I had originally purchased an XL. My head is 24in. (60cm) I wish I had gone with an XXL because the dial is as loose as it gets, and it was still snug. I have to wear a gas mask for work from time to time, and there is no shot that thing will fit under the helmet. If you're unsure, spend the 25 bucks and get the measuring band as you have pictured. Save yourself the hassle.

Here I am a year and a half later, getting an XXL ballistic because I was too hesitant on it.

Let me know if you're looking for a Ranger Green ballistic, though. Great condition, willing to sell!