Educational analysis only, not insurance or tax advice; all Medicare premium, deductible, and IRMAA figures reflect the 2026 program year, and employer plan benchmarks reflect the 2025 KFF Employer Health Benefits Survey.
TL;DR — Quick Verdict
- Employer size decides everything: at 20 or more employees the group plan pays first and you can safely delay Part B; under 20, Medicare pays first whether or not you enrolled.
- The 2026 standard Part B premium is $202.90 per month with a $283 annual deductible; a two-year enrollment gap adds a permanent 20% surcharge, taking the premium to $243.50 per month.
- Any Medicare enrollment — including premium-free Part A — drops your HSA contribution limit to zero, forfeiting up to $5,400 in annual self-only contributions for someone 55 or older.
- Comparison result: a single worker at a large employer contributing $1,440 a year toward coverage (KFF, 2025) usually beats Part B plus Medigap; a family contributor at $6,850 usually does not.
- Recommendation: confirm your employer’s nationwide headcount in writing, stop HSA contributions six months before you file for Medicare, and keep the CMS-L564 employment letter on file.
Roughly one in five Americans past 65 is still working, and most of them carry a group health plan that quietly changes character on their 65th birthday. The Centers for Medicare & Medicaid Services sets the 2026 standard Part B premium at $202.90 per month — nearly 10% above the 2025 rate of $185.00 — which makes “just keep the employer plan” look like the obvious move. It often isn’t. Whether your UnitedHealthcare or Aetna group plan pays first or second depends on a single number: how many people your employer employs nationwide. Get that number wrong and claims get repriced retroactively, an HSA becomes an excess-contribution problem, and a 10%-per-year Part B surcharge attaches for life.
This analysis walks through the coordination-of-benefits rules that determine payment order, prices out four common worker profiles against 2026 Medicare figures and 2025 KFF employer benchmarks, models the HSA forfeiture cost month by month, and identifies the five coordination mistakes that generate the largest permanent losses.
The 20-Employee Rule Decides Who Pays First
Medicare Secondary Payer rules assign payment order by employer size, not by preference. When an employer has 20 or more employees, the group health plan is the primary payer and Medicare pays second on what remains. When the employer has fewer than 20 employees, that order flips: Medicare becomes primary, and the group plan drops to secondary.
That inversion is where the money leaks. A secondary payer pays only the balance it would owe after the primary payer’s share. If you work at a 12-person firm and never enrolled in Part B, Medicare pays nothing — there is no coverage in force — and the group plan calculates its payment as though Medicare had already paid its portion. Routine claims come back paid at a fraction of billed charges. Larger claims can be effectively rejected.
Headcount is counted across every location nationwide, including part-time employees, not just your worksite. A regional office of six inside a 400-person company is a large-employer situation. Two 15-person entities under common ownership may or may not aggregate, depending on the controlled-group analysis your benefits administrator applies. Ask HR for the determination in writing before your Initial Enrollment Period closes, and cross-check it against the enrollment deadlines and late penalty costs that apply if the answer turns out to be “fewer than 20.”
Spousal coverage follows the same test. If you are covered under your spouse’s active employment plan, it is the spouse’s employer headcount that governs — a detail that catches couples where one spouse works for a small consultancy.
What Medicare Actually Costs in 2026
Before comparing anything, price the Medicare side accurately. CMS published the following amounts effective January 1, 2026.
Source: Centers for Medicare & Medicaid Services, 2026 Medicare Parts A & B Premiums and Deductibles, and the Federal Register notice of Part B actuarial rates for 2026.
Two figures deserve emphasis for still-working enrollees. First, IRMAA begins at $109,000 of modified adjusted gross income for single filers and $218,000 for joint filers, based on the 2024 return — and wages count. A worker earning $150,000 who enrolls in Part B at 65 pays $527.50 per month, not $202.90, which changes the arithmetic of every comparison below. Second, the standard premium is the starting figure; the IRMAA surcharge rules and reduction requests process is the only route back down after a genuine income drop.
Large Employer vs Small Employer: Which Coordination Path Costs Less?
Consider two workers, both 66, both single, both earning $95,000 — below the IRMAA threshold. Worker A is at a 350-person manufacturer. Worker B is at a 14-person architecture firm. Both have identical group plans: KFF’s 2025 national average of $9,325 in total single-coverage premium, of which the worker contributes $1,440 annually, with a $1,886 general annual deductible.
Modeled by Real Cost Report using 2026 CMS premium and deductible figures and 2025 employer benchmarks from the KFF Employer Health Benefits Survey.
Worker B pays $2,434.80 more per year for the privilege of working somewhere small. That is not optional spending. Declining Part B to save it converts the group plan into a secondary payer with nothing in front of it, which is the single most expensive coordination error available.
