You measure your head size with a textile centimeter. If you don't have a fabric centimeter to hand, you can also use a piece of string. You can then measure this piece of rope later.

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With the centimeter you measure a finger thickness above your ears and eyebrows. See also the image below. The measured quantity is the measure. Example: if you measure 59 cm, the hat size is 59 cm.

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Because hats can shrink under the influence of moist elements, such as rain, it is better to take a slightly larger size than a size that is too small or just fitting.