All helmet manufacturers orientations are that a bicycle helmet only offers sufficient protection if it fits properly. It is therefore essential to check if, once secured, you feel protected and comfortable by simulating head rotations. The helmet should not hurt anywhere and the strap around the neck should not prevent from any movement.
Every head is different, but most helmets have built-in adjustment systems and removable interior reinforcements.
To find out which size is best, measure your head circumference at the level of your forehead, about 2.5 cm above your eyebrows and ears. You can use a flexible tape measure (a shoelace, a cell phone cable or a cord will do as well) and refer to the average sizes that we find in our partner brands.