How do I know if my mountain bike helmet fits correctly?

A properly fitted helmet sits level on your head, covering your forehead with about one to two finger-widths of space above your eyebrows. When the retention system is adjusted, the helmet should not shift more than an inch in any direction. The chin strap should form a V below your earlobes and allow one to two fingers between the strap and your chin. There should be no pressure points or gaps between the pads and your head. If the helmet rocks forward over your eyes or backward off your forehead when unstrapped, it's the wrong size or shape for your head.