Are butterfly knives legal in California?

California Penal Code 653K defines pocket knives related terms:

For the purposes of this section, "switchblade knife" means a knife having the appearance of a pocketknife and includes a spring-blade knife, snap-blade knife, gravity knife or any other similar type knife, the blade or blades of which are two or more inches in length and which can be released automatically by a flick of a button, pressure on the handle, flip of the wrist or other mechanical device, or is released by the weight of the blade or by any type of mechanism whatsoever.

Ballisongs or Butterfly knives fall under that category, they're the gravity knives for the purpose of the PC.

And then PC 12020 defines what is legal to carry and what not.

Knives that are legal for the street carry are most likely illegal in schools and public buildings. Anything over 2.5" is illegal.

Also, local towns and cities may have more restrictive laws compared to CA state laws.

Detailed analysis and quotes of the relevant PCs in the attached link.