Patient Responsibility

Medicare Coinsurance: Why You Owe 20%

Name: Medicare Coinsurance: Why You Owe 20%
Creator: Barley Billing Team
Published: 2026-03-26

Written by Barley Billing Team, Medicare Billing Experts | Fact-checked against primary CMS sources | Last reviewed September 11, 2026

Are you wondering why you owe 20% of your Medicare bill?

"Why do I still owe money after Medicare paid?"

"My statement says I owe a coinsurance amount"

"I have to pay 20% and it seems like a lot"

Let's walk through what coinsurance means and whether you can get help paying it.

What This Means

Your Medicare Summary Notice or Explanation of Benefits shows a “coinsurance” amount. This is not a denial. Medicare approved your claim and paid its share — the coinsurance is the portion you owe.

For most Part B services (doctor visits, outpatient care, medical equipment), Medicare pays 80% of the approved amount and you pay the remaining 20%. This 20% is your coinsurance.

Why This Happens

This is standard Medicare cost-sharing. After you meet your annual Part B deductible ($283 in 2026), you pay 20% coinsurance on most Part B services. This is how Original Medicare is designed.
There is no annual cap on coinsurance in Original Medicare. Unlike Medicare Advantage plans, Original Medicare does not have an out-of-pocket maximum. If you have many or expensive services, your coinsurance can add up throughout the year.
Part A has different coinsurance rules. For hospital stays, you pay nothing for days 1-60 (after the Part A deductible of $1,736 per benefit period in 2026), then a daily coinsurance for days 61-90 ($434 per day in 2026), and $868 per day for each “lifetime reserve” day (days 91-150, up to 60 days over your lifetime). Skilled nursing facility stays have $0 coinsurance for days 1-20 and $217 per day for days 21-100 in 2026.

Should You Appeal?

This is not a denial

Coinsurance is a standard part of Medicare cost-sharing. Medicare approved the service and paid its portion — the coinsurance is your share. You cannot appeal the 20% rate itself; that is set by law, and no appeal will change it.

The arithmetic is a different matter, and it is appealable. Whether your deductible had already been met and how the coinsurance amount was computed are both formal initial determinations under 42 CFR 405.924(b)(4) and (b)(5), which means they carry the same redetermination rights, and the same 120-day deadline, as an outright denial. So verify the amount before you pay: check that the coinsurance is 20% of the Medicare-approved amount rather than of the provider’s full charge, that the service was billed accurately, and that any deductible you had already satisfied was credited.

One thing to know before you reach for a calculator. A hospital outpatient bill — an emergency room visit, outpatient surgery, observation, imaging done at a hospital — usually carries two separate lines: the hospital’s own facility charge, where your share follows a separate schedule set by regulation rather than a flat 20%, and the doctor’s professional charge, where the 20% does apply. So on a hospital bill, a facility figure that is not 20% of the approved amount is normal and not by itself a sign of error. The facility rate itself is fixed by regulation and 42 CFR 405.926(b) keeps that rate out of the appeals process — but that is the rate, not your claim. Whether it was applied to you correctly is still reviewable under 405.924(b)(5), the same as any other coinsurance computation.

What To Do Next

Verify the amount. For a doctor’s or other professional service, check that the coinsurance is 20% of the Medicare-approved amount — not of the provider’s full charge. The approved amount is listed on your Medicare Summary Notice. On a hospital outpatient facility line, compare against the notice rather than against 20%, for the reason above.
Check if you have supplemental coverage. If you have a Medigap (Medicare Supplement) policy, it covers some or all of your Part B coinsurance — every standardized plan letter does. Plans A, B, C, D, F, G, M, and N cover it at 100%, Plan K at 50%, and Plan L at 75%; Plans K and L then pay 100% for the rest of the calendar year once you have met both their annual out-of-pocket limit ($8,000 for Plan K and $4,000 for Plan L in 2026) and your yearly Part B deductible ($283 in 2026). Contact your Medigap insurer to file a claim.
Look into the Qualified Medicare Beneficiary (QMB) program. Of the four Medicare Savings Programs, QMB is the one that pays Medicare cost-sharing — Part A and Part B premiums, deductibles, coinsurance, and copays. Federal law also forbids providers from billing QMB enrollees for those amounts. SLMB and QI pay only your Part B premium. Contact your State Health Insurance Assistance Program (SHIP) at shiphelp.org or call 1-800-MEDICARE to apply or check eligibility.
If you’re having trouble paying, ask your provider about payment plans. Many providers offer interest-free payment arrangements for Medicare patients.
Consider Medigap for future protection. If you are in Original Medicare and do not have supplemental coverage, a Medigap policy can protect you from high coinsurance costs. Note that the best time to enroll is during your Medigap Open Enrollment Period.

