What Is Medicare Gap Insurance and How Does It Work?

Medicare gap insurance, formally called Medicare Supplement Insurance or Medigap, is a private policy that pays many of the out-of-pocket costs Original Medicare leaves behind, including deductibles, copayments, and coinsurance. Medicare Part B, for example, covers 80% of most outpatient services after you meet an annual deductible of $283 in 2026, and a Medigap policy can pick up the remaining 20%.¹ The benefits are standardized by federal law, so each lettered plan offers the same core coverage no matter which insurer sells it. Premiums are where insurers compete.

How a Medigap Policy Pays Your Bills

Medigap is secondary insurance. When you see a doctor or go to the hospital, Medicare processes the claim first and pays its share. Your Medigap insurer then receives the claim automatically and pays the remaining covered costs according to your plan’s benefits. In most cases you don’t file anything yourself.

A practical example. Say you’ve already met your $283 Part B deductible for 2026 and you have a follow-up outpatient visit. Medicare pays 80% of the approved amount, and you owe the remaining 20% coinsurance.² If you have Plan G, your Medigap policy pays that 20%, so your out-of-pocket cost is zero beyond the deductible and your monthly premium. Plan N works similarly, except you’d owe a small copayment of up to $20 for the office visit.³

On the hospital side, the Part A deductible is $1,736 per benefit period in 2026. Plans B through N cover this deductible either in full or in part, which protects you from a large upfront bill each time you’re admitted.¹

All Medigap policies are also guaranteed renewable. Your insurer cannot drop you because of health problems or because you file claims, as long as you keep paying your premiums.⁴

What Medigap Does Not Cover

Medigap only covers services that Medicare itself approves. If Medicare doesn’t cover something, such as routine dental care, vision exams, or hearing aids, your Medigap plan won’t cover it either.

Prescription drugs sit outside Medigap entirely. Medigap policies cannot include drug coverage, so if you want it, you’ll need to enroll in a separate Medicare Part D plan.⁴

You also cannot use Medigap alongside a Medicare Advantage plan. Medigap is built to supplement Original Medicare only. If you’re enrolled in Medicare Advantage, a Medigap policy won’t pay any of your costs, and it’s illegal for an agent to sell you one while knowing you’re in Medicare Advantage.⁴⁵ To use Medigap, you have to be back in Original Medicare.

One area where Medigap goes further than Original Medicare is emergency care abroad. Original Medicare generally does not cover healthcare outside the United States, but Plans C, D, F, G, M, and N pay 80% of medically necessary emergency care overseas after a $250 annual deductible, up to a $50,000 lifetime limit.⁶ Plans A, B, K, and L do not include this benefit.⁷

The Standardized Lettered Plans

Federal law requires every Medigap policy to be standardized. A Plan G from one company covers the same benefits as a Plan G from any other company. What differs between insurers is the premium, the customer service, and any extra perks.⁸ There are 10 plans in most states, labeled A through D, F, G, and K through N.⁷

Most comprehensive (Plans F and G). Plan G covers virtually all Medicare-approved out-of-pocket costs except the Part B annual deductible. Plan F also covers that deductible but is only available to people who became eligible for Medicare before January 1, 2020.⁹
Moderate coverage (Plans B, C, D, M, and N). These cover many of the same costs but leave certain items to the policyholder. Plan C, like Plan F, is closed to anyone newly eligible for Medicare on or after January 1, 2020. Plan N requires copayments of up to $20 for office visits and up to $50 for emergency room visits that don’t result in a hospital admission.³
Cost-sharing plans (Plans K and L). Lower premiums, in exchange for splitting costs with you until you hit an annual out-of-pocket cap. In 2026, the cap is $8,000 for Plan K and $4,000 for Plan L. After you reach the cap, the plan pays 100% of covered costs for the rest of the year.¹⁰
Basic coverage (Plan A). Covers hospital coinsurance, the first three pints of blood, hospice coinsurance, and Part B coinsurance. Every insurer that sells Medigap must offer at least Plan A.

Plans F and G are also sold in high-deductible versions. You pay a lower premium and cover the first $2,950 of Medigap-eligible expenses yourself in 2026 before the plan starts paying.¹¹ That can make sense if you’re relatively healthy and want lower monthly costs with a safety net against large bills.

Because of the Medicare Access and CHIP Reauthorization Act, anyone who became newly eligible for Medicare on or after January 1, 2020, cannot buy Plan C or Plan F, both of which covered the Part B deductible. If you were Medicare-eligible before that date but hadn’t enrolled yet, you can still purchase them.⁹ For everyone else, Plan G is the closest alternative and is now the most popular choice.

