California Code, Civil Code - CIV § 3287

(a) A person who is entitled to recover damages certain, or capable of being made certain by calculation, and the right to recover which is vested in the person upon a particular day, is entitled also to recover interest thereon from that day, except when the debtor is prevented by law, or by the act of the creditor from paying the debt. This section is applicable to recovery of damages and interest from any debtor, including the state or any county, city, city and county, municipal corporation, public district, public agency, or any political subdivision of the state.

(b) Every person who is entitled under any judgment to receive damages based upon a cause of action in contract where the claim was unliquidated, may also recover interest thereon from a date prior to the entry of judgment as the court may, in its discretion, fix, but in no event earlier than the date the action was filed.

(c) Unless another statute provides a different interest rate, in a tax or fee claim against a public entity that results in a judgment against the public entity, interest shall accrue at a rate equal to the weekly average one year constant maturity United States Treasury yield, but shall not exceed 7 percent per annum. That rate shall control until the judgment becomes enforceable under Section 965.5 or 970.1 of the Government Code, at which time interest shall accrue at an annual rate equal to the weekly average one year constant maturity United States Treasury yield at the time of the judgment plus 2 percent, but shall not exceed 7 percent per annum.