For in-between measurements use the next size up.
*Handy hint: measure a few heads if buying for a group of people to get an idea of the sizes you need.
Heads come in all sizes, and in a surprising variety of shapes.
Although high quality hats are adaptable, and will usually conform to differences in shape with a little wearing, it is important that the size be correct for the head.
To determine your hat size, measure the circumference around your head, keeping the tape level and firm, across the temples and above the eyebrow ridges.