When the chin strap is buckled, a bicycle helmet should have a snug but comfortable fit. You should not be able to move the helmet from side to side or up and down.
The helmet should sit level on the head (not tilted back) and rest low on the forehead.
The bottom edge of the helmet should be 1 to 2 finger widths above the eyebrow.
Another technique to check proper positioning: your eyes should be able to see the very edge of the helmet when looking upward.
The straps of the helmet need to be even and should form a "Y" that comes together right at the bottom of the earlobe. The straps should be snug against the head.
The buckled chin strap should leave only enough room to allow one finger to be inserted between the buckle and chin. When opening your mouth, you should be able to feel the helmet pull down on your head.