The best way to measure your head is to use a measuring tape and measure around your hairline, starting at the forehead, around the ears, nape of the neck and back up again.
When you come to your ears, measure next to your head, not over the ears. A small adult head measures 20.5”-21.5”, medium is 22”-23” and a large head size is 23”-24”, occasionally 25”.
If your hairline grows extra low in the back, measure where you would want a hat or turban to sit. You might want to measure several times to see if you get the same result.
Usually, children around 9 or 10 years of age can wear small adult sizes. Our hat and headscarf descriptions include head sizes based on this way of measuring.