When choosing your new helmet, you must check the circumference of your head. Grab a tape measure and place it around your head at about 2 cm above your brow ridge, measure the widest area of the head by wrapping the tape in line with the temples, then on the rear of the skull and again towards the forehead.
Take note of the precise measure (without rounding up or down) and compare it with the size chart below.

If in doubt, for example because your measure is in-between sizes, try on the helmet of the smaller size. You should do this because a helmet starts adjusting to the configuration of your face after wearing it for just a few hours. As a consequence, a helmet that feels slightly loose when you first wear it may prove to be too large after just a few weeks of use, thus compromising your safety.
Consider also the possibility of swapping the inner-lining for another in a smaller or larger size, where possible! This would allow you to perfectly customise your helmet for a perfect fit!