How to Coordinate Medicare and an Employer Health Plan After 65 in 2026: Primary/Secondary Rules, When to Delay Part B, and the Traps That Cost the Most

Roughly one in four Americans age 65 to 69 is still working, and the number is climbing. If that's you — or you're planning it — the question of what to do about Medicare is genuinely complicated. Auto-enroll is not a thing for most people turning 65 in 2026 (only Social Security recipients get pulled in automatically), and the wrong choice quietly locks in a 10%-per-year premium hike that you carry to the grave.

The rules break cleanly along a single fault line: whether your employer has 20 or more employees. Everything downstream — whether you take Part A, whether you delay Part B, how the 8-month Special Enrollment Period works after you retire, and whether IRMAA hits you — flows from that one number. Here's the 2026 decision framework, with the specific traps that cost people the most.

Before the strategy, the sticker prices. These are the numbers that get plugged into every scenario below:

• Part A premium (2026): $0 for most workers (you or your spouse have 40 quarters of Medicare-taxed work). If you don't qualify for premium-free Part A, the premium runs up to $518 per month.
• Part B standard premium (2026): $202.90 per month.
• Part B IRMAA thresholds (2026): Surcharges start at $109,000 modified AGI for single filers and $218,000 for married filing jointly. The IRMAA lookback uses your 2024 tax return. Total Part B premium with IRMAA runs $284.10 to $689.90 per month across five income brackets.
• Part D IRMAA surcharges (2026): Between $14.50 and $91.00 per month on top of your Part D plan premium.
• Late enrollment penalty: 10% permanent premium hike per full 12 months you delayed Part B without qualifying employer coverage.
• Special Enrollment Period after employment ends: 8 months to enroll in Part B without penalty.
• Employer size threshold: 20 employees, measured over 20 weeks in current or prior year.

Federal coordination-of-benefits rules assign primary vs. secondary payer status based on employer size. This matters more than any other single fact.

Part A is free for the vast majority of workers turning 65, since 40 quarters (10 years) of Medicare-covered work qualifies you and your spouse for premium-free hospital insurance. Because it's free and it sits behind your employer coverage as backup, most people should take it as soon as they're eligible.

The one situation where you should not take Part A: you're contributing to a Health Savings Account. Enrollment in any part of Medicare — including premium-free Part A — disqualifies you from HSA contributions from that month forward. And here's the sting: if you sign up for Social Security after 65, Part A is backdated up to 6 months. Contributions you made to the HSA in those retroactive months become excess and are subject to tax and penalty.

Part B is the expensive one — $202.90 per month standard, potentially triple that with IRMAA. Whether to enroll in it or delay comes down to who's paying first.

You have coverage through an employer with 20+ employees (yours or a spouse's)

Delay Part B. Your employer plan is primary; Part B would be paying secondary on already-covered claims and would deliver almost no value to justify the premium. When the employment ends, your 8-month SEP opens the month after — sign up during that window to avoid the permanent penalty.

You have coverage through an employer with fewer than 20 employees

Enroll in Part B during your Initial Enrollment Period (the 7-month window that starts 3 months before your birthday month). Your employer plan will pay only after Medicare — going without Part B leaves you with no real primary coverage. This is the mistake that causes the biggest financial damage of any Medicare misstep.

You're on retiree coverage, COBRA, or the ACA Marketplace

Enroll in Part B on time. None of these count as "active-employee coverage" and none of them delay the late enrollment penalty. The Marketplace will actually stop paying Advance Premium Tax Credits toward your plan once you become eligible for Medicare, whether you enroll or not.

Your income is high enough to trigger IRMAA

The IRMAA math changes the calculus a bit but doesn't change the rule. If you're subject to the highest IRMAA tier, Part B premiums approach $700/month, and paying that for coverage that duplicates your active-employee employer plan is a waste. Delay Part B until retirement, then enroll during the SEP. IRMAA still applies once you enroll, but you didn't pay it for years of overlap.

When you delay Part B while working at a large employer, the 8-month SEP is your safety valve. It works like this:

1. Your active-employee group health plan ends (retirement, termination, plan cancellation) — or your employment ends, whichever comes first.
2. The month after, an 8-month window opens.
3. During that 8 months, you can enroll in Part B without the late enrollment penalty. Coverage typically starts the month after you enroll.
4. Miss the window, and you wait for the next General Enrollment Period (January 1 to March 31) and pay the permanent 10%-per-year late enrollment penalty on Part B forever.

To get this right in practice: as soon as you know your last day of active employment, contact Social Security (which administers Part B enrollment) and file form CMS-40B along with employer-completed form CMS-L564 to prove you had qualifying coverage. Do this before your last day if possible so coverage lines up seamlessly.

