To measure the circumference of your head, use a cloth measuring tape (or, as REI experts recommend, use a piece of string and measure that), wrapping the device around the widest part of your head. Then you can reference the helmet manufacturer’s sizing chart to find the one that fits. Sizing is broken out in ranges, from extra-small up to extra-large, though some brands only offer helmets in small, medium, and large. And a few brands provide a one-size-fits-all approach, particularly for toddlers and kids, and use a flexible dial or an elastic fit system to assure a custom-like fit. And a few helmets provide foam pads of different thickness, which you adjust to get the perfect fit.  When stuck between sizes, REI recommends that you go with the smaller size. And if you plan on cold-weather cycling, be sure that the helmet can adjust to the thickness of extra layering (skull caps, headbands, etc). Dixon says that “your helmet should feel snug but not tight, and should stay put if you shake your head up and down and back and forth.” And you should be able to fit two fingers between the buckled chin strap and your chin.