Verdict
At 20 or more employees, delay Part B and keep the group plan — the $2,434.80 annual Part B premium buys nothing you don’t already have, and the 8-month Special Enrollment Period protects you at retirement. Under 20 employees, enroll in both Part A and Part B during your Initial Enrollment Period regardless of how good the group plan looks. The extra $2,434.80 is the price of having a primary payer at all, and no Special Enrollment Period rescues a missed Initial Enrollment Period in the small-employer scenario.
The HSA Collision Nobody Budgets For
Here is where high earners lose the most money, and it has nothing to do with premiums. IRS Publication 969 states the rule plainly: beginning with the first month you are enrolled in Medicare, your HSA contribution limit is zero. Not prorated for that month — zero. Any part of Medicare triggers it, including premium-free Part A that arrived automatically because you claimed Social Security.
Revenue Procedure 2025-19 sets the 2026 ceilings at $4,400 for self-only high-deductible coverage and $8,750 for family coverage, with a $1,000 catch-up for those 55 and older who are not enrolled in Medicare. A 60-year-old with family coverage forfeits $9,750 in annual tax-advantaged contribution room the month Part A begins. At a 32% marginal federal rate, that is roughly $3,120 in lost deduction value per year, before any state benefit.
The trap has a second jaw. When you file for Medicare or claim Social Security after 65, premium-free Part A is backdated up to six months — never earlier than the month you turned 65. Contributions made during that retroactive window become excess contributions subject to a 6% excise tax for every year they remain in the account. Someone who contributed the full self-only amount monthly and then filed in July has six months of deposits reclassified after the fact.
The defensive move is mechanical: stop all HSA contributions, including employer deposits, at least six months before you intend to apply for any part of Medicare. Existing balances stay yours and can pay Part B premiums, Part A deductibles, and most cost-sharing tax-free — though not Medigap premiums. Workers weighing whether the HDHP is still worth holding should read that alongside the Part A deductible and benefit period gaps that the balance will eventually cover.
What Most People Get Wrong
Five errors account for the majority of permanent, unrecoverable losses in this area.
Treating COBRA as employer coverage
CMS classifies COBRA as continuation coverage, not active group health plan coverage. The consequence: your 8-month Special Enrollment Period clock starts when active employment ends, not when COBRA ends, and it keeps running the entire time you are on COBRA. Someone who takes 18 months of COBRA and then applies for Part B has missed the window by ten months. Correct action: enroll in Part B within eight months of the last day of active employment, even if COBRA is still paying claims.
Assuming retiree coverage extends the deadline
Retiree health benefits from a former employer are not based on current employment, so they create no Special Enrollment Period. A worker who retires at 63, keeps retiree coverage, turns 65, and waits will land in the General Enrollment Period of January 1 through March 31 with a penalty attached. Correct action: enroll at 65 unless the coverage derives from someone’s active job.
Miscounting employer size
Employees at every location count, including part-timers, and the threshold applies to the calendar-year pattern rather than a single day’s headcount. Correct action: get the determination in writing from HR and retain it.
Skipping the CMS-L564 documentation
The Special Enrollment Period requires proof. Form CMS-40B is the Part B application; Form CMS-L564 is the employer’s certification of coverage dates. Employers that have merged, dissolved, or outsourced benefits often cannot complete it years later. Correct action: request the completed L564 while you still work there, and keep pay stubs showing health premium deductions as backup.
Declining Part A when it is free
Most workers who qualify for premium-free Part A should take it at 65 — unless they are contributing to an HSA, in which case the calculation above governs. Correct action: run the HSA forfeiture math against expected inpatient exposure before declining.
Is Delaying Part B Worth It? Conditional Logic
Delay makes financial sense under a narrow set of conditions, all of which must hold simultaneously.
Delay Part B if your employer — or your spouse’s — has 20 or more employees, the coverage is based on current active employment, your worker contribution is below roughly $2,700 annually (the 2026 Part B premium of $2,434.80 plus the $283 deductible), and you are prepared to file within eight months of leaving. High earners have an additional reason: at $150,000 of MAGI the Part B premium is $527.50 monthly, or $6,330 per year, which almost always exceeds a group plan contribution.
Enroll in Part B now if the employer has fewer than 20 employees, the coverage comes from retiree benefits or COBRA rather than active work, you are covered under a spouse’s plan that itself falls below the threshold, or your group plan’s out-of-pocket exposure has drifted upward. KFF found the 2025 average single deductible at small firms was $2,631 against $1,670 at larger firms, with more than half of small-firm workers facing at least $2,000.
Family contributors face a different arithmetic entirely. At the 2025 average family contribution of $6,850, dropping to individual Medicare coverage while a spouse moves to marketplace or their own employer plan can save several thousand dollars annually — the calculation depends on what the spouse’s alternative costs, and on whether you would pair Medicare with a supplement. Anyone modeling that path should first compare the Medigap vs Medicare Advantage annual cost comparison and then narrow further using the Supplement Plan G vs Plan N cost comparison, since the supplement premium is what determines whether the switch clears.
