Measure the circumference of your head at your temples and across your forehead. Generally speaking, an adult size small helmet fits from 51/52-56 cm, a medium fits 55/56-59/60cm, and a large fits 59/60-62/62 cm. Children's helmets tend to be model-specific, with a different model geared toward a different age/head size.

To know what size helmet you need, measure the circumference of your head around the temples as shown in the image below:


See our size charts here: https://www.specialized.com/size-guide