If you or your spouse work for a small business, Medicare works differently in one big way: who pays your medical bills first. People often mix this up with a second rule, about when you can sign up for Part B. They are separate, and the answers are different.
Two separate rules
| Question | Employer with 20 or more employees | Employer with fewer than 20 |
|---|---|---|
| Who pays first? | The job plan pays first, Medicare second | Medicare pays first, the job plan second |
| Can you use the Part B Special Enrollment Period later? | Yes | Yes |
| Are job-coverage months left out of the late penalty? | Yes | Yes |
Who pays first: the 20-employee rule
Medicare pays second to a job-based plan for people 65 and older only when the employer has at least 20 employees and the coverage comes from current work. A multi-employer plan also counts if at least one of the employers in it has 20 or more (42 CFR 411.172).
How employees are counted: an employer has 20 or more employees if it had 20 or more employees for each working day in each of 20 or more calendar weeks in the current calendar year or the year before (42 CFR 411.170). So a business that hovers around 20 may count as 20 or more based on last year.
Below that, Medicare.gov is clear: with coverage through an employer that has fewer than 20 employees, "Medicare pays first, and the... group health plan pays second."
There's a related protection that works only at the larger size. Social Security says that, by law, group health plans of employers with 20 or more employees have to offer workers 65 and older, and their spouses, the same health benefits as younger workers.
Why it matters: your plan might not pay
Check before you skip Part A and Part B
Medicare.gov tells people at small employers: "Because the company has less than 20 employees, your job-based coverage might not pay for health services if you don't have both Part A and Part B."
If your plan expects Medicare to pay first and you don't have Medicare, the plan might not pay for your care. Medicare.gov's advice is to ask the employer that provides your health insurance whether you need to sign up for Part A and Part B when you turn 65.
- If the plan says it needs Medicare to pay first, signing up during your Initial Enrollment Period around your 65th birthday is the safe route. Already past it? You can sign up any month while you're still covered through work.
- If the plan says it pays as usual without Medicare, you can keep it and use the Special Enrollment Period later.
Check your Part B Special Enrollment Period in 2 minutes. Start the free Part B check →
The Special Enrollment Period still works at any size
Here's the part many people miss. Social Security's manual says Congress decided that working people 65 and older with job-based coverage from employers of any size won't face enrollment limits or penalties for the months they had that coverage. "Thus, even though Medicare is secondary payer only when an individual is enrolled in a plan that covers 20 or more employees, an SEP and premium surcharge relief are possible for all those covered under a GHP of any size based on current employment" (POMS HI 00805.751).
The proof doesn't need to mention size, either. The employer's Section B on CMS-L564 just shows your coverage and work dates.
Example: Maria, 66, works at a 12-person dental office. Its plan told her it pays as usual without Medicare, so she kept it and skipped Part B. Her job and coverage end in October 2026. Her window runs November 2026 through June 2027, and the last day is June 30, 2027. Her months on the dental office's plan don't count toward a late penalty. See the Special Enrollment Period rules for the full details.
Self-employed or a very small business
Self-employed people have the same Special Enrollment Period rights as employees. Your plan counts as a group health plan if at least one employee besides you is eligible for it. A plan that isn't available to any employees, such as one only for self-employed people, is not a group health plan and doesn't give you a Special Enrollment Period (POMS HI 00805.290).
Medicare.gov's advice if you're self-employed, or your coverage isn't available to everyone at the company: ask your insurance provider whether it is employer group health plan coverage, as the IRS defines it. If it isn't, sign up for Medicare when you turn 65.
HSA warning
If your job plan is a high-deductible plan with a Health Savings Account (HSA), know these rules before you apply for any part of Medicare:
- The high-deductible plan from current work can qualify you for the Special Enrollment Period. The HSA itself is not a group health plan.
- You can't contribute to an HSA once Medicare Part A or Part B coverage begins (SSA Publication 05-10043).
- Premium-free Part A starts up to 6 months before the month you apply, but not before the first month you were eligible. So Social Security advises stopping HSA contributions at least 6 months before you apply. Medicare.gov says you and your employer should stop contributing 6 months before you retire or apply for Social Security benefits.
- You can still use money already in your HSA after you enroll, for deductibles, premiums, copayments or coinsurance.
Example: if you apply for Medicare in June 2027, Part A could start as early as December 2026, as long as you were 65 by then. HSA contributions would need to stop before that. For the tax details, Social Security points to IRS.gov.
If you already have Part A and are only adding Part B, see CMS-40B instructions. If you don't have Part A yet, see how to submit your forms.