Federal workers occupy their own category; the FEHB program interacts with Part B differently than private group coverage, which the FEHB vs Part B analysis for federal employees addresses directly. Workers who lose employer coverage before 65 face a separate problem entirely and need bridge coverage after losing employer insurance.
The Penalty That Never Stops Compounding
Medicare.gov states the Part B late enrollment penalty as an extra 10% of the standard premium for each full 12-month period you could have enrolled but did not. Two features make it worse than it sounds. It is permanent for as long as you hold Part B, and it is recalculated against the standard premium each year — so it grows every time CMS raises rates.
Modeled by Real Cost Report from the 2026 standard premium of $202.90; the 24-month figure of $243.50 is stated directly by Medicare.gov (verify at medicare.gov). Later-year amounts will exceed these as the standard premium rises.
Project a 30% surcharge across a 20-year retirement and the loss clears $15,000 in nominal terms even before further premium increases. Part D carries its own separate penalty of roughly 1% of the national base beneficiary premium per month of uncovered gap, which is why anyone delaying should confirm their group drug benefit is creditable and understand Part D plan selection and formulary comparison before the group coverage terminates. Relief exists but is narrow: Form CMS-10797 requests a Special Enrollment Period for exceptional circumstances, and for those over 65 equitable relief generally applies only when a federal employee gave incorrect information.
Frequently Asked Questions
Does my employer plan’s drug coverage let me skip Part D?
Only if the plan is certified creditable, meaning it pays at least as much as standard Part D on average. Employers must notify you annually, typically before October 15. Keep that notice. Without proof of creditable coverage, the Part D penalty runs at roughly 1% of the national base beneficiary premium for every uncovered month and lasts as long as you hold Part D.
Can I keep my employer plan and add Part B at the same time?
Yes, and at large employers some workers do it for the extra layer. Weigh the $2,434.80 annual Part B premium at the 2026 standard rate against what the group plan already covers. The combination rarely pays off unless the group deductible is high — KFF put the 2025 small-firm average single deductible at $2,631 — or you anticipate significant outpatient utilization.
My spouse is 62 and on my employer plan. What happens when I go on Medicare?
Medicare covers individuals only. If you drop the group plan entirely, your spouse loses coverage and needs a marketplace plan or their own employer’s plan until 65. Losing group coverage is a qualifying event for a marketplace Special Enrollment Period. Many couples keep the family plan running — averaging $6,850 in worker contributions in 2025 per KFF — precisely to bridge that gap.
Do my wages count toward IRMAA while I’m still working?
They do. IRMAA uses modified adjusted gross income from two years prior, so 2026 premiums reflect 2024 income including wages. A working enrollee above $109,000 single or $218,000 joint pays between $284.10 and $689.90 monthly for Part B. Retiring reduces income, but the surcharge lags two years unless you file Form SSA-44 citing work stoppage as a life-changing event.
How We Researched This Article
Every Medicare premium, deductible, coinsurance, and income-related monthly adjustment amount in this article comes from the Centers for Medicare & Medicaid Services release of 2026 Parts A and B amounts, cross-checked against the Federal Register notice establishing 2026 Part B actuarial rates. The late enrollment penalty structure and the stated $243.50 figure for a 24-month delay come from Medicare.gov’s guidance on avoiding late enrollment penalties. Health Savings Account contribution ceilings and the Medicare enrollment rule reducing the limit to zero come from IRS Revenue Procedure 2025-19 and IRS Publication 969.
Employer plan benchmarks — average premiums, worker contributions, deductibles, and the small-firm versus large-firm split — come from the 2025 KFF Employer Health Benefits Survey, which drew on 1,862 interviews with non-federal public and private firms. Payment-order rules follow the Medicare Secondary Payer provisions administered by CMS.
A distinction between measured and modeled figures matters here. All premiums, deductibles, coinsurance amounts, IRMAA tiers, penalty percentages, HSA limits, and KFF averages are measured — published directly by the named source. The four-row worker comparison, the penalty projection table beyond the 24-month row, the HSA forfeiture value at a 32% marginal rate, and the $2,700 break-even threshold are modeled by Real Cost Report using those inputs, with arithmetic shown in the body text.
Three limitations apply. Employer benchmarks are national 2025 averages against 2026 Medicare figures, since the 2026 KFF survey had not published at the time of writing; individual plans vary widely by industry, region, and funding arrangement. Penalty projections for years beyond 2026 assume a flat standard premium and therefore understate the true cost, because CMS raised the premium 9.7% for 2026 and has raised it in most recent years. Controlled-group aggregation for the 20-employee test depends on ownership facts specific to each employer and cannot be generalized. Research last conducted July 2026. All figures were verified against named primary sources before publication.