Can I collect interest on back child support in CA, if so how is it calculated

If I understand your question, you also need to know how payments are applied to child support arrears and the interest that has accrued on those arrears. Interest on any overdue installment accrues at the rate of 10% per year, or 0.83% per month. When payments are made, they are credited as follows, per California Code of Civil Procedure 695.221: "Satisfaction of a money judgment for support shall be credited as follows: (a) The money shall first be credited against the current month's support. (b) Any remaining money is next to be credited against the accrued interest that remains unsatisfied. (c) Any remaining money shall be credited against the principal amount of the judgment remaining unsatisfied. If the judgment is payable in installments, the remaining money shall be credited against the matured installments in the order in which they matured." So, any payment gets applied to the current month's support, if any is still due, THEN to the accrued interest on overdue support, and ONLY gets applied to the underlying overdue principal AFTER any accrued interest has been paid in full.