The percentage of costs of a covered health care service you pay (20%, for example) after you’ve paid your deductible. Let’s say your health insurance plan’s allowed amount for an office visit is $100 and your coinsurance is 20%. If you’ve paid your deductible: You pay 20% of $100, or $20. The insurance company pays the rest. If you haven’t met your deductible: You pay the full allowed amount, $100. Example of coinsurance with high medical costs Let’s say the following amounts apply to your plan and you need a lot of treatment for a serious condition. Allowable costs are $12,000. Deductible: $3,000 Coinsurance: 20% Out-of-pocket maximum: $6,850 You’d pay all of the first $3,000 (your deductible). You’ll pay 20% of the remaining $9,000, or $1,800 (your coinsurance). So your total out-of-pocket costs would be $4,800 — your $3,000 deductible plus your $1,800 coinsurance. If your total out-of-pocket costs reach $6,850, you’d pay only that amount, including your deductible and coinsurance. The insurance company would pay for all covered services for the rest of your plan year. Generally speaking, plans with low monthly premiums have higher coinsurance, and plans with higher monthly premiums have lower coinsurance.