcher to the required measurement by twisting its handle, and allow the hat to dry on the device. The hat will stretch to the correct size and remain permanently formed to that size.
Making a hat smaller
Wash a shrinkable hat in a dishwasher or washing machine. Wearing the wet hat afterward will let it conform to the proper size.
Put foam hat tape inside the hat. Add the foam hat tape as needed until the hat fits snugly.
Wear a wet hat on your head to let it conform to the proper size. Wear it until it dries.
Before washing a hat in a washing machine or dishwasher, check the hat's label or ask the retailer because some hats will be permanently ruined, such as fur hats.