When Is Medicare Primary vs. Secondary to Employer Insurance?

"Primary" and "secondary" sound like insurance jargon, but they decide which plan pays first — and that decides how much money stays in your pocket. Get it wrong and a bill you thought was covered can land on you.

Here's how Medicare decides whether it pays first or second when you also have employer coverage.

The 20-employee rule

Medicare's rule is simple on the surface: if your employer has 20 or more employees, the employer plan pays first and Medicare pays second. If your employer has fewer than 20, Medicare pays first and the employer plan pays second.

That rule applies to current employees and their spouses who are covered by the employer plan. Retiree coverage, COBRA, and severance plans usually flip the order — Medicare becomes primary.

What "primary" actually means

The primary plan pays as if it's the only coverage you have. The secondary plan may cover some or all of what the primary plan didn't pay, depending on its coordination-of-benefits rules.

Important: secondary coverage is not guaranteed to pay everything the primary plan leaves behind. It only pays up to its own benefit limits.

Large employer (20+): employer plan is primary

• You can usually delay Part B without a late penalty.
• Your group plan processes claims first.
• Medicare Part A is still usually worth taking at 65 because it's premium-free for most people.
• If you keep working, you get an 8-month Special Enrollment Period for Part B when employment or coverage ends.

Small employer (under 20): Medicare is primary

• You generally need to enroll in both Part A and Part B at 65.
• If you skip Part B, the employer plan can pay as if Medicare had already paid its share — leaving you with the rest.
• Ask HR to confirm employer size in writing before your Initial Enrollment Period ends.

Common exceptions

• Retiree coverage: Medicare is primary, regardless of employer size.
• COBRA: Medicare is primary; COBRA is not creditable for delaying Part B.
• Disability under 65 with employer coverage: different rules apply based on employer size and Medicare disability status.
• TRICARE or VA: these programs have their own primary/secondary rules.

How to protect yourself

Ask your benefits administrator for a written confirmation of employer size and a copy of the plan's coordination-of-benefits policy. Then confirm the same details with Medicare or a licensed broker before your enrollment window closes.

Educational only. Coordination-of-benefits rules can vary by plan and state; confirm your specific situation with your employer and a licensed agent.