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Coinsurance, Explained With Real Numbers

SmartHealthMatch team · Reviewed by a licensed health insurance advisor · Updated August 2026

In short: Coinsurance explained in plain English: how the percentage works, what it's a percentage of, and a step-by-step dollar example from deductible to OOP max.

Coinsurance is the plan term people nod along to and then quietly misunderstand. It sounds like a synonym for copay. It is printed as a friendly-looking number — 20%, 30% — that seems like a small share of something. And then a bill arrives that is larger than expected, and the percentage turns out to have been applied to a bigger figure than anyone pictured.

The concept itself is not complicated. What trips people up is what the percentage is a percentage of, and where in the payment sequence it applies. Here is both, with the arithmetic written out.

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What coinsurance is

Coinsurance is cost-sharing expressed as a percentage. After you have met your deductible, you and the plan split covered costs — you pay your percentage, the plan pays the rest — until your spending reaches the plan's out-of-pocket maximum.

The convention is that the percentage named is your share. A plan advertising "20% coinsurance" means you pay 20% and the plan pays 80%. Occasionally a plan document lists it the other way around as an "80/20 plan," with the plan's share first. When in doubt, look at which number sits next to the words "you pay" in the summary of benefits.

That distinguishes it from a copay, which is a flat dollar amount — $35 for an office visit, $20 for a generic prescription — that does not move with the size of the bill. A copay is predictable by design. Coinsurance is proportional by design, which means it is small on small bills and large on large ones.

Where it sits in the payment order

Almost every question about coinsurance answers itself once you can see the sequence. For covered, in-network care on a typical plan:

1. Premium. Paid monthly whether or not you use care. It never counts toward anything below.

2. Deductible. You pay the full negotiated cost of covered services until you have satisfied it. (Preventive care and any copay-based services usually sit outside this step.)

3. Coinsurance. Deductible satisfied, the percentage split begins.

4. Out-of-pocket maximum. Once your deductible, copays, and coinsurance together reach this ceiling, the plan pays 100% of covered in-network care for the rest of the plan year.

Coinsurance, in other words, only lives in the gap between step 2 and step 4. That is a genuinely important point, and we will come back to it. If the deductible and out-of-pocket maximum are still fuzzy, our guide to deductible vs. out-of-pocket max works through those two numbers on their own.

The part people get wrong: a percentage of what?

This is where the surprise bills come from. Coinsurance is calculated on the plan's allowed amount — the rate the insurer and the provider have negotiated — not on the price the provider bills.

That usually works in your favor. A hospital may bill $12,000 for an outpatient procedure while the plan's allowed amount for that procedure is $5,400. Your 20% is 20% of $5,400, or $1,080 — not $2,400. The negotiated discount happens before your share is figured, which is a substantial and underappreciated part of what in-network coverage buys you.

It cuts the other way when the provider is not in the network. Then there may be no negotiated rate at all, the plan applies a higher out-of-network coinsurance percentage against a separate deductible, and in some cases the provider can bill you for the remaining balance. The percentage on the brochure is doing far less work than the network is. Our explainer on how networks actually work covers why the same procedure with the same doctor can produce two very different bills.

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A worked example, dollar by dollar

Illustrative figures only — real plans vary by state, carrier, and year — but the mechanics hold everywhere. Say your plan has a $2,500 deductible, 20% coinsurance, and a $7,500 out-of-pocket maximum. In March you have an outpatient surgery with an in-network allowed amount of $22,000.

Step 1 — The deductible. You pay the first $2,500 of the allowed amount. Paid so far: $2,500. Remaining allowed amount: $19,500.

Step 2 — The split. Coinsurance now applies to that $19,500. Your 20% share would be $3,900; the plan's 80% share is $15,600.

Step 3 — Running total. $2,500 deductible plus $3,900 coinsurance is $6,400. That is under your $7,500 ceiling, so you owe all of it. Total for the surgery: $6,400 of a $22,000 procedure.

Step 4 — The rest of the year. You have $1,100 of coinsurance exposure left before the ceiling closes. If a follow-up in June produces another $8,000 in covered charges, your 20% would be $1,600 — but you pay only $1,100, because that is where the out-of-pocket maximum stops it. From that point on, covered in-network care costs you nothing for the remainder of the plan year.

Two things are worth noticing. Your effective share of a $22,000 event was about 29% once the deductible was included — higher than the 20% headline. And the number that ultimately capped your exposure was not the coinsurance percentage at all; it was the out-of-pocket maximum.

Why the percentage matters less than it looks

Because coinsurance only operates between the deductible and the ceiling, its practical impact depends entirely on how wide that gap is. Compare two illustrative plans with identical deductibles:

	Plan A	Plan B
Deductible	$3,000	$3,000
Coinsurance (your share)	10%	30%
Out-of-pocket maximum	$9,000	$5,000
Cost of a $10,000 event	$3,700	$5,000
Cost of a $60,000 event	$8,700	$5,000
Worst case for the year	$9,000	$5,000

Plan A's 10% coinsurance is the more attractive number on the page, and it does win a moderate year. But in a serious year Plan B costs $3,700 less, because its ceiling is lower. The generous percentage was doing its best work in exactly the range where the stakes were smallest.

