What Should You Look for in a Helmet?
All helmets sold in the U.S. must be certified by the Consumer Product Safety Commission (CPSC). Double-check to make sure your helmet is CPSC-certified. Two other certifications come from the Snell Foundation and ASTM; you may find any or all of these stickers inside a helmet.
Make sure your helmet fits! A helmet does no good if it's falling off your head. How do you get a good fit?
First, adjust the fit pads or ring. Most helmets come with extra foam fitting pads to customize the fit. Use thicker pads on the sides and back if there is space between your head and the helmet. If you have a "one size fits all" model with a fitting ring, adjust the fit by tightening the ring.
Next, adjust the straps. Put the helmet on, keeping it level on your head. Adjust the rear straps, then the front straps, to get a "Y fitting" where the straps come together just under your ear. Then adjust the chin strap so it is comfortably snug. Now adjust the rear stabilizer if the helmet has one.
Are you done? Shake your head around. Place your palm under the front edge of the helmet, 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 down more than an inch? If so, tighten the neck strap. When you are done, your helmet should feel level, comfortable, and solid on your head. It should not bump on your glasses (if it does, tighten the neck strap). If your helmet still does not fit, you may need a different size, or you may need to keep adjusting your helmet with foam pads. On cold days, stocking caps can be worn under most helmets by removing the pads.
Helmet too far forward
Helmet too far back
Helmet correctly positioned and fastened
Photos from City of Fort Collins, Colorado website.
Cheaper helmets usually aren't much heavier than expensive ones, and most cyclists don't notice a difference. If you think you need an ultra-light helmet, test-ride a regular one first.
Helmets generally cost between $25 and $100. The extra money won't buy you much in the way of extra protection, though, so just pick a helmet that fits well.
If your helmet took an impact in a crash, you should discard it and buy a new helmet. Even if the old one looks fine, the foam's ability to absorb an impact may have been damaged. You should also replace your helmet every five years, since the protective foam can get brittle as it ages.