A GOOD FIT

A helmet is the single best way to prevent brain injury when riding a bicycle. A bicycle helmet should be snug, level and stable on your head and cover most of your forehead before any adjustments are made.

Place the helmet on your head, and without buckling the chin strap, shake your head to see if it shifts from side to side.
If the helmet moves a lot, it's too loose and either needs to be tightened with pads or the ring at the back and base of the helmet ... or you need a smaller helmet.
If your forehead isn't covered, you probably need a larger helmet.
With the helmet level on your head, you should see the very edge or rim of your helmet when you look up past your eyebrows. This is about two finger widths above your eyebrow.
The straps should meet and form a "V" right under each earlobe. To check this, form a "V" with two fingers around your ear.
Once the straps are adjusted in a "V," buckle them. The straps should be snug but not too tight. Adjust until you can put no more than two fingers between the strap and your chin.

-- National Highway Traffic Safety Administration