Measuring for motorcycle helmet size is as simple as looking for a good fitted hat. The difference is in how the helmet fits over the rider's head. The best tool for this is a seamstress or tailor's tape measure. It is flexible enough to wrap around the rider's skull and marked in useful increments for determining an accurate size. Use it to find which size of helmet suits you:
Wrap the measuring tape around the fullest part of the head - this will be just above the ears and about a half-inch above the eyebrows for most - and take the measurement at the forehead. To get the most accurate measure, have a friend help with this step. Take the measurement and compare to the size chart on the page for the desired motorcycle helmet (you might have to scroll down). Each manufacturer has sizing which is specific to its own models, so only rely on the size chart produced for the motorcycle helmet being considered.
Some variance occurs, even between different models from the same manufacturer, which is why Red Ash Designs offers a "Free First Exchange" program if your helmet does not fit properly.
When a new helmet is first worn, it should actually be slightly tight, with the interior coming into contact with most of the head, but not so restrictive that it causes any pain. There should be no "hot spots" - places where the helmet's interior puts pressure on specific points of the skull or face - but it should not move around freely. With time, a helmet adjusts to match the shape of the rider's head as it is worn, "breaks-in" and loosens a little. However, it should never become loose enough to easily turn from side to side.
Put the helmet on - it should be a little tight as it goes on over the head.
The helmet should sit on the head evenly with the eye port's upper edge just above the eyebrows and offer good peripheral vision to see side-to-side.
Put a finger between the helmet interior and the head. If it easily fits, try a smaller size. Some helmet models allow a change in the cheek pads for better fitment, so consider this when checking for proper sizing.
Finally, when deciding on a helmet style - it's mostly up to personal choice for color, patterns and shield tint but remember the brighter and easier to see a helmet the more likely drivers spot a rider. The face shield should not obscure the rider's vision in any way and tinted visors should only be used for sunny days.
A well-fitting motorcycle helmet contributes to a safer and more comfortable ride. If the helmet does not fit right, it can cause pain which often leads to dangerous distractions, and it may not completely protect the rider's head in a crash. Therefore, finding a motorcycle helmet which fits the rider correctly is very important to the helmet's twin missions of comfort and safety. Spend enough time finding the right helmet and you will find that the helmet almost disappears while wearing and yet it is right where it needs to be should the ride go wrong. Be smart. Be safe. Ride with a correctly sized motorcycle helmet and enjoy the road for years to come.