# Dental coordination of benefits (COB)

> When a patient has two dental plans, how much does each plan pay and what does the patient owe? Traditional COB vs non-duplication, with worked examples.

URL: https://dentovio.com/dental-coordination-of-benefits

Last verified: 2026-08-30

Dentovio is an independent publisher, not an insurance carrier, benefits administrator, the ADA, or any government agency. This page was drafted with AI assistance and verified against the primary sources linked here. It has not been reviewed by a credentialed dental billing specialist or attorney. These pages describe how the two most common coordination-of-benefits methods and the birthday rule work in general terms; the plan documents and each state's adopted COB rules control the actual order and amounts, and a self-funded employer plan is generally governed by ERISA rather than those state rules. Educational reference only — not legal, benefits, or billing advice; confirm the order of benefits and the secondary plan's coordination method with each carrier before quoting a patient.

## Direct answer

When a patient has two dental plans, the primary plan pays first under its own terms, as if no other coverage existed. Under traditional coordination of benefits, the secondary then pays the balance of the allowed amount the primary left unpaid — up to what the secondary would have paid on its own — which often brings the patient share of the allowed charge to $0. Under a non-duplication clause, the secondary pays only the amount by which its own benefit exceeds the primary's payment, which is frequently $0 when both plans cover the service at the same percentage. Which plan is primary comes from the state's adopted version of the NAIC Coordination of Benefits Model Regulation; the non-duplication method itself is a plan-document term the model regulation does not define, so the plan document controls it — and a self-funded employer plan is generally governed by ERISA rather than the state's COB rules, though not categorically: a self-funded governmental or church plan is not an ERISA plan at all.

## The two modeled methods

- **Traditional (standard) COB** — the primary pays first under its own terms; the secondary pays the balance of the allowed amount the primary left unpaid, capped at what the secondary would have paid standing alone. Formula: secondary pays = min(secondary standalone benefit, allowed − primary paid).
- **Non-duplication** — the secondary pays only the amount by which its standalone benefit exceeds the primary's actual payment; if the primary paid as much or more, the secondary pays $0. Formula: secondary pays = max(0, secondary standalone benefit − primary paid).

A plan's standalone benefit is its coverage percentage applied to the allowed amount after its remaining deductible, capped by its remaining annual maximum. Maintenance-of-benefits and carve-out plans are not modeled here. Their reductions are defined by each plan document rather than a single published formula, so read the plan's COB provision — or ask the carrier which method it applies — instead of estimating.

## Worked examples ($200 fee, $160 allowed unless noted)

| Example | Method | Primary pays | Secondary pays | Patient | Demonstrates |
| --- | --- | --- | --- | --- | --- |
| Traditional, both plans at 80% | Traditional | $128 | $32 | $0 | On a $160 allowed amount, the primary pays $128 (80%) and traditional COB lets the secondary pick up the remaining $32. |
| Non-duplication, both plans at 80% | Non-duplication | $128 | $0 | $32 | Same claim, but the secondary's own benefit ($128) does not exceed what the primary paid ($128), so it pays $0 and the patient owes $32. |
| Non-duplication, richer secondary (50% / 80%) | Non-duplication | $80 | $48 | $32 | The secondary pays only its excess over the primary: $128 − $80 = $48. Non-duplication still leaves the 20% neither plan covers. |
| Traditional, $50 primary deductible left | Traditional | $88 | $72 | $0 | The primary pays 80% of $110 after its deductible ($88); traditional COB lets the secondary absorb the rest, deductible included. |
| Traditional, secondary annual maximum used up | Traditional | $128 | $0 | $32 | A secondary with $0 of annual maximum left pays nothing under either method — the patient owes whatever the primary left. |

## What varies by plan and state

- Which plan is primary comes from the state's adopted version of the NAIC model regulation. The non-duplication method is a plan-document term the model regulation does not define or endorse — it caps combined payment at 100% of the total allowable expense and leaves the method to the plan. Most states adopt some edition of the model, but editions and amendments differ, and this page deliberately states no per-state COB rule: which state adopted which edition was not verified against current state code and is outside what this cluster publishes.
- Self-funded employer plans are generally governed by ERISA, which supersedes state laws relating to an employee benefit plan and bars a state from deeming the plan an insurer (29 U.S.C. § 1144(a), (b)(2)(B)). Such a plan can use non-duplication or its own order-of-benefits rules even where the state's adopted COB rules say otherwise; the plan document controls. "Generally" is doing work: a self-funded governmental or church plan is not an ERISA plan at all, so state law can reach it.
- Both plans are treated here as recognizing one allowed amount. In practice each plan may apply its own allowable or fee schedule, which changes the arithmetic.

## Which plan is primary for a child?

The birthday rule — the parent whose birthday falls earlier in the calendar year holds the primary plan, month and day only. See the [birthday-rule page](https://dentovio.com/dental-insurance-birthday-rule/index.html.md) for the tie-break and the court-decree exception.

## Sources

- [NAIC Coordination of Benefits Model Regulation (MDL-120)](https://content.naic.org/sites/default/files/model-law-120.pdf) — Model order-of-benefit-determination rules most states adopt, including the birthday rule, the longer-coverage tie-break, and court-decree sequencing.
- [NAIC model laws index](https://content.naic.org/model-laws) — Where NAIC publishes current model text and state-adoption information. This cluster states no per-state COB rule, so no adoption row is published here.
- [29 U.S.C. § 1144 — ERISA preemption of state law](https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title29-section1144&num=0&edition=prelim) — Why a self-funded employer plan is generally governed by ERISA rather than a state's COB rules, and can follow its own coordination method and order of benefits. The section never uses the words self-funded; the rule comes from the courts reading its preemption, saving, and deemer clauses together.
- [ADA guidance on coordination of benefits](https://www.ada.org/resources/practice/dental-insurance/ada-guidance-on-coordination-of-benefits) — Practice-facing definitions of traditional COB, non-duplication, and the maintenance-of-benefits variants this calculator declines to model.

Research confidence: medium. The method mechanics above were read against the NAIC model regulation text and the ADA's practice guidance. Confidence stays medium because state adoption of the model was not verified against current state code — which is why no per-state COB rule appears anywhere on this page.

## Related

- [Dental insurance birthday rule](https://dentovio.com/dental-insurance-birthday-rule/index.html.md)
- [Dental billing hub](https://dentovio.com/dental-billing/index.html.md)
- [Dental claim denial codes](https://dentovio.com/dental-claim-denial-codes/index.html.md)
- [Payer documentation requirements](https://dentovio.com/dental-claim-documentation/index.html.md)
- [Dental prompt-pay laws by state](https://dentovio.com/dental-prompt-pay-laws/index.html.md)