Coinsurance

Coinsurance is the share of the cost of a covered service that an insured person pays once the deductible has been satisfied, expressed as a percentage. HealthCare.gov defines it as "the percentage of costs of a covered health care service you pay (20%, for example) after you've paid your deductible," and its worked illustration makes the base explicit: on a visit whose allowed amount is $100 with 20% coinsurance, you pay $20 if the deductible has been met and the whole $100 if it has not.

The word carries a second, unrelated meaning in property insurance, and the two have nothing in common beyond the spelling. In a homeowners or commercial property policy, a coinsurance clause is a condition requiring the owner to carry coverage equal to a stated percentage of the building's replacement cost. It is not a share of each claim; it is a penalty for underinsuring, and it changes the payment on a partial loss. Both senses are addressed on this page because a reader who meets one and reasons from the other will get the answer badly wrong.

🔑 The base is the allowed amount, and that single fact explains most surprising medical bills. A plan's coinsurance percentage is applied to the price the plan has agreed with the provider, not to the number on the provider's original invoice. In network, those two figures are reconciled for you and the difference disappears. Out of network there may be no agreed price at all, so a provider can bill more than the plan's allowed amount and look to the patient for the difference. That difference is a separate charge from coinsurance, it is not a percentage of anything, and it generally does not count toward the out-of-pocket maximum, which is why an out-of-network bill can defeat the protection a plan appears to offer. Federal law now bars that practice in defined circumstances, chiefly emergency care and certain services delivered by out-of-network clinicians at in-network facilities, with the protections written in parallel into the Public Health Service Act, the Employee Retirement Income Security Act and the Internal Revenue Code. Outside those circumstances the exposure is real.

Coinsurance and the deductible are sequential, not simultaneous. Below the deductible you generally pay the whole allowed amount; above it you pay your percentage and the plan pays the rest; above the out-of-pocket maximum you pay nothing further for covered in-network care. That three-stage structure is why the same procedure can cost three different amounts in the same year depending only on when in the year it happens.

Percentages differ by service, and by network status, within one plan. A plan commonly applies one percentage in network and a higher one out of network, and may apply a different percentage to a category such as durable medical equipment or imaging. Original Medicare's outpatient coverage works on the same principle, charging a flat share of the Medicare-approved amount for most services, and it is the clearest illustration of why an unbounded percentage matters: a share of an unbounded number is itself unbounded, which is the reason supplemental coverage exists at all.

Now the property-insurance sense, which is a different animal. A coinsurance clause requires the owner to insure the dwelling to at least a stated percentage of its replacement cost, commonly in the range of 80% to 90%. Insure it for less and the policy does not simply pay less on a total loss, which is capped at the policy limit anyway; it reduces the payment on partial losses, which is where the clause actually bites. The recovery is the greater of the actual cash value of the damaged property or a proportion, and the proportion's denominator is the amount you were required to carry, not the full replacement cost. Getting either of those two details wrong understates the payment, which is a common failing in general explanations of the rule. The practical consequence is that a policy bought years ago and never revisited can quietly fall out of compliance as construction costs rise, with no notice and no change in the premium to signal it.

1. The deductible is satisfied first. Until then you generally pay the full allowed amount for covered services, apart from anything the plan pays ahead of the deductible.

2. The plan prices the service at its allowed amount, the negotiated rate rather than the provider's list price.

3. You pay your percentage of that amount and the plan pays the remainder.

4. The amount you paid counts toward the out-of-pocket maximum.

5. Coinsurance stops once the out-of-pocket maximum is reached, and the plan pays the whole of the covered in-network bill for the rest of the plan year.

A hypothetical, health sense, showing what the percentage is a percentage of. A plan has 20% in-network coinsurance and the deductible has already been met. An imaging centre in the network bills $6,500 for a scan; the plan's allowed amount for it is $4,000. The coinsurance is 20% of the allowed amount, so 0.20 × 4,000 = $800, and the plan pays the remaining $3,200. The $2,500 gap between the billed charge and the allowed amount is written off, because the provider agreed to the plan's price as a condition of being in network.

Change one fact: the same scan at an out-of-network centre, where the plan has agreed no price. Suppose the plan still allows $4,000 and applies a higher out-of-network share of 40%, so you owe 0.40 × 4,000 = $1,600, and the provider may additionally look to you for the $2,500 it billed above the allowed amount. Your total is $4,100 rather than $800, and the balance-billed portion generally does not count toward the out-of-pocket maximum. Figures are illustrative.

A hypothetical, property sense, showing the clause on a partial loss. A home costs $500,000 to rebuild. The policy carries an 80% coinsurance clause, so the required amount of insurance is 0.80 × 500,000 = $400,000. The owner insured it for $300,000. A kitchen fire causes $40,000 of damage. The proportion is 300,000 ÷ 400,000 = 0.75, so the proportional recovery is 0.75 × 40,000 = $30,000. The insurer pays the greater of that figure or the actual cash value of what was damaged; if the depreciated value of the damaged kitchen is $24,000, the proportional figure wins and the settlement is $30,000. The owner absorbs the remaining $10,000, plus the deductible, on a loss the policy limit would comfortably have covered. Note what did the damage: not the size of the claim, but a number chosen years earlier and never revisited.

Pros

• It keeps some of the cost of care visible to the patient without exposing them to the whole of it, which is the design intent.
• It is proportional, so a cheap service produces a cheap share. A flat copayment does not.
• It is bounded from above on a health plan. Once the out-of-pocket maximum is reached, coinsurance stops entirely for covered in-network care.
• Plans with meaningful coinsurance generally carry lower premiums, which is a reasonable trade for someone who rarely uses expensive services.

Cons

• It is unknowable in advance, because you cannot price a service until you know the plan's allowed amount for it and how much of it will be billed.
• On an expensive episode it grows without limit until the out-of-pocket maximum, and that maximum is often several thousand dollars away.
• Out of network it is applied to a number the plan chose rather than the one you were billed, and the gap may be yours as well.
• It resets with the plan year, so an illness spanning December and January runs the sequence twice.
• The property-insurance meaning shares only the word, and a reader who carries the health intuition across will misread their own policy.