Sources

CMS: 2026 Medicare Parts A & B Premiums and Deductibles — primary source for the 2026 Part A deductible ($1,736), Part B deductible ($283), Part B premium ($202.90), Part A days 61-90 coinsurance ($434/day), lifetime reserve days ($868/day), and SNF days 21-100 coinsurance ($217/day) cited above.
Medicare.gov: Medicare costs — beneficiary-facing summary of premiums, deductibles, and coinsurance.
Medicare.gov: What Part B covers — list of Part B-covered services subject to the 20% coinsurance.
Medicare.gov: Compare Medigap plan benefits — which Medigap plan letters cover Part B coinsurance and at what percentage (Plans K and L are 50% / 75%; the rest, including C and F, are 100%, with small office-visit/ER copays for Plan N), the 2026 Plan K / Plan L out-of-pocket limits, and the rule closing Plans C and F to people first eligible for Medicare on or after January 1, 2020.
Medicare.gov: Medicare Savings Programs — eligibility and benefits for QMB, SLMB, QI, and QDWI.
KFF: Medicare Advantage in 2026 — source for the 2026 Medicare Advantage out-of-pocket ceilings ($9,250 in-network, $13,900 combined for PPO plans) referenced in the FAQ above. CMS sets these annually under 42 CFR 422.100(f)(4)–(5) but publishes the figures in a plan-facing memorandum rather than on its public pages.
CMS: Qualified Medicare Beneficiary (QMB) Program — federal balance-billing prohibition for QMB enrollees.

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Frequently Asked Questions

Is this a denial?

No. A coinsurance charge means Medicare approved and paid its share of your claim. The coinsurance is the portion you are responsible for — typically 20% of the Medicare-approved amount for Part B services.

Why is my coinsurance so high?

Coinsurance is a percentage, not a flat fee. If you had an expensive service — such as an MRI, surgery, or extensive lab work — 20% of a large bill can be a significant amount. There is no annual cap on Part B coinsurance in Original Medicare.

Can I get help paying my coinsurance?

Yes. Every standardized Medigap (Medicare Supplement) plan letter covers Part B coinsurance to some degree — Plans A, B, C, D, F, G, M, and N cover it at 100%, Plan K at 50%, and Plan L at 75%. Plan N's 100% still leaves you a copayment of up to $20 for some office visits and up to $50 for an emergency room visit that doesn't end in admission. Plans C and F are closed to anyone who first became eligible for Medicare on or after January 1, 2020, so if you came to Medicare after that date, Plan G is the nearest equivalent still sold. The Qualified Medicare Beneficiary (QMB) Medicare Savings Program also covers Part A and Part B cost-sharing in full, and federal law bars providers from billing QMB enrollees for those amounts. The other two Medicare Savings Programs (SLMB and QI) pay your Part B premium but do not pay coinsurance. Full Medicaid in some states picks up Medicare cost-sharing as well.

Does Medicare Advantage handle coinsurance differently?

Yes. Medicare Advantage plans may charge copays instead of coinsurance for some services, and every plan must set an annual in-network out-of-pocket maximum that no plan may exceed (the 2026 mandatory ceiling is $9,250 in-network and $13,900 combined in-network plus out-of-network for PPO plans; many plans set their own MOOP lower). Original Medicare has no out-of-pocket maximum, which is why a Medigap supplement is often paired with it.

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This information is for educational purposes only and is not legal or medical advice. Always verify with your doctor's office and insurance company.