If you are 65 or older and still covered by an employer health plan — either through your own job or a spouse's — you are in a situation Medicare calls a coordination of benefits scenario. It sounds bureaucratic, but the financial stakes are real. Depending on how your employer plan and Medicare interact, you could end up with excellent coverage at a reasonable cost, or you could find yourself stuck with bills that neither insurer wants to pay. Understanding the actual cost of staying on employer coverage, compared to transitioning to Medicare or Medicare Advantage, is one of the most consequential financial decisions you will make in your 60s.
The first thing to understand is that employer coverage does not automatically become secondary to Medicare just because you turn 65. The rules depend entirely on the size of your employer. If you work for — or are covered through — a company with 20 or more employees, your employer plan remains the primary payer and Medicare acts as secondary coverage. In that situation, keeping employer coverage alongside Medicare Part A, which is free for most people who paid Medicare taxes for at least 10 years, can make sense. Many people in this group delay enrolling in Part B to avoid the $185 monthly premium in 2025. However, if your employer has fewer than 20 employees, Medicare becomes the primary payer by law. If you have not enrolled in Part B in that scenario, your employer plan may pay as if Medicare had already covered its share — leaving you responsible for the difference. That gap can reach thousands of dollars per hospital stay, and your employer plan is under no legal obligation to make you whole.
The actual dollar cost of employer coverage varies enormously, but national averages tell a useful story. According to the KFF Employer Health Benefits Survey, the average total premium for employer-sponsored single coverage reached approximately $8,951 in 2023, with employees paying an average of $1,401 of that annually. For family coverage, total premiums averaged $23,968, with employees contributing around $6,575 per year. These figures do not include out-of-pocket costs like deductibles, copays, and coinsurance — which for employer plans averaged over $1,700 per year for single coverage. When you add it all up, a senior on employer single coverage may be spending $3,000 to $5,000 or more per year in premiums and cost-sharing before they see a single medical benefit. And that assumes the employer is still subsidizing a significant share of the premium. Retirees who continue coverage through COBRA pay the full unsubsidized premium — that $8,951 average — plus a 2% administrative fee, making COBRA one of the most expensive coverage options available to anyone over 65.
Now compare that to what Medicare Advantage offers. According to CMS.gov data, the average Medicare Advantage plan premium in 2025 is approximately $17 per month, and a significant share of plans available nationally carry a $0 monthly premium. CMS reported that for 2025, there are more than 5,000 Medicare Advantage plan options available across the country, with an average of 43 plans per county. Many of these plans bundle prescription drug coverage under Part D along with dental, vision, and hearing benefits that traditional employer plans often charge extra for or exclude entirely. For a healthy 67-year-old in a metropolitan area, switching from employer coverage to a $0-premium Medicare Advantage plan with a $3,500 annual out-of-pocket maximum may represent thousands of dollars in annual savings — though individual results depend heavily on your health needs, your doctors' network participation, and the specific plans available in your ZIP code.
That said, Medicare Advantage is not the right fit for everyone, and the comparison is not purely about premium dollars. Employer plans typically offer broader provider networks, especially for people who travel frequently or live in rural areas where Medicare Advantage networks can be thin. Employer coverage also tends to have more predictable cost-sharing structures for people managing complex, ongoing conditions. If you are receiving treatment at a specialized cancer center or seeing a subspecialist who does not participate in any local Medicare Advantage network, staying on employer coverage — or transitioning to Original Medicare with a Medigap supplement — may be the smarter clinical and financial choice even if the monthly premium is higher. Network adequacy is a real concern: CMS star rating data for 2025 shows that approximately 40% of Medicare Advantage enrollees are in plans rated 4 stars or higher, but that also means 60% are in plans rated below that threshold, where access and care coordination can vary considerably.
Medigap, also called Medicare Supplement Insurance, is the other major alternative worth understanding in this comparison. Medigap plans are sold by private insurers and wrap around Original Medicare Parts A and B to cover most or all of your cost-sharing. Plan G, currently the most comprehensive option available to new Medicare enrollees, typically costs between $100 and $200 per month depending on your age, gender, location, and the insurer. Plan G covers the Part A hospital deductible of $1,676 in 2025, all Part A coinsurance, and all Part B coinsurance after you pay the annual Part B deductible of $257 in 2025. For someone with frequent medical needs, Plan G can effectively cap annual out-of-pocket spending at just that $257 deductible — which compares very favorably to employer plan deductibles that often run $1,500 to $3,000 or higher. The tradeoff is that Medigap plans do not include drug coverage, so you would also need a standalone Part D plan, adding roughly $30 to $60 per month in most regions. Even so, the combined cost of Part B plus Plan G plus a Part D plan often comes in below what a retiree pays for COBRA or a retiree health plan premium.
