Coinsurance is the percentage of health care costs you're responsible for after you've met your annual deductible. For example, with an 80/20 plan, your insurer pays 80% of eligible costs and you pay the remaining 20%, until you hit your out-of-pocket maximum. After that point, your insurer covers 100% for the rest of the plan year. Coinsurance is commonly applied to more significant services like surgeries, specialist care, and hospital stays. Unlike a copay, which is a flat fee per visit, coinsurance is a percentage of the total bill, so your share of costs can vary significantly depending on the service.