Your objective: Snug, Level, Stable
You want the helmet to be comfortably touching the head all the way around, level and stable enough to resist even violent shakes or hard blows and stay in place. It should be as low on the head as possible to maximize side coverage, and held level on the head with the strap comfortably snug.
Be Prepared for the Worst
Heads come in many sizes and shapes. You should be prepared for the possibility the helmet you are trying to fit may not be compatible with this particular head. And unfortunately, you can expect to spend ten to fifteen minutes to get your helmet properly fitted.
First, Use the Fit Pads or Ring
Most helmets come with at least one set of foam fitting pads and if you got a second set of thicker pads they can be used to customize the shape.
For starters, you can often remove the top pad entirely or use the thinnest ones. This lowers the helmet on the head bringing its protection down further on the sides. It reduces the flow of cooling air but probably not enough to notice.
Adjust the side fit pads by using thicker pads on the side if your head is narrow and there is a space, or thinner pads in the back for longer heads. You may also move pads around particularly on the "corners" in the front and rear. Leaving some gaps will promote air flow. The pads should touch your head evenly all the way around with-out being too tight. The helmet should sit level on the head, with the front one finger width above the eyebrows, or if the rider uses glasses, just above the frame of the glasses. If you walk into a wall, the helmet should hit before your nose does!
Some helmets use a fitting ring instead of pads. With these "one size fits all" models you begin by adjusting the size of the ring. Some of them may require the ring so tight for real stability on your head they feel binding, but if loosening the ring produces a sloppy fit that helmet is not for you.
Then, Adjust the Straps
Now put the helmet on and fasten the buckle. Be sure the front is in front! You want to adjust it to the "Eye-Ear-Mouth" test developed by the Bicycle Coalition of Maine. When you look upward the front rim should be barely visible to your eye, the Y of the side straps should meet just below your ear, and the chin strap should be snug against the jaw so when you open your mouth very wide you should feel the helmet pull down a little bit.
With the helmet in position on your head, adjust the rear (nape) straps, then the front straps to locate the Y fitting where the straps come together just under your ear. You may have to slide the straps across the top of the helmet to get them even on both sides. Then adjust the chin strap so it is comfortably snug. Now adjust the rear stabilizer if the helmet has one. It keeps the helmet from jiggling in normal use and makes it feel more stable, but only a well-adjusted strap can keep it on in a crash.
When you think the straps are right, shake your head around violently. Then put your palm under the front edge and push up and back. Can you move the helmet more than an inch or so from level, exposing your bare forehead? Then you need to tighten the strap in front of your ear. Now reach back and pull up on the back edge. Can you move the helmet more than an inch? If so, tighten the nape strap. When you are done, your helmet should be level, feel solid on your head and be comfortable. It should not bump on your glasses (if it does, tighten the nape strap). You should forget you are wearing it most of the time, just like a seat belt or a good pair of shoes. If it still does not fit that way, keep working with the straps and pads, or try another helmet.
We hope it worked for you.
For more information, please contact Sergeant Neil Hennelly at firstname.lastname@example.org
Bicycle Helmet Safety Institute
A consumer-funded program of the Washington Area Bicyclist Association www.helmets.org