One of the most important — and most misunderstood — aspects of leaving employer coverage is the enrollment window you receive. When your employer-sponsored coverage ends, whether because you retire, your spouse retires, or you lose eligibility for another reason, you receive a Special Enrollment Period that allows you to sign up for Medicare Part B without penalty. This SEP lasts 8 months from the date your coverage or employment ends, whichever comes first. Critically, COBRA coverage does not count as employer-sponsored coverage for SEP purposes. If you retire and elect COBRA instead of enrolling in Medicare, your 8-month SEP clock is still running from the day your active employment ended. Missing that window means you will face a Part B late enrollment penalty of 10% added to your premium for every 12-month period you were eligible but did not enroll, and that penalty is permanent. On a $185 monthly premium, a two-year delay adds roughly $37 per month — for the rest of your life. A three-year delay adds $55.50 per month. These are not trivial numbers over a 20-year retirement.
Once you have enrolled in Part B and your employer coverage has ended, you also have a guaranteed-issue right to purchase any Medigap plan sold in your state without medical underwriting. This is a one-time window that most people only receive at initial Medicare enrollment. If you miss it and try to buy Medigap later, insurers in most states can reject your application or charge higher premiums based on your health history. There are meaningful exceptions: a number of states have enacted birthday rules that give beneficiaries an annual 30-day window to switch Medigap plans without underwriting. Those states currently include California, Idaho, Illinois, Kentucky, Louisiana, Maine, Maryland, Missouri, Nevada, New Jersey, New York, Oklahoma, and Oregon. If you live in one of these states, you have more flexibility to switch Medigap plans in future years — but for everyone else, the guaranteed-issue window at initial enrollment is a one-time opportunity that should not be wasted by delaying a decision.
For people who are still working past 65 and genuinely satisfied with their employer coverage, there is no requirement to enroll in Medicare right away — as long as the employer has 20 or more employees and the coverage is considered creditable. You can delay both Part B and Part D enrollment without penalty while you have that qualifying employer coverage. But the moment that coverage ends, the clock starts. The practical guidance: contact your employer's HR department and your State Health Insurance Assistance Program counselor at least six months before you plan to retire. SHIP counselors provide free, unbiased Medicare counseling in every state and can help you model the actual cost comparison between your specific employer plan and the Medicare options available in your area. You can find your local SHIP counselor through Medicare.gov or by calling 1-800-MEDICARE (1-800-633-4227). There is no sales pitch involved — SHIP counselors are funded by the federal government specifically to help beneficiaries navigate these decisions without a conflict of interest.
Data Snapshot: According to CMS.gov enrollment data, more than 33 million Americans were enrolled in Medicare Advantage plans as of early 2025, representing roughly 54% of all Medicare beneficiaries — a dramatic increase from just 24% a decade ago. The shift reflects both the growing availability of $0-premium plans and the expansion of supplemental benefits that traditional Medicare does not cover. CMS also reports that the number of Medicare Advantage plans offering a $0 premium has grown substantially year over year, giving beneficiaries in most counties a genuine no-cost alternative to employer coverage that includes drug, dental, and vision benefits in a single plan.
The bottom line for any Medicare-eligible senior still on employer coverage is this: do not assume that what you have is automatically better or worse than Medicare. Run the actual numbers. Add up your annual premium contribution, your deductible, your average copays and coinsurance, and any premiums for dental or vision riders. Then compare that total to the cost of Medicare Part B at $185 per month in 2025 plus either a $0-premium Advantage plan or a Medigap Plan G with a Part D drug plan. In many cases, especially for retirees no longer receiving an employer premium subsidy, Medicare options come out significantly ahead on total annual cost. In others — particularly for people with complex conditions who need broad network access or who see specialists at academic medical centers — a robust Medigap plan may be worth the higher premium. The decision is personal, but it should always be made with real numbers, not assumptions, and ideally with the help of a SHIP counselor who knows the plans available in your specific county.