Co-insurance is the percentage split between you and your insurer for eligible expenses after you have met your deductible. It is expressed as a ratio such as 80/20, meaning the insurer pays 80% and you pay 20%. Co-insurance rates can vary by benefit category — for example, a plan might reimburse 80% for prescription drugs but only 50% for dental. Some premium plans offer 100% co-insurance (the insurer pays the full eligible amount after the deductible). Co-insurance applies to the "reasonable and customary" amount, so if a service costs more than what the insurer considers reasonable, you pay the difference on top of your co-insurance share. Understanding your co-insurance rate is essential for estimating your true out-of-pocket costs throughout the year.