COB Secondary Claim Calculator
When a patient has two insurance plans, how much should the secondary plan actually pay? Run the standard coordination-of-benefits math and see every step.
Secondary claims are where coordination of benefits (COB) stops being a definition and starts being arithmetic. Under the common maintenance-of-benefits ("lesser-of") method, the secondary plan first works out what it would have paid on its own, then pays no more than the balance the primary plan left behind — so the two plans together never pay more than the primary plan's allowable. Enter the primary EOB figures and the secondary plan's terms to see the calculation step by step.
Calculate the secondary payment
How coordination of benefits decides who pays first
Before any math, COB rules decide the order of the plans. The most common rules: a plan covering the patient as an employee pays before a plan covering them as a dependent; an active-employee plan pays before a retiree or COBRA plan; and for dependent children of parents who are divorced or separated, the plan of the parent with custody (or the custodial parent's spouse) generally pays first.
For dependent children covered under both parents' plans, the birthday rule decides: the plan of the parent whose birthday (month and day only — the year does not matter) falls earlier in the calendar year is primary. If both parents share the same birthday, the plan that covered a parent longer is primary. The birthday rule applies when the parents are together or when no court order assigns responsibility; a court decree or custody arrangement can override it.
One warning about the math above: not every plan uses the maintenance-of-benefits method shown here. Some plans use a non-duplication clause (the secondary pays only the excess of its normal benefit over the primary payment) or "carve-out" variants, and Medicare secondary-payer calculations follow their own statutory formula. The result here is the standard estimate — the secondary plan's EOB is the final word. Secondary claims also carry their own timely-filing clocks, which usually run from the primary EOB date; our payment posting team works COB balances daily so secondary claims go out with the primary EOB attached, the first time.
Disclaimer: This calculator is educational only. It is not medical, legal, or billing advice for a specific case, and it does not guarantee coverage or payment. COB methods vary by plan document and, for Medicare, by statute; verify the secondary plan's coordination method before relying on an estimate.
Frequently asked questions
What is the birthday rule in coordination of benefits?
For a dependent child covered by both parents' plans, the plan of the parent whose birthday (month and day, ignoring the year) falls earlier in the calendar year is primary. If both parents have the same birthday, the plan that has covered its subscriber longer is primary. Court orders and custody arrangements can change the order.
Why did the secondary plan pay less than its normal benefit?
Under the maintenance-of-benefits (lesser-of) method, the secondary payment is capped at the balance remaining after the primary plan's allowable and payment. The two plans combined never pay more than the primary allowable, so when the primary already paid most of the allowable, the secondary pays only the small remainder — sometimes zero.
What must be attached to a secondary claim?
The primary plan's explanation of benefits (EOB) or remittance advice. Secondary payers need the primary allowed amount, paid amount, and adjustment/denial reason codes to run their COB calculation; claims sent without the primary EOB are routinely denied or pended.
Is the COB calculation the same for Medicare as secondary payer?
No. Medicare's secondary-payer calculation is set by statute and regulation and does not simply mirror commercial maintenance-of-benefits math. Use this calculator for commercial-to-commercial coordination, and work Medicare secondary claims under the Medicare Secondary Payer rules.