The MET Crackerjack is simply the most popular of all our youth helmets developed with the very same rigorous testing and specifications as our adult line of products. Young riders deserve the best not only in safety but exceptional design and a style they'll love to be seen in. One of the MET Crackerjack's greatest features is the adjustable sizing. The helmet can be increased in size from 52 to 57cm ensuring the helmet will grow along with the child. When buying a children's bike helmet the same guidelines apply as if you were buying a helmet for yourself - it needs to be comfortable durable and the correct size You should also ensure that you choose a helmet colour/design that your child will love to ensure that they'll want to wear it every time they jump on their bike. SIZING You can determine your size by measuring the circumference of your head approximately 3 cm above your eyebrows.