Helmet sizing is pretty basic; the sizes are determined by the circumference of your head and are measured in centimeters or inches. The best way to measure your head is to use a fabric tape measure or a cutout tape measure. Take the tape measure and measure the circumference of your head just above your eyebrows (clear of your hair) and that measurement is your helmet size. One of the MOST important things to keep in mind is that not all helmets fit the same. Head shape is a factor in your helmet selection and will vary by brand of helmets. In most cases, the helmet you choose will fit with no problem, but sometimes, depending on the shape of your head, the shape of the helmet may not be compatible. In these rare cases, you may have to try a different brand or model.
For kids, it is important not to purchase a helmet too big for their head. A helmet that is too loose or large is not going to be safe and can do more harm in the long run. Kids heads grow slowly compared to the rest of their bodies so make sure you order a helmet that fits. The only time you may want to bump up a size is if the head is right in-between sizes. Then you may opt to select a larger size.

Measuring Tape: To determine the correct size, wrap the tape around your head, just above your eyebrows. Note your size at the point of overlap. This is your helmet size. And please note that centimeter measurements can be converted to inches by dividing the value by 2.54.