Choosing the wrong Medicare supplement insurance plan can cost you thousands of dollars a year in unexpected out-of-pocket expenses. With 10 standardized plan types available in most states, the differences matter — and knowing what to look for makes all the difference before you sign up.
What Medicare Supplement Insurance Actually Does
Original Medicare (Parts A and B) covers roughly 80% of approved medical costs. That leaves you responsible for deductibles, coinsurance, and copayments that can add up fast — especially with hospital stays or specialist visits.
Medicare supplement insurance plans (also called Medigap) fill those gaps. You pay a monthly premium to a private insurer, and in return, the plan covers some or all of the costs Medicare leaves behind. These plans work alongside Medicare, not instead of it.
The 10 Standardized Plan Types
In most states, Medigap plans are labeled with letters: A, B, C, D, F, G, K, L, M, and N. Each letter represents a specific set of benefits — and every insurer offering Plan G, for example, must provide the same core coverage. The difference between carriers is price and customer service, not the benefits themselves.
Here's a quick breakdown of the most popular options:
- Plan F — The most comprehensive coverage available, but only for those who were eligible for Medicare before January 1, 2020. Covers the Part B deductible, excess charges, and nearly all out-of-pocket costs.
- Plan G — Now the top choice for new enrollees. Covers everything Plan F does except the Part B deductible ($257 in 2025). Often provides better value because premiums are lower.
- Plan N — Lower premiums than Plan G, but you pay up to $20 copays for office visits and $50 for emergency room visits. A smart fit if you're generally healthy and want to trade some cost-sharing for savings.
- Plan K and Plan L — Share costs with you up to an annual out-of-pocket limit. Good for people comfortable with some exposure in exchange for significantly reduced premiums.
- Plan A — The most basic option. Covers only Medicare Part A coinsurance and hospital costs, Part B coinsurance, blood (first 3 pints), and hospice care coinsurance.
How Premiums Are Calculated
Premium pricing isn't one-size-fits-all. Insurers use one of three rating methods:
- Community-rated — Everyone pays the same premium regardless of age.
- Issue-age-rated — Your premium is locked in based on the age you enroll. Younger enrollment means lower lifelong rates.
- Attained-age-rated — Premiums start low but increase as you get older. This is the most common structure — and the one to watch carefully.
A Plan G premium might run $90–$160/month for a 65-year-old in a lower-cost state, and $180–$280/month in a high-cost area like New York or California. Always compare multiple carriers for the same plan letter.
Enrollment Timing Is Critical
Your Medigap Open Enrollment Period begins the month you turn 65 and are enrolled in Medicare Part B. During this 6-month window, insurers cannot deny you coverage or charge higher premiums based on health conditions. Once this window closes, you may face medical underwriting — meaning a pre-existing condition like diabetes or heart disease could result in denial or significantly higher costs.
Missing this window is one of the most expensive mistakes new Medicare enrollees make.
What Medigap Doesn't Cover
Even the most comprehensive Medicare supplement insurance plans have limits. None of the standard plans cover:
- Prescription drugs (you'll need a separate Part D plan)
- Dental, vision, or hearing
- Long-term care
- Private-duty nursing
If you need drug coverage, enroll in a standalone Part D plan during your Initial Enrollment Period to avoid a late enrollment penalty.
How to Compare Plans the Right Way
Don't just call one insurer and accept their quote. Compare across carriers for the exact same plan letter, check the insurer's financial stability rating (look for A or higher from AM Best), and review their rate increase history in your state. Some companies raise rates aggressively after year one.
Mercoly makes this process straightforward by letting you compare and find trusted Medicare supplement insurance providers in one place, saving you hours of research across multiple websites.
Also ask whether the insurer offers a household discount — many carriers reduce premiums by 5–12% if two people in the same household enroll.
Making the Final Call
Once you've identified the plan letter that fits your health needs and budget, prioritize the carrier with the most stable rate history and strong customer service reviews. Price matters, but so does knowing your insurer will be easy to work with when you actually need care.
Start your Medicare supplement insurance plan comparison today and lock in your coverage before that open enrollment window closes.