Simple definition
Coinsurance is the percentage of a covered cost you keep paying after your deductible is met. If your plan lists 20% coinsurance, the insurer pays 80% of the approved amount and you pay 20% — continuing until you hit your out-of-pocket maximum, after which the plan covers everything.
Why it matters
Meeting your deductible isn't the finish line, and that surprises people. On a large bill, 20% of a serious hospital stay is still a serious number. Coinsurance is why the out-of-pocket maximum — not the deductible — is the figure that tells you your genuine worst case for the year.
Real-life example
You've met your $2,000 deductible. A procedure is approved at $10,000. With 20% coinsurance you owe $2,000 of it and the plan pays $8,000 — unless that pushes you to your out-of-pocket maximum, at which point the plan picks up the rest of the year.
Formula
Your share = approved amount × coinsurance rate
Common mistakes
- Assuming costs stop once the deductible is met.
- Reading the out-of-pocket maximum as optional fine print rather than your real worst case.
- Going out of network, where coinsurance is higher and some costs may not count toward your maximum at all.
- Confusing coinsurance with a copay — one is a percentage, the other a flat fee.
Pro tips
- Judge a plan by its out-of-pocket maximum, since that's the number that caps a bad year.
- Confirm a provider is in network before scheduled care; the coinsurance difference can be large.
- Ask for the approved amount, not the billed amount — your share is calculated on the former.
- Request an itemized bill and check it against your explanation of benefits before paying.
Related Money Dictionary terms
- DeductibleThe amount you pay out of pocket for covered costs before your insurance starts chipping in.
- CopayA fixed dollar amount you pay for a specific service, like a doctor visit, at the time you receive it.
- Out-of-Pocket MaximumThe most you will pay for covered care in a year, after which insurance covers eligible costs fully.
- PremiumThe regular payment you make to keep an insurance policy active, usually billed monthly, quarterly, or yearly.
- ClaimA formal request you file with your insurer to be paid for a covered loss or medical expense.
- Explanation of BenefitsA statement from your insurer showing what a service cost, what they paid, and what you may owe, though it is not a bill.
Frequently asked questions
What's the difference between coinsurance and a copay?
A copay is a fixed dollar amount for a service, like $30 for an office visit. Coinsurance is a percentage of the cost, so what you owe scales with the size of the bill. Plans often use both for different services.
When does coinsurance stop?
When you reach your out-of-pocket maximum for the plan year. After that, the plan pays 100% of covered, in-network care for the rest of the year.
Why did I owe more than my coinsurance percentage?
Common causes are out-of-network care, a service the plan doesn't cover, or balance billing by a provider. Compare the bill to your explanation of benefits and ask the insurer about anything that doesn't match.
Knowing what Coinsurance means is knowledge — the first half. A brick gets placed when you act on it: find your plan's coinsurance rate and out-of-pocket maximum and write both down.
Sources & references
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Plain-English education — not personalized legal, tax, or investment advice.