You can easily determine the size of your helmet by measuring your head’s circumference and checking the following table:
Take care to measure from the middle of your forehead just above the temple area.
You must always try a helmet on before purchasing it.
A helmet which is too big, even if apparently more comfortable, does not offer the same grade of safety as a helmet that fits properly and snugly.
Different helmet models or even versions of the same model, may have a different fit which is the result of the continuing efforts made to improve comfort and safety.