How to find the right hat / cap size.
How to measure correctly:
Take a tape measure and place it around the head at the height of the back of the head and the middle forehead. The measure in centimeters is your hat size. Alternatively, you can measure your head circumference with a shoelace or other string and then read off the length with an folding rule. You'll get a cork strip with your headwear (put behind sweat band to narrow hat size). Masks are one-size and can be narrowed with a simple knot.