Paddle sports helmets are designed to protect against impact with rocks and other hard objects to avoid head injuries. They may look similar to helmets sold for rock climibing, skiing, windsuring, or other sports, but there are nuances in the design of these helmets that are specific to the types of impact that can occur in whitewater settings.
Adjustable helmets (NRS Havoc Livery) are designed to fit a wide range of riders, so it is usually not necessary to take a head measurement before buying one. Helmets that are not adjustable (WSRI) in size require an accurate head circumference measurement, which is taken from the center of the forehead and around the head about 1.5 cm above the ears, which is normally the widest part of your head. Take note of this measurement and refer to the sizing chart for each helmet model for size selection.