Measure your head as shown, around the widest part of your head. This is generally right above your eyebrows and ears. If you plan on wearing a Balaclava (we suggest it), you should measure your head with it on.
We have significantly better results with a flexible tape measure, but if one is not available you can use a string to measure your head, and then transfer that to your measuring device.
Then reference the chart below to find which helmet size you should use. If you fall between sizes (IE if your head measures 56.5 cm) we would recommend using the smaller size.