Measure your head carefully, don't guess or measure the inside of your favourite hat.
To measure the circumference of your head, with a tape measure, start in the centre front of your forehead, measuring around your head, ensuring the tape measure sits comfortably on your occipital bone at the back.
Do not pull the tape measure too tightly.
If you don't not have a tape measure, measure round your head in the same way using a piece of string and lay out the length you have measured against a ruler.
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