Knitted or crocheted hats should be 1" - 1.5" less than the actual head circumference, since the nature of the material is to stretch. If you are making a hat out of 100% cotton, you may want to make the circumference larger than the recommended hat circumference since cotton is not as stretchy as most acrylics and other natural fibers. These are only suggested sizes, and you may find that sizes other than those listed here work better for you.