One boundary to note on geography. Massachusetts, Minnesota, and Wisconsin do not use the standard letter-designated plan system. They created their own standardized Medigap benefits before the federal system existed and were allowed to keep them. If you live in one of those three states, check with your state insurance department for the plan names and structures where you live.¹²

Who Can Buy Medigap and When

You need both Medicare Part A and Part B before any insurer will sell you a Medigap policy. Medigap is not standalone health insurance; it only fills gaps in what Original Medicare already covers.¹³

Most buyers are 65 or older, and the enrollment rules are built around that age. If you qualify for Medicare before 65 because of a disability or end-stage renal disease, your options depend on where you live. Some states require insurers to sell Medigap to younger Medicare beneficiaries; others let insurers refuse or charge significantly more. If you’re under 65, check your state’s rules before shopping.⁹

The Six-Month Open Enrollment Window

Timing matters more than almost anything else with Medigap. The Medigap Open Enrollment Period starts the first day of the month you turn 65 and are enrolled in Part B, and it runs for six months. During this window, insurers must sell you any Medigap plan they offer in your state at the standard rate, regardless of your health history.⁹

If you delay Part B because you still have employer coverage, your open enrollment period doesn’t start until you actually sign up for Part B. Once those six months expire, you lose the federal protection. Insurers can then review your medical history, charge more, or refuse to sell you a policy. Federal law also lets Medigap insurers refuse to cover pre-existing conditions for the first six months after your policy begins, though that waiting period disappears if you had at least six continuous months of prior creditable coverage, such as an employer group plan or another Medigap policy.

Guaranteed Issue Rights

Certain life events give you the right to buy a Medigap policy without medical underwriting, even after your open enrollment period has passed. These guaranteed issue rights kick in when you lose other coverage involuntarily, for example when employer retiree benefits end, your Medicare Advantage plan leaves your area, or your insurer goes bankrupt.

Under federal rules, guaranteed issue rights entitle you to buy Medigap Plan A, B, C, D, F, G, K, or L from any insurer in your state.⁹ The deadline is tight: generally no more than 63 days after your prior coverage ends. You’ll need to provide proof of the previous coverage. Act as soon as you know it’s ending.

There’s also a first-time-trial protection built around Medicare Advantage. If you joined a Medicare Advantage plan for the first time and switch back to Original Medicare within 12 months, you have the right to buy a Medigap policy with guaranteed issue protections.⁴

How Premiums Are Priced

Because the benefits are identical across insurers for a given lettered plan, the real shopping question is the premium and how it changes over time. Insurers use one of three pricing methods:

Community-rated. Everyone in the same area pays the same premium for the same plan, regardless of age. Your premium won’t rise because you get older, though it can still increase for inflation or general rate hikes.
Issue-age-rated. Your premium is based on your age when you first buy the policy. Someone enrolling at 65 gets a lower rate than someone enrolling at 72. Aging doesn’t push it up, but inflation and other factors can.
Attained-age-rated. Your premium is based on your current age and rises as you get older. These often start with the lowest premiums but become the most expensive over time.

An attained-age policy that looks cheap at 65 can turn into a budget problem at 80. If two plans have similar starting rates, favor community-rated or issue-age-rated pricing. The premium trajectory over 15 to 20 years matters more than the first-year cost.

Some insurers offer discounts for paying annually, using electronic funds transfer, being a non-smoker, or covering multiple people in the same household. Tobacco users, on the other hand, can face surcharges of up to 50% on their premiums outside the open enrollment period.

Switching Medigap Plans Later

Unlike Medicare Advantage and Part D, Medigap has no guaranteed annual switching window at the federal level. If you want to change plans after your initial open enrollment period, you’ll need to apply for a new policy, and the insurer can subject you to medical underwriting. If you’ve developed health conditions since buying your current plan, you could be denied or charged more.

Apply for the new policy first and keep your current one in force until you’re approved. Canceling early risks leaving you without supplemental coverage if the new insurer turns you down.

Some states offer additional protections. A number of them use a “birthday rule” that gives Medigap policyholders a short window around their birthday each year to switch to any plan with the same or lesser benefits without medical underwriting. Contact your state insurance department to see whether any switching protections apply where you live.

When comparing plans, look past the initial premium. A new policy priced lower today but on an attained-age basis could end up costing more over time than the one you already have.

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