The medicare 20 employee rule decides one thing: whether your job’s health plan pays your medical bills first, or Medicare does. It turns on the size of the employer that sponsors the coverage. If that employer had 20 or more employees, the group health plan is the primary payer and Medicare pays second. If the employer is smaller, Medicare becomes primary the moment you are eligible — whether you enrolled or not. That second scenario is where people get hurt.
They keep working, skip Part B, and later find their claims were never fully covered. Understanding the medicare 20 employee rule before your 65th birthday is how you avoid that gap.
The Rule Behind Medicare 20 Employee Rule
The medicare 20 employee rule comes from the Medicare Secondary Payer statute. CMS calls this category “Working Aged.” It applies when you are 65 or older and covered by a group health plan through your own current employment or your spouse’s.
CMS states the threshold plainly. If the employer has 20 or more employees, the group health plan pays first and Medicare pays second. Confirm the payer order for your own situation on the CMS page below.
Medicare Secondary Payer — CMS
The counting is stricter than most people assume. Under the medicare 20 employee rule, the employer must have employed 20 or more people for each working day in each of 20 or more calendar weeks. That test looks at the current calendar year or the preceding one.
Full-time and part-time employees both count. Headcount is what matters, not how many people enrolled in the plan. An employer with 40 workers and 6 plan members still meets the medicare 20 employee rule.
Multi-employer plans have their own wrinkle. If your small employer participates in a multi-employer plan and any participating employer has 20 or more employees, the plan is generally primary for everyone age-entitled. Some small employers request a Small Employer Exception instead. Ask your benefits administrator which applies to you.
Small Employer Exception — CMS
Your Choices at 65
You have three broad paths. You can take Part A only and keep the employer plan. You can take Part A and Part B together. Or you can delay both and rely on the group plan alone.
Which path is safe depends entirely on employer size. Ask your HR department for the employer’s headcount in writing before you decide anything.
| Employer size | Who pays first | What to do |
|---|---|---|
| 20 or more employees (current employment) | Group health plan pays first; Medicare pays second | You may delay Part B without penalty. A Special Enrollment Period protects you later. Confirm the plan is creditable for drug coverage. |
| Fewer than 20 employees | Medicare pays first; group plan pays second | Enrolling in Part A and Part B at 65 is usually necessary. Without Part B, the plan may pay only what it owes as secondary. |
| Small employer in a multi-employer plan with a 20+ participant | Usually the group plan pays first | Get the plan administrator’s answer in writing. Do not assume based on your own employer’s size. |
| COBRA or retiree coverage at any employer size | Medicare pays first | This is not current employment. Enroll in Part B on time. Delaying here creates a penalty. |
Part A is premium-free for most people with enough work credits. Part B carries a premium. CMS set the 2026 standard Part B premium at $202.90 per month, with higher amounts for higher incomes.
2026 Medicare Parts A & B Premiums and Deductibles — CMS
The Penalty If You Get It Wrong
The Part B late enrollment penalty is 10 percent of the standard premium for each full 12-month period you could have had Part B and did not. It is added to your monthly premium.
Two full years late means a 20 percent penalty. Four full years means 40 percent. The penalty generally continues for as long as you have Part B — not for a fixed term.
The Part D penalty works differently. It is 1 percent of the national base beneficiary premium, times the number of full uncovered months without creditable drug coverage. The 2026 national base beneficiary premium is $38.99.
So 20 uncovered months is 20 percent of $38.99, rounded to the nearest ten cents. That is roughly $7.80 per month added to your drug plan premium. Because the base premium changes annually, your penalty amount can change too.
Avoid late enrollment penalties — Medicare.gov
Neither penalty is a one-time charge. Both are designed to last for life. This is the practical reason the medicare 20 employee rule matters so much at 65.
Medicare 20 Employee Rule: the Paperwork
Two forms do the work. Form CMS-40B is the Application for Enrollment in Medicare Part B. You sign that one yourself.
CMS-40B: Application for Enrollment in Medicare Part B — CMS
Form CMS-L564 is the Request for Employment Information. Your employer’s benefits or HR representative completes and signs Section B. You complete Section A. It proves you had group coverage based on current employment.
CMS-L564: Request for Employment Information — CMS
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Send both forms together to Social Security. You can mail, fax, or upload them; SSA’s Part B page lists the current submission options.
The deadline is the Special Enrollment Period. It runs while you are still covered by current employment and for 8 months after that coverage or the employment ends, whichever comes first. If your employer will not complete the CMS-L564, ask Social Security what alternative proof it will accept.
The medicare 20 employee rule is what makes that Special Enrollment Period available to you. Without qualifying current-employment coverage, there is no SEP to use. Do not wait until the eighth month to start collecting signatures.
The Traps People Fall Into
COBRA is the most common trap. COBRA is not coverage based on current employment. Medicare pays first, and COBRA months do not extend your Special Enrollment Period.
Retiree coverage is the same story. It pays second to Medicare regardless of how large the former employer is. The medicare 20 employee rule only reaches active employment.
HSA contributions are the next trap. The IRS says your HSA contribution limit is zero beginning with the first month you are enrolled in Medicare, including any retroactive months.
Publication 969, Health Savings Accounts — IRS
Retroactivity is the part people miss. If you apply for Part A more than six months after turning 65, coverage is backdated up to six months, but never before the month you turned 65. Contributions made during those backdated months become excess contributions.
That is why SSA advises stopping HSA contributions six months before you apply. Ask your tax preparer how Form 8889 handles any excess you already contributed.
The last trap is assuming small-employer coverage is enough. Under the medicare 20 employee rule, a fewer-than-20 employer’s plan pays second. Your claims may be reduced as if you had Part B, even when you do not.
Frequently Asked Questions
Does the medicare 20 employee rule count part-time workers?
Yes. CMS counts both full-time and part-time employees toward the threshold. It also counts employees who are not enrolled in the health plan at all.
My employer dropped below 20 employees this year. What changes?
The test looks at the current calendar year and the preceding one. Coverage may stay primary based on last year’s headcount. Ask your plan administrator to confirm your payer status in writing, then contact Social Security.
Do I still need Part D if my employer plan is primary?
Not necessarily. What matters is whether your drug coverage is creditable. Your plan must send you a creditable coverage notice each year — keep it, because it is your evidence against a Part D penalty.
Can a late enrollment penalty ever be removed?
Medicare has a reconsideration process, and there are equitable relief provisions in limited circumstances. No outcome is guaranteed. Contact Social Security at 1-800-772-1213 or your State Health Insurance Assistance Program to ask what applies to your case.
Key Takeaways: Medicare 20 Employee Rule
- Employer size decides everything. Whether medicare 20 employee rule is a choice or a requirement turns on the 20-employee rule.
Compare Your Options
Once you know what medicare 20 employee rule means for you, the official Plan Finder shows what is actually sold in your ZIP code, and a SHIP counselor will walk it with you for free — no commission, no sales call.
Official Sources & Resources
- Social Security — Medicare enrollment: https://www.ssa.gov/medicare
- IRS Publication 969 (HSAs): https://www.irs.gov/publications/p969
- Medicare.gov: https://www.medicare.gov
- CMS.gov: https://www.cms.gov
- Find your SHIP counselor: https://www.medicare.gov/talk-to-someone
Checked against the official sources above in September 2026. Rules and dollar figures change; if a notice you received disagrees with this page, the notice wins — and please tell us. General information, not legal, financial or medical advice.