Many people like to measure the circumference of their head and buy accordingly. My personal recommendation is to try on several to properly fit a helmet. The tape measure can be a bit decieving, sort of like other clothes. It makes sense to try on the clothes before you buy them, because the sizes are a bit different despite using the same measurements.
The helmet should be snug on the head when it fits properly, with very little wiggle room.. Be sure to attach the straps while trying them out for both comfort and ease. When the helmet is fully strapped on, try to move it back and forth without moving your head. If properly fit, your helmet will allow for almost no movement. When it’s fairly snug, it’s best.
If buying a helmet for a youngster it is very tempting to buy a size too big to allow room for growth, even though it may not properly fit. While this makes perfect sense when buying a kid a pair of pants or a shirt, it doesn’t with important safety equipment. A loose helmet can actually be more dangerous, and is not a proper fit.
It is strongly recommended that you buy a DOT approved helmet in addition to making sure it’s properly fit. These helmets must meet certain safety regulations. A bit more spendy yes, but you only have one brain. This is the single most important piece of safety equipment you can use for motorsports, so don’t be too cheap. A properly fit helmet can save your life.
If you have a kids four wheeler, you must put them in a helmet. Saftey first!