Coinsurance is your share of a cost expressed as a percentage. If the plan pays 80% and you pay 20%, that 20% is coinsurance, and it usually kicks in after the deductible is satisfied.

The difference from a copay matters more than it looks. A copay is knowable in advance: $50 is $50. Coinsurance is a percentage of a number you don’t know yet — and in a hospital that number can be large. Twenty percent of an imaging bill is a different animal from twenty percent of an office visit.

Where it bites hardest: imaging, surgery, hospital stays, specialty drugs. Anything where the underlying price is high and variable.

How to bound it: the out-of-pocket maximum is the ceiling on your combined deductible, copays and coinsurance for in-network care in a plan year. Out-of-network care often doesn’t count toward it, which is how people end up past the “maximum” and still paying.

In New York: coinsurance is a percentage of the plan’s allowed amount, never of the provider’s charge, and that difference is the whole argument on a disputed bill. For emergency care, and for an out-of-network clinician treating you inside an in-network hospital, New York’s surprise billing law limits you to in-network cost sharing, so the percentage is calculated on the in-network allowed amount even when the provider was not. Disputes go to the Department of Financial Services, which runs an independent dispute resolution process.

Also called: cost sharing percentage. Reference: Coinsurance on Wikipedia — general definition, not New York specifics.

More in Glossary of health coverage and billing terms

  • Qualifying life event — The change that opens a 60-day window to enrol outside the marketplace season, and the clock that starts the day it happens.
  • Essential health benefits — The ten categories every marketplace and small group plan must cover, and the reason a plan that skips one is not really insurance.
  • Presumptive eligibility — A temporary approval that starts care immediately while a full Medicaid application is processed, and the one thing it does not carry.
  • Out-of-pocket maximum — The out-of-pocket maximum caps your cost sharing for covered in-network care each plan year. What counts toward it, what never does, and the New York rules.
  • Formulary — A formulary is a plan's list of covered drugs, sorted into tiers that set your cost. What on-formulary means, how exceptions work, and New York's NYRx list.

This page explains how the system works. It is not medical advice. More.