Bike Helmets: Get the Right Fit
Allow time for helmet shopping. Be prepared to go to more than one store if the first store doesn't have the right size.
Make sure the helmet meets safety standards. Look for a U.S. Consumer Product Safety Commission (CPSC) sticker inside the helmet. All helmets sold after 1999 must carry this sticker. Keep in mind that bike visors are not rated for shattering in a fall.
The helmet should fit your head. That means it should touch your head at all points, and feel comfortably snug, but not tight. You should not be able to move the helmet more than about an inch in any direction. You should not be able to pull it off your head. Make sure the junction of the front and back straps is under the ears.
Make sure the helmet protects the forehead.
Besides a good fit, your helmet should have air vents to help cool your head during a ride. The larger the front vents, the better the flow of air on your head.
If you have long hair, you might consider a helmet with a "ponytail port." If you have no hair, you might consider a helmet with minimal top vents to avoid tan lines on the top of your head.