While TurboTaxDwayneM's answer may be accurate for most cases, I have to add an important consideration to amend that answer. I have bolded an important phrase below in the information from the California state. There are circumstances in which your SDI is taxable.
"SDI benefits are taxable only if paid as a substitute for unemployment insurance (UI) benefits. This could occur if a person was receiving UI benefits and then became disabled. When SDI benefits are received as a substitute for UI benefits, the SDI is taxable by the federal government but is not taxable by the State of California."
