When I bought my Bell RS1, I bought at a store where I could try it on. I bought a large, even though a medium fit, because it was more comfortable, and the med. felt too snug. In retrospect, large was too loud because there were gaps in the padding. Purchasing these $20 replacement pads in the med size, I was able to change my helmet size back to a med, literally! Bell uses the same shell for both. Check the charts and tune your fit.