Medigap 101: Coverage, Costs & Enrollment Deadlines

Medicare Supplement (Medigap) plans pay the deductibles and 20% coinsurance Original Medicare leaves behind—letting you see any doctor who accepts Medicare with virtually no surprise bills. Failing to enroll when you are within your initial six-month open-enrollment window can impact your ability to enroll, as well as your plan premium.

Medicare Supplement 101 • Why Choose a Medicare Supplement Plan? • Plan Letters, Made Simple • Your Six-Month Window • Premium Reality Check • Rating Methods, Demystified • Medicare Supplement vs. Medicare Advantage • Who Isn't a Good Fit • Decision Checklist • Find the Plan that Fits

Medicare Supplement 101 

Medicare Supplement (aka Medigap) is sold by private insurers and only teams up with Original Medicare. Think tag team: Original Medicare pays first; Medicare Supplement jumps in for many of the out-of-pocket bits—deductibles, coinsurance, and certain hospital costs.

Heads-up: Medicare Supplement doesn't ride along with Medicare Advantage. They're two different routes or plan types—you can choose Original Medicare + Medicare Supplement or Medicare Advantage, not both.

Why choose a Medicare Supplement plan?

• Fewer “gotchas,” more predictability. Most Medicare Supplement plans cover the 20% Part B coinsurance and Part A hospital costs Original Medicare leaves behind, so surprise bills don’t ambush your budget.
• Any Medicare doc, anywhere. If a provider accepts Original Medicare, they accept your Medicare Supplement plan—perfect for snowbirds, road-trippers, and people whose specialist is a ZIP Code away.
• Minimal hoops. If Original Medicare covers it, Medicare Supplement plans typically don’t pile on extra referrals or prior auth requirements.

Quick reality check: You’ll still pay the Part B premium ($202.90 in 2026) and its deductible ($283 in 2026). Exact coverage depends on the letter you pick (think G, N, or High-Deductible G), but compared with Original Medicare alone, a Medicare Supplement plan makes most of the gaps disappear. Medicare Supplement plans, however, only cover what Original Medicare Parts A and B cover. They do not cover most prescription drugs.

Medicare Supplement plan letters, made simple

And not to be confused with Medicare "Parts"

Medicare Supplement (Medigap) plans are federally standardized by letter—A, B, C, D, F, G, K, L, M, and N—not the same thing as Medicare Parts A, B, C, or D.

Here’s the key: a Plan G in Peoria covers the exact same benefits as a Plan G in Portland. The only real differences are price and customer service from the insurer. (Heads-up: if you became Medicare-eligible after Jan 1, 2020, Plans C and F generally aren’t available to you.)

The crowd-favorites

Plan G: the "sleep-well" pick

• What it covers: Every major gap except the Part B deductible ($283 in 2026).
• That means no Part A hospital deductible, no coinsurance, and it even covers Part B excess charges.
• Why people choose it: After you meet that first $283, you're basically at $0 out-of-pocket for Medicare-approved services. Maximum predictability, minimum surprises.

Plan N: the "save-on-premium" pick

• What it covers: Similar backbone to G, but with copays.
• That means you'll typically see office-visit copays, ER copays (waived if admitted), and no coverage for Part B excess charges.
• Why people choose it: Generally lower premiums than G. Great trade-off if you don't visit specialists often and your doctors accept Medicare assignment.

High-Deductible Plan G: the “catastrophe cap” pick

• What it covers: Same benefits as Plan G after you meet a $2,950 annual deductible (2026).
• Why people choose it: Rock-bottom monthly cost with a firm ceiling for “what if” years. It’s popular with ultra-healthy retirees who want a low premium and a safety net.

Other letters, in brief

• Plan A: Bare-bones (and usually cheapest)
• Plan K / Plan L: You pay about 50% / 75% of most cost-shares until you hit a built-in annual cap, then the plan picks up the rest.

Bottom line: same alphabet soup everywhere, but you’re shopping on price and service, not on different benefits. Pick the letter that fits your risk tolerance, then compare carriers for the best value.

Your six-month golden ticket

Your Medicare Supplement Open Enrollment Period starts the month your Part B kicks in (usually the month you turn 65) and lasts six months. During this window, insurers must sell you any plan at the best rate—no health questions, no surcharges. This can sometimes be called your “guarantee issue” period.

