What Medigap is
Medigap (Medicare Supplement Insurance) is a private insurance policy that fills the gaps in Original Medicare. It only works WITH Original Medicare — not Medicare Advantage. If you have an MA plan, you can't have Medigap.
Medigap pays AFTER Medicare. When you get a doctor visit: Medicare pays its share (80% of approved amount), then your Medigap pays its share of what's left (the 20% coinsurance, the deductible, etc., depending on the plan letter).
Medigap plans are sold by private insurers (UnitedHealthcare, Humana, Mutual of Omaha, BlueCross, AARP, etc.) but the BENEFITS of each plan letter are federally standardized. A Plan G from UHC covers exactly the same things as a Plan G from Humana. The only differences are the premium and the customer service.
The 10 plan letters
There are 10 standardized Medigap plans, lettered A through N (with some letters skipped or no longer sold). The most common today:
Plan G: covers the Part A deductible, all Part B coinsurance and copays, skilled nursing coinsurance, hospice, foreign travel emergency. You pay only the $257 Part B deductible. After that, Medicare + Plan G = no medical bills. This is the most popular new plan.
Plan N: similar to G but you pay a $20 office-visit copay, $50 ER copay (if not admitted), and you don't get coverage for Part B 'excess charges' (the 15% over Medicare rate that non-participating doctors can bill). Lower premium than G.
Plan F: identical to G except it ALSO covers the Part B deductible. But Plan F is closed to anyone who became Medicare-eligible after 1/1/2020 (MACRA closed it). If you were on Medicare before 2020, you can keep Plan F.
Plan K and L: 'cost-sharing' plans where YOU pay 50% (K) or 25% (L) of certain coinsurance until you hit an annual OOP max. Lower premium, more skin in the game.
Plan A and B: very limited; rarely chosen. Plan C and D: similar to F and G, also closed to new enrollees.
Why Plan G dominates
Plan G covers virtually everything except the small Part B annual deductible. For new Medicare enrollees, Plan G hits the sweet spot of 'comprehensive coverage' without the closed Plan F.
Plan N is the cheaper alternative for people OK with $20 office-visit copays and the chance of excess-charge exposure. In states that ban excess charges by law (NY, MN, PA, MA, RI, OH, CT, VT), Plan N's downside is muted.
The pricing reality
Federally standardized benefits doesn't mean federally standardized PRICES. Each state regulates Medigap separately, and within each state, carriers price independently. The result: in any given state, the cheapest Plan G carrier and the most expensive Plan G carrier can differ by $1,500-$2,500 PER YEAR for identical coverage.
Example (real California 65-year-old female non-tobacco): Cheapest Plan G in 2026 = $135/month. Most expensive Plan G = $295/month. Same federally standardized coverage. Difference of $1,920 per year, $19,200 over 10 years, $38,400 over 20.
This is why we built the Medigap Rate Explorer (free) and the Compare My Medicare tool ($49) — the only thing that matters is finding the cheapest carrier for Plan G in YOUR state at YOUR age.
Three rate methods carriers use
Attained-age: premium goes up as you age. Cheapest at 65, most expensive at 85+. Most common. Example: $135/mo at 65, $310/mo at 85.
Issue-age: premium based on how old you were when you bought the policy. Doesn't go up with age (only with inflation). Slightly higher starting premium, more predictable long-term. Less common.
Community-rated: premium is the same regardless of age. Highest starting premium, most stable long-term. Required in NY, CT, MA, VT, ME, WA. Optional in others.
Why this matters: a cheap attained-age policy at 65 might be expensive at 85. An issue-age or community-rated policy might cost more upfront but save money over decades. Most people pick attained-age and just shop for cheapest year by year — which usually wins on total cost.
The Medigap Open Enrollment window — your one shot
When you first enroll in Part B at age 65, you get a 6-month Medigap Open Enrollment period. During this window, you have 'guaranteed-issue' rights: any Medigap carrier MUST sell you any plan they offer at the standard rate, regardless of your health.
After this 6-month window, in most states, carriers can use medical underwriting — review your health history, deny coverage, or charge higher premiums. Some health issues (recent heart attack, cancer in remission) can make Medigap uninsurable in many states.
A few states have year-round guaranteed issue or annual switching windows: New York and Connecticut have year-round guaranteed issue. Massachusetts, Oregon, California, Maine, Missouri, and Washington have specific birthday rules or anniversary windows. Check your state.
Bottom line: shop hardest during your initial 6-month window. Lock in the cheapest carrier whose Plan G you'd be comfortable with for years.
Pre-65 disability beneficiaries
If you're getting Medicare before 65 due to SSDI, ESRD, or ALS, federal law does NOT require Medigap carriers to sell to you at standard rates. About 30 states have their own laws requiring access (some at higher rates), but in many states, Medigap is unavailable or only available at very high premiums for under-65 disability beneficiaries.
When you turn 65, you get a fresh 6-month Medigap Open Enrollment window. Use it.