A good-fitting helmet shouldn't compress your head, but should remain well positioned when/if your head is shaken. You can use a tape string or our Head Sizer to measure the circumference of your head, passing over your ears and staying 0.78 inches above your eyebrows. Many of our helmets have a knob to adjust and fine-tune the fit for additional comfort and safety. Ideally, with the helmet on, you should be able to twist the helmet forward and back and see your eyebrows moving along with it. If the helmet slides forward or back too easily and/or without eyebrow movement, it's not providing the ideal fit.