Miss it and you’re rolling the dice on underwriting. (Think trying to buy flood insurance during a storm.)

Premium reality check

Premiums vary widely based on your ZIP Code, age, and insurer. Plan G typically costs more per month than Plan N, which costs more than High-Deductible G. But here's the key: shopping across insurers for the same letter plan can save you money per year on identical coverage, since every insurer sets its own price for the same standardized benefits.

Why premiums rise over time

Community-rated (aka no-age-rated)

• What it means: Your base premium doesn’t change because you got a year older. A 65-year-old and a 75-year-old pay the same base rate for the same plan, same insurer, same area.
• How it moves: It can still rise for things like medical inflation or company-wide adjustments—just not because you had a birthday.
• Why people like it: Smoother ride if you plan to keep the policy for the long haul.
• The trade-off: The starting price at 65 can be a bit higher than other methods; you’re paying for stability up front.

Issue-age-rated

• What it means: Your premium is locked to the age you were when you bought it. Buy at 65? You keep that younger-age base rate forever.
• How it moves: It won’t climb just because you age, but it can go up for inflation or claims trends.
• Why people like it: Great for early birds who enroll at 65 and want a predictable base.
• The trade-off: Waiting costs more—if you buy at 70, you lock in a higher base than someone who bought at 65.

Attained-age-rated

• What it means: Your premium is tied to your current age—it starts lowest at 65 and steps up as birthdays roll by.
• How it moves: Expect age-based increases + inflation adjustments. Think gentle staircase that can get steeper in later years.
• Why people like it: Low entry price at 65.
• The trade-off: Pay later vibes—those age bumps add up. Switching plans down the road may require underwriting.

Quick reality check: No rating method has a price-freeze button. All can see increases for inflation, benefits changes, or insurer-wide adjustments. Your job is to pick the curve you’d rather ride—steady (community), age-locked at purchase (issue-age), or low-now/high-later (attained-age).

Medicare Supplement plans at-a-glance

Medicare Supplement vs. Medicare Advantage:

Quick-look comparison

Who often passes on Medicare Supplement (and why)

1. $0-premium chasers: If a local Medicare Advantage plan fits the budget and looks decent, they’d rather keep the monthly spend near zero.

2. “I barely see a doctor” crowd: Ultra-healthy folks who roll the dice on low premiums now (with a quiet note to Future Them that health can change).

3. Wrapped-and-covered retirees: Veterans or former employees with TRICARE/VA/employer retiree coverage that already fills most gaps.

4. Rich-MA-market residents: Those living in counties where Medicare Advantage plans are stacked with low Maximum Out-Of-Pockets and generous extras (like Florida)—hard to say no to that value combo.

Decision-help checklist:

1. Name your must-haves

Doctor freedom? Snowbird travel? Fewer surprise bills? Let these steer your letter (G, N, or High-Deductible G).

2. Gut-check the premium

Can your budget handle a higher monthly premium in exchange for dodging that 20% coinsurance on big bills?

3. Price three carriers (minimum)

Same benefits, different prices. Get at least three quotes before you commit.

4. Peek at the pricing method

Community-rated (steady), Issue-age (locks at the age you buy), or Attained-age (rides the birthday escalator).

5. Circle your six-month guaranteed window

Your Medicare Supplement open enrollment starts when Part B begins. Enroll then, before underwriting can say “no.”

Turn “I think so” into “That’s my plan” with ALEX®

You don’t have to memorize every letter, deductible, or acronym to choose confidently. Tell ALEX, our friendly digital guide, what matters—your health needs, budget comfort zone, and must-haves—and it maps you to the plan type that fits (Medicare Supplement vs. Medicare Advantage), then shows real-world costs and trade-offs without the jargon.

• Health needs: ALEX checks your doctors and meds so networks and formularies don’t surprise you later.
• Budget math: ALEX lays out premiums, typical copays, and MOOP in one view.
• Priorities: Prefer freedom to roam or one-card simplicity? ALEX highlights the path that matches you.

Ready to find the plan that fits, minus the guesswork?

Get started with ALEX