How To Fit Helmet

Regardless of your current level of cycling expertise, it is critical that you know how to fit a helmet properly. An improperly fitted helmet may be little better than wearing no helmet at all. These tips should help ensure a correct and comfortable fit.

1. Initial Set-up. Upon purchasing a new helmet or setting up a helmet for first time use, you will have the opportunity to make a few large, onetime adjustments. The most important criteria are that the helmet fit low on the head and fit snugly. Most helmets come with a set of adhesive pads that can be attached to the inside of the helmet to take up any extra space between the helmet and your head. When attaching the pads, make sure the helmet ultimately fits evenly against the whole head. The helmet should not move at all even when you shake your head vigorously.
2. Straps. The second step is to adjust the straps. Begin by buckling the straps under your chin to determine what adjustments must be made. Helmets will have a rear and a front strap that are joined together under the ear, forming a Y shape. A plastic piece will join the two straps. Neither strap should have slack - if one strap is too long, pull on both sides of the strap (on each side of the helmet) until the desired length is achieved. The straps should fit comfortably underneath each ear. The straps should also fit comfortably under the chin. It should be loose enough that you can open your mouth wide before it pulls the helmet down, but tight enough that the helmet will not slip off even when violently jarred against a wall or other obstacle. Loosen or tighten the chin straps until this adjustment is reached.
3. Additional Adjustments. Some models will have additional features that allow more precise adjustments than just pads and straps alone. Many models have a knob on the back that adjusts the snugness of the helmet against the head. This should be used in conjunction with proper pad placement. Fancier models may also have locking mechanisms that allow wearers to preserve their adjustments from one ride to the next.

Reference:

Properly Fitting a Bicycle Helmet