Secondary Insurance Payment Calculator

Coordination of benefits method

Take these figures from the primary payer’s remittance. If you are not sure which plan is primary, work that out first with the primary payer decision tool.

Primary payer — allowed amount (USD)

Primary payer — amount paid (USD)

Secondary — what it would have paid as primary (USD)

Billed charge (USD) — Medicare Secondary Payer only

Secondary allowed amount (USD) — Medicare Secondary Payer only

Secondary should pay

Patient owes

Enter the primary allowed amount, the primary payment, and what the secondary would have paid as primary.

What Coordination of Benefits Actually Decides

Coordination of benefits does not decide whether a second plan pays. It decides how much, and the answer depends entirely on which of three methods the secondary plan uses. The same claim, with identical numbers, can pay the full remaining balance under one method and nothing at all under another.

This is the single most misread line on a secondary remittance. A secondary that pays zero looks like a denial, gets worked like a denial, and generates an appeal that was never going to succeed — because the plan calculated correctly and simply owed nothing.

Before any of this applies, the order has to be right. If the primary and secondary are reversed, every number below is wrong and the claim needs reprocessing, not appealing. That is a CO-22 conversation, and the order itself is decided by the rules in the primary payer decision tool.

The Three Methods

Method	What the secondary pays	Typical result
Standard COB (lesser-of)	The lesser of its own obligation or the balance the primary left	Patient often owes nothing
Non-duplication	What it would have paid as primary, minus what the primary actually paid	Frequently pays $0
Medicare Secondary Payer	The lowest of three separate tests	Usually the remaining balance

Standard COB is the method most people assume is in force. The secondary looks at what the patient still owes, compares it to what it would itself have paid, and pays the smaller of the two. On a claim where the primary allowed $200 and paid $160, leaving $40, a secondary that would have paid $150 as primary pays the $40. The patient owes nothing.

Non-Duplication: The One That Pays Zero

Non-duplication changes the arithmetic completely. Instead of looking at the balance, the secondary looks at itself: what would I have paid as primary, and has the primary already paid that much?

Take the same claim. Primary allowed $200 and paid $160. The secondary would have paid $150 as primary. Under non-duplication it calculates $150 minus $160, gets a negative number, and pays nothing. The patient still owes the $40.

Nothing went wrong. The plan's position is that the patient has already received benefits equal to or better than what this plan promised, so it owes nothing further. It is common in self-funded employer plans, and it is the reason a patient with two insurance cards can still receive a bill.

Non-duplication does pay when the primary underperforms. If that same primary had allowed $200 but paid only $120, the secondary would pay $150 minus $120, or $30 — leaving the patient $50 of the original $80.

The Medicare Secondary Payer Formula

When Medicare is the secondary payer it does not use either commercial method. It runs three tests and pays the lowest result:

The billed charge minus the primary payment.

What Medicare would have paid had it been primary — its allowed amount less the patient's deductible and coinsurance.

The higher of the Medicare allowed amount or the primary allowed amount, minus the primary payment.

Test three is usually the binding one, and that is why Medicare as secondary so often pays exactly the balance the primary left. Work an example: billed $500, primary allowed $300 and paid $240, Medicare allowed $280 and would have paid $224 as primary. Test one gives $260. Test two gives $224. Test three gives $300 minus $240, or $60. Medicare pays $60 — the balance — and the patient owes nothing.

The formula produces a result that surprises people in the opposite direction too. When the primary allows less than Medicare would have, test two can bind and Medicare pays more than the patient owed, because its obligation is measured against its own fee schedule rather than the primary's.

What the Patient Actually Owes

After both payers have processed, the patient owes the portion of the primary's patient responsibility that neither plan covered. The contractual adjustment — the gap between what you billed and what the primary allowed — is never billable to the patient and never becomes their problem, regardless of what the secondary does.

That distinction is where balance billing complaints come from. A practice that treats a zero-paying secondary as a reason to bill the full charge is billing an amount the contract already wrote off. Only the deductible, coinsurance and copay the primary assigned can travel to the patient, and only up to the allowed amount. To break that figure down for a patient, use the patient responsibility calculator.

Where Secondary Claims Actually Go Wrong

The primary's adjudication was not transmitted. A secondary claim needs the primary's allowed amount, payment and adjustment reasons carried in the COB loop. Without them the secondary has nothing to coordinate against and rejects the claim — frequently as CO-23. The mechanics of assembling that claim are covered in how to bill secondary insurance claims.

The balance was calculated from the billed charge. The secondary coordinates against the primary's allowed amount, not what you billed. Starting from the charge inflates the expected payment and produces phantom underpayments.

A correct zero payment was appealed. Non-duplication zeros are not denials. Time spent appealing them is time not spent on claims that would actually pay.

A genuine underpayment was assumed to be COB. The opposite error. If the secondary paid less than its own method requires, that is recoverable — check the arithmetic against your contract with the underpayment calculator.

Frequently Asked Questions

How much should secondary insurance pay?

Under standard coordination of benefits the secondary pays the lesser of two numbers: what it would have paid had it been primary, or the balance the primary left the patient. It never pays more than the patient still owes, and it never pays more than its own obligation on that service.

Why did my secondary insurance pay nothing?

Almost always because the plan uses non-duplication coordination of benefits. Under that method the secondary pays only the difference between what it would have paid as primary and what the primary actually paid. If the primary already paid as much as or more than the secondary would have, the secondary owes nothing and the entire remaining balance falls to the patient. It is not a denial and there is nothing to appeal.

What is non-duplication coordination of benefits?

A COB method that subtracts the primary payment from what the secondary would have paid as primary, rather than paying the leftover patient balance. It is common in self-funded employer plans. The same claim that pays in full under standard COB can pay zero under non-duplication, which is why knowing which method the plan uses matters more than the coverage itself.

How does Medicare calculate payment as a secondary payer?

Medicare pays the lowest of three amounts: the billed charge minus the primary payment; what Medicare would have paid had it been primary; or the higher of the Medicare allowed amount and the primary allowed amount, minus the primary payment. In most cases the third test is the binding one, which is why Medicare as secondary often pays exactly the balance the primary left.

Can I bill the patient after the secondary pays?

Only for what is left of the patient responsibility the primary assigned, and only up to the allowed amount. The contractual adjustment is never billable to the patient. If the secondary pays nothing under non-duplication, the deductible or coinsurance the primary assigned is still the patient's.

Does secondary insurance cover the primary deductible?

Usually yes under standard COB, because the deductible is part of the balance the primary left. Under non-duplication it often does not, because the calculation ignores the balance and looks only at what the secondary would have paid on its own.