The practical order of operations when comparing plans: annual premium plus out-of-pocket maximum first, because together they define your worst-case year. Then the deductible, because it sets your cost for a moderate year. Then coinsurance, which fine-tunes the middle. A low coinsurance percentage is a real benefit — it is just not the number to lead with.

Details worth checking on any specific plan

Coinsurance rules are less uniform than the headline percentage suggests. Before enrolling, look for these in the plan documents:

Different percentages for different services. Many plans use one rate for most care and another for imaging, outpatient surgery, or specialty drugs. The number on the brochure is usually the most favorable one.
Prescription tiers. Lower drug tiers often use flat copays while specialty tiers switch to coinsurance — which can turn a high-cost medication into a percentage of a very large number.
A separate prescription deductible. Some plans run one for drugs alongside the medical deductible, which delays when drug coinsurance begins.
Out-of-network terms. A distinct percentage, a separate deductible, a separate and much higher maximum, or no coverage at all. Plan type drives this; PPO vs. HMO vs. EPO lays out the differences.
Family versus individual accumulation. Whether each person's coinsurance runs against their own deductible and maximum or against the family's changes the math for households considerably.

All of this is disclosed. Marketplace plans publish a standardized Summary of Benefits and Coverage, and our walkthrough on how to read an SBC shows where each line lives. Reading it takes a few minutes and is the single highest-return thing you can do before enrolling.

A note on plan type

Cost-sharing structure is one input, not the whole decision. ACA marketplace plans are guaranteed issue, cannot exclude pre-existing conditions, must cover the ten essential health benefits, and are required to cap in-network out-of-pocket spending — and if your household income qualifies for a premium tax credit, or for the cost-sharing reductions that lower deductibles and coinsurance on silver-level plans, the marketplace is frequently the strongest option available. For many households it simply wins.

Privately sold underwritten plans are a different product. They are medically underwritten rather than guaranteed issue, so approval depends on health history, and they may limit or exclude pre-existing conditions. Some are not required to cover the essential health benefits, and cost-sharing terms — including how coinsurance is calculated and whether the annual ceiling behaves the way an ACA plan's does — can differ meaningfully. For some healthy households outside subsidy range they are a reasonable fit; for anyone with an ongoing condition, they are frequently the wrong tool. Compare both honestly rather than assuming either one.

The bottom line

Coinsurance is your percentage share of the plan's negotiated rate, applied after the deductible and capped by the out-of-pocket maximum. Read it as one term inside a sequence rather than a standalone feature, confirm which services it applies to and at what rate, and check the network before any scheduled care — because in-network status determines the number the percentage is multiplied by, and that number matters more than the percentage does.

Frequently asked questions

Does coinsurance start before or after the deductible?

On most plans, after. You pay the full negotiated cost of covered services until the deductible is satisfied, and coinsurance begins on covered costs after that point. A handful of services may be an exception — some plans apply coinsurance to certain categories, such as specialty drugs or imaging, without requiring the full deductible first. The summary of benefits for the specific plan will show which services say the deductible does not apply.

What does 20% coinsurance actually mean in dollars?

It means that after your deductible, you pay 20 percent of the plan's allowed amount for a covered service and the plan pays the other 80 percent. The allowed amount is the negotiated in-network rate, not the price on the hospital's chargemaster, so 20 percent of a $12,000 billed procedure might be 20 percent of a $5,400 allowed amount instead. Your share keeps accruing until you reach the plan's out-of-pocket maximum, after which covered in-network services are paid at 100 percent for the rest of the plan year.

Is a lower coinsurance percentage always the better plan?

Not on its own. Coinsurance only operates in the space between the deductible and the out-of-pocket maximum, so a plan with generous 10 percent coinsurance and a very high out-of-pocket maximum can expose you to more in a bad year than a plan with 30 percent coinsurance and a low ceiling. Compare annual premium plus out-of-pocket maximum first, then use the coinsurance percentage to judge the middle range where moderate medical years actually land.

Does coinsurance apply to out-of-network care?

It depends entirely on the plan. Many HMO and EPO plans do not cover routine out-of-network care at all, so there is no coinsurance — there is just the bill. PPO plans typically do cover it, but at a higher coinsurance percentage, against a separate and larger out-of-network deductible and maximum. In some situations the provider may also bill you for the difference between their charge and the plan's allowed amount, which is why verifying network status before scheduled care matters more than the percentage itself.

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Educational information only — not an offer of insurance, and not legal, medical, or tax advice. Plan availability, benefits, and premiums vary by state and are set solely by the insurance carrier. Underwritten plans require carrier approval, are not guaranteed issue, and may limit or exclude pre-existing conditions. Savings are not guaranteed. Marketplace coverage is available at HealthCare.gov.