There are a few things that are important to consider when buying a bicycle helmet:
Fit. The helmet should sit straight and firmly on the head and feel comfortable. It's supposed to cover the forehead, the back of your head and the top of your head.
Size. It is important that the helmet has a perfect fit, and it should not be able to fall off or move on your head. The size is obtained by measuring the circumference of your head where it is the widest.
Safety. Make sure your helmet meets the basic safety requirements. A CE mark on the helmet shows that the manufacturer has received a certificate that the helmet meets the safety standards required by the Swedish legislation.