If you are making the headband for a child with fine silky hair, consider using a narrow elastic so the headband can be held in place with children's barrettes.
The Elastic: Sewing Elastic - Who, What, When, Where,Why and How to Use Sewing Elastic
Measure the head of the person who will be wearing the headband, in the area of the head that the head band will be worn. Depending on the stretchability of the elastic you are using, subtract just enough to make the elastic snug on the head, allowing for the ends of the elastic to overlap by 1/2". Do not make the elastic so tight it is uncomfortable. The Fabric:
The Length equals -- The length of the elastic times 1.5 (times 2 if you are using a very fine fabric)plus 1/2" for turning under at the ends of the fabric tube (later in the directions).
The Width of the Fabric -- The width of the elastic times 3. For example; If you are using 1" wide elastic, the fabric would be 3". If the elastic is 1/2" wide, the fabric will be 1 1/2" wide