Part D has its own late enrollment penalty — 1% of the national base beneficiary premium for every month you were eligible but didn't enroll, added to your Part D premium permanently. The way to avoid it while working: your employer's prescription drug coverage must be "creditable," meaning at least as good as standard Part D.

Ask HR for a creditable coverage notice — plans that offer drug benefits are required to issue one every fall. Keep the notice with your records. When you eventually enroll in Part D during your Part D SEP (2 months after employer creditable coverage ends), you'll need that history to avoid the penalty.

Medicare uses your tax return from 2 years prior to determine IRMAA. So Part B premiums in 2026 are based on your 2024 modified adjusted gross income. For anyone approaching or in a high-income year, this creates a planning opportunity: income spikes in 2024 (Roth conversions, capital gains, a bonus, a home sale, deferred compensation, RSU vesting) will show up as higher Part B and Part D premiums in 2026.

Two levers that help:

• Delay Part B enrollment if you have qualifying employer coverage, so you're not paying IRMAA during overlap years.
• File form SSA-44 if you had a "life-changing event" (retirement, divorce, death of a spouse, loss of pension) that reduced income after the tax year Medicare is looking at. SSA can adjust IRMAA down to reflect current income. This does not help with market-driven income swings — only qualifying life events.

Scenario 1: 65, large employer, HSA saver

Delay Part A (to keep HSA contributions), delay Part B, delay Social Security (to avoid the retroactive Part A trap). Notify HR you're staying on the group plan. Six months before your intended retirement date, stop HSA contributions. Enroll in Part A and Part B during the SEP after retirement. This preserves 100% of your HSA contribution room and avoids all penalties.

Scenario 2: 65, large employer, no HSA

Enroll in Part A during your Initial Enrollment Period (it's free and adds hospital backup). Delay Part B. Ask HR for a creditable coverage letter for drugs. Enroll in Part B during the 8-month SEP after retirement. Enroll in Part D within 2 months of losing creditable drug coverage.

Scenario 3: 65, small employer (fewer than 20)

Enroll in Part A and Part B during your Initial Enrollment Period. If you're on an HSA-eligible high-deductible plan, this ends HSA contributions but is worth it — your small-employer plan expects Medicare to pay first. Compare your employer plan to a Medicare Supplement + Part D combo to see which delivers better coverage as secondary.

Scenario 4: 65, spouse still working at a large employer

You can be covered under your spouse's active-employee group plan and delay Part B. The SEP opens when your spouse retires or the plan ends. If your spouse works past your 65, this is often the cheapest path — no Part B premium during those years.

Scenario 5: 65, on the ACA Marketplace, still self-employed

Enroll in Medicare (Parts A and B) at 65. The Marketplace and Medicare do not coordinate — Advance Premium Tax Credit ends the first month you're Medicare-eligible, and staying on the Marketplace means paying full unsubsidized rates and possibly repaying subsidies at tax time. Medicare is almost always cheaper.

1. Working at a small employer and skipping Part B. The bill is not "your employer plan minus a co-pay." It's your employer plan minus what Medicare would have paid, if you had Medicare. Enroll.
2. Relying on COBRA to hold the SEP open. COBRA doesn't. The 8-month window starts when active employment ends. Miss it and you pay 10% more for Part B forever.
3. Contributing to an HSA after enrolling in Part A. Even the premium-free version disqualifies HSA contributions. And the retroactive 6-month Part A backdating when you start Social Security after 65 catches thousands of people every year.
4. Staying on the ACA Marketplace with subsidies after Medicare eligibility. Subsidies end whether you enroll or not, and staying enrolled means unsubsidized premiums plus subsidy clawback at tax time.
5. Assuming Part D creditable coverage. Ask HR every year and keep the letter. A "no" from HR before you turn 65 changes the enrollment math immediately.

1. Confirm employer size. Ask HR whether the employer had 20+ employees for 20 weeks in the current or prior year. Get the answer in writing.
2. Confirm creditable Rx coverage. Get the current-year creditable coverage notice from HR.
3. Decide about HSA contributions. If yes to HSA, delay Part A and Social Security. If no, enroll in Part A.
4. Enroll in Part B or delay based on the 20-employee rule. Delay if 20+; enroll if under 20.
5. Save your creditable coverage documentation. Keep every annual letter — you'll need them if you ever enroll in Part D.
6. Set a calendar alert 4 months before your intended retirement date. That's when you'll want to file CMS-40B and CMS-L564 with Social Security.
7. Model total costs. A quick before/after comparison of employer coverage vs. Medicare + Supplement + Part D usually pays for itself in avoided surprises.