Few people would choose to ride in a car with no seat belts. So why hop on a bike without a bike helmet? Helmets simply make sense in all riding conditions, particularly since at least 21 states carry laws requiring them.

Helmets have grown more style-conscious, typically provide superb ventilation and in most cases weigh less than the omelet you had for breakfast—all of which makes it tough for anyone to dream up an excuse for not wearing one. Here are some tips for choosing a model well-suited to your needs.

Find the Right Size

A good fit is vital. Sport helmets many times simply offer a single, adjustable size. In other categories, helmets come in small, medium, large or extended sizes.

To determine your size, wrap a flexible tape measure around the largest portion of your head—about 1 inch above your eyebrows. Or, wrap a string or ribbon around your head, then measure the length of string with a straight-edge ruler or yardstick. Look for a helmet that matches your measurement.

How can you tell if you are a small, medium or large? On the product page for each helmet offered on REI.com, the size range is listed at the bottom of the "Description" section.

Manufacturers sometimes vary in their interpretation of what dimensions are encompassed by the terms small, medium and large. Your best guide: Use the numbers shown on the "Description" tab for individual products on REI.com. We offer some general parameters for adults below.

• Small: 20"-21.75" (51cm-55cm)
• Medium: 21.75"-23.25" (55cm-59cm)
• Large: 23.25"-24.75" (59cm-63cm)
• Extra-small, extra-large: Below 20" (51cm), above 24.75 (63cm)
• One size fits all (men): 21.25"-24" (54cm-61cm)
• One size fits all (women): 19.75"-22.5" (50cm-57cm)

Most children's helmets are one-size-fits-all variety with a range of 18"-22.5" (46cm-57cm).

What if you are right on the border between sizes? The general advice is to choose the smaller size.

Adjust the Fit

Whatever your size, simple adjustments can customize the fit. Nearly all helmets offer an internal universal-fit sizing ring (one size fits all) and adjustable straps. Occasionally some helmets, including some kids' helmets, still offer a selection of fit pads to accomplish this task.

To adjust the fit, first expand the sizing ring before you place a helmet on your head. Once the helmet is in place, reach behind your head and tighten the ring (by pushing a slider or twisting a dial) until you achieve a snug fit.

Right: Helmet is level

A good-fitting helmet should be snug but not annoyingly tight. It should sit level on your head (not tilted back) with the front edge no more than 1 inch above your eyebrows. Your forehead should be protected. Push the helmet from side to side and back to front. If the helmet shifts noticeably, adjust the sizing ring (or pads) to snug the fit.

Wrong: Helmet is tilted back

Next, buckle and tighten the chinstrap. Push up on the front edge of the helmet, then up on the back edge. If the helmet moves significantly in either direction, tighten the chinstrap and try again. Adjust the straps around both ears to achieve a comfortable fit. Finally, with the chinstrap buckled, open your mouth. If the helmet doesn't press against your forehead as you do so, tighten further and repeat. Just don't tighten the strap excessively and create discomfort.

When to Replace a Helmet

Any time your helmet is involved in an accident, it's likely to get damaged. Since damage isn't always easy to spot visually, replace the helmet after any significant impact, even if everything "looks" OK. It is generally recommended that a helmet be replaced after 5 years, even if it has remained crash-free. Pollution, UV light and weathering can potentially weaken its components over time.

A few helmet tips:

• If your helmet includes an owner's manual, read it before your first ride.
• Avoid using chemical solvents to clean a helmet. Manufacturers recommend only the use of a soft cloth or sponge, plus mild soap and water.
• Do not store a helmet in an attic, garage, car trunk or other area where heat can accumulate. Excessive heat may cause bubbles to form on helmet parts. Do not wear a heat-damaged helmet.
• Avoid loaning your helmet to others. As a savvy rider, you want to know exactly what kind of treatment your helmet has experienced during its entire lifespan so you can knowledgeably assess its integrity over time.

Which Type? Sport, Road or Mountain?

Cycling helmets come in 3 basic styles—sport, road and mountain bike helmets. All types are designed to protect riders from impact while being light, comfortable and stylish.

• Sport helmets ($35-$55): Economical models that work well for commuter, road and mountain bikers as well as skateboarders and inline skaters.
• Road bike helmets ($60-$230): Their appeal lies in their low weight, generous ventilation and aerodynamic design.
• Mountain bike helmets ($50-$130): Built to offer maximum protection from trailside obstacles. Typically these helmets include visors and more extensive rear-head coverage. Special retention systems provide a firm and secure fit on rough terrain. Cyclocross riders often seek out these helmets.