When choosing a bike helmet, a good fit is vital. Most helmets come in small,
medium, large or extended sizes. To find your size, wrap a flexible tape measure around the largest portion of your head—about 2.5cm. above your eyebrows. Or, wrap a string or ribbon around your head, then measure the length of string with a straight-edge ruler or tape messure. Look for a helmet size that matches your measurement. The size range is listed under the "Specs" tab on each product page.
General sizing parameters for the ZUMA:
Small / Medium: (54cm–58cm) 230grams
Between sizes? Either opt for the smaller size or wear a cycling cap or beanie to improve the fit of the larger helmet. .
Helmet sizing and fitting instruction can be found here - HELMET SIZING