Ask an owed parent what they’re owed and they’ll quote the principal. Ask the ledger and the answer is usually dramatically larger — because California support arrears accrue 10% simple annual interest under CCP §685.010, installment by installment, from each payment’s due date.
How the math actually runs. Each monthly payment that went unpaid is treated as its own judgment. A missed $800 payment from January 2016 has been accruing $80/year for a decade — it’s a $1,600 obligation now, by itself. Stack 60 or 120 missed months, each with its own accrual start, and a $40,000 principal balance routinely audits out at $65,000–$80,000. Payments that were made get applied under the statutory order — generally to current support first, then accrued interest, then principal (see Family Code §4373 and related rules) — which means partial payers often barely touched principal for years.
Getting the number certified. If the local child support agency (LCSA) is on the case, request an official audit and arrears statement — California Child Support Services performs these at no cost and its accounting is presumptively reliable in court. In private cases, the owed parent files for a determination of arrears in the family law case, supported by a payment-history declaration and an interest calculation worksheet; the court’s order then fixes the figure for enforcement.
Why fixing the number matters: every enforcement tool — wage assignment for arrears, bank levy, real property lien, license action, tax intercept — runs on a stated amount. A judicially determined arrears figure ends the “I paid her cash sometimes” defense, because §3651(c) bars retroactive modification and the burden of proving undocumented payments sits on the payor.
The negotiation implication: owed parents are routinely offered lump-sum settlements against principal, priced as if interest didn’t exist. Run the audit first. A $25,000 offer against a $40,000 principal sounds like 62 cents on the dollar; against the true $70,000 balance it’s 35 cents. You cannot negotiate a number you’ve never calculated — and in this corner of the law, the calculation is always in your favor.
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