This guide is for general information only and is not legal advice. Laws change — verify current rules with your state DOT or a licensed attorney before relying on this for any legal matter. Read full disclaimer.
The rule
California requires a bicycle helmet for every rider under 18 — in practice, every rider except adults aged 18 and over. The same rule extends to children carried as passengers in a child seat or trailer. Fine of up to $25 for a first offense; courts often waive or reduce the fine on proof of helmet purchase and completion of a bicycle-safety course. See Cal. Veh. Code § 21212.
If you're 18 or older, California doesn't require you to wear a helmet by state law. Riders under 18 (and any child carried as a passenger) must wear a helmet that meets the CPSC standard.
Cities can be stricter than the state floor, and bike-share systems often require helmets regardless of age. If you're renting or commuting in a major city, confirm the local rule before you ride.