Some brands may vary from these measurements but you can still use them as a guide.
HOW TO MEASURE
To choose the correct size for you, measure your head as follows:
1. HEAD CIRCUMFERENCE
To determine your head (hat) size, measure around your head (circumference) as shown in the image
One size hats are designed to fit a head circumference of 58cm.One size caps are adjustable and have been designed to fit head circumference of between 22.5 inches (57cm) and 23 inches (59cm).
*These charts are for reference only. Fit may vary depending on the construction and materials