If you are replacing the center pad simply to freshen up your helmet choose the standard size for your helmet. ie: Small Helmet = S9, Medium Helmet = M9 etc... When replacing the center pad to fine tune the helmets fit choose a thicker pad to tighten the fit and a thinner pad to loosen the fit. ie: Install a S13 in a Small Helmet to tighten the fit and a S5 to loosen the fit. Purchase the same size pad as the helmet size it will be installed in.