To work properly, a bike helmet must fit correctly and be worn properly.
Sizing your helmet
In order to determine the size you need, you need to measure
the circumference of your head.
Simply wrap a measuring tape around your head, being
sure to locate the tape just above your ears and keeping the tape level front
to back.
Once you've determined the proper size, you can check
to make sure the helmet fits correctly.
If the helmet you own now doesn't fit on your head
with just a little space between your head and the headband when the fit system
is loose, you should replace it with one that does.
You should always wear the helmet positioned low
enough in the front to protect your forehead, but not so low as to hinder your
vision.