How Medicare Works with Employer Insurance
Employer size matters. Here's how to coordinate.
20 or more employees
If your (or your spouse's) employer has 20+ employees and you're actively working, the employer plan is primary and Medicare is secondary. You can usually delay Part B without penalty and enroll later during a Special Enrollment Period when the job or coverage ends.
Under 20 employees
Medicare is primary. That means the employer plan expects Medicare to pay first — and if you haven't enrolled, you could be responsible for the amount Medicare would have paid. In these situations, most people enroll in Parts A and B on time even if they keep the employer plan.
HSA warning
You can't contribute to a Health Savings Account once you enroll in any part of Medicare, including premium-free Part A. If maximizing HSA contributions matters to you, coordinate the timing carefully — and remember that if you take Social Security at or after 65, you're auto-enrolled in Part A with retroactive coverage up to 6 months.
Retiree coverage is different
Retiree health plans are not "active employer coverage." They do not protect you from Part B late-enrollment penalties and they do not trigger a Special Enrollment Period. If you're offered retiree coverage, treat it as a supplement to Medicare, not a replacement.
COBRA is different too
COBRA lets you continue employer coverage after you leave a job — but it doesn't count as creditable coverage for Part B, and it doesn't extend your Special Enrollment Period beyond 8 months after the employer coverage ends.
Get everything in writing
Ask HR for a written confirmation that your coverage is creditable for both Part B and Part D. Save these letters. If you ever need to appeal a late-enrollment penalty, they're the evidence you'll rely on.