In order to make sure the helmet fits as well as possible, we need some measurements: The width and depth of your head shown as measurement 6 and 13 from this chart: https://en.wikipedia.org/wiki/Human_head#Average_head_sizes

Those two measurements makes it a lot easier to size the helmet for your head and give you a great fit. Some helmets may benefit from other measurements as well to make sure nothing is to tight, but that will be handled on a case-to-case basis.

The measurement should be taken in a 'straight line' going through the head, and a normal width (ear to ear) measurement is around +/-15 cm. It's advised to measure more than once to make sure you have an accurate measurement.

If you have a caliper then that is the easies way for you to get these measurements, but if not there are other options available. You can lie down on the floor and put your head against the door frame or a cardboard box, then either close the door or move another cardboard box until it touches you head and then